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Copying Status: Public Domain. However, no liability is accepted for the correctness and safety any procedures or advice given here. This FAQ is distributed in the hope that it will be useful. This file can be found at http:www.minidisc.org.
7 cm (2.75") x 6.75 cm (2 21/32") x 0.5 cm (3/16") The disc inside is 64 mm in diameter.
Since MD stores audio using a data reduction technique (see ATRAC, below) there are considerable differences between the audio signal from a CD and that of an MD digitally recorded from it. Whether or not these differences are audible however depends to a large degree upon who is doing the listening, most MD users consider the sound quality of modern MD equipment to be essentially the same as CD. But even those who can hear differences usually find them to be undisturbing and inconsequential, frequently being unable to say which is which. (Note that a proper A/B comparison of the two formats necessitates using the same DAC. If an outboard DAC is unavailable, or the MD unit (such as a portable) does not provide digital out, the MD unit's monitor mode can be used to do D/A conversion of external digital signals, thereby allowing comparison with a common DAC.)
A small double-blind test made by the ABX Company found that subjects did not find a difference between ATRAC processed music and its original, but could readily spot the difference when a special test signal was used for the comparison.
However, one significant difference with DAT is that differences with the original increase in each subsequent copy generation, even when recording digitally. Each time the MD is played, a full 16-bit data stream is regenerated from the compressed data. Each time it records, the input data stream is recompressed. The artifacts of the compression process build up from generation to generation. See ATRAC below.
Yes. DCC uses a lower-loss compression algorithm than MD (4: 1 vs. 5: 1), and thus could sound better. MD should be compared to DCC with the same DAC, for fair results. Also note that most tests compared the first generation Philips home DCC recorder with the first generation Sony portable MD recorder (MZ-1).
ATRAC (Adaptive TRansform Acoustic Coding) divides the 16 bit 44.1 KHz digital signal into 52 sub-bands in the frequency domain (after a Fast Fourier Transform). The sub-bands in the low frequencies are finer than the ones in the high frequency range. A psycho-acoustic transfer function that takes advantage of the masking effect and the absolute hearing threshold then removes enough information to reduce the data stream to 1/5th of the original size. Each channel receives that treatment separately (the Sony MZ-1 portable MD recorder features one ATRAC encoder/decoder chip per channel). PASC (Precision Adaptive Sub-band Coding) divides the digital signal into equally spaced sub-bands and removes less information (to only 1/4th of the original size). PASC is essentially the MPEG Layer 1 audio standard (can be decompressed with MPEG Layer 1 players after a trivial preprocessing step).
Both are data compression algorithms, used to store the information content from a stream of 16-bit samples in fewer bits. The purpose of compression is to reduce the rate at which the disk has to deliver or record bits, and to reduce the total number of bits stored. There are many compression algorithms. The ones used for computer data (for example in archiving programs) are lossless; the result of decompression is identical to the input.
PASC and ATRAC are both "lossy" algorithms. In order to get greater compression, they do not attempt to preserve every bit of the original data, but rather only the acoustically "important" bits. Considerable cleverness goes into finding the sounds masked by properties of the human auditory system, ones that you would not hear even if they were reproduced. By all accounts the two schemes do amazing well, considering they operate in real time.
For a stereo signal it's 292162.5 bits/sec. ATRAC compresses 512 incoming 16 bit samples (1024 bytes) into one ATRAC ``sound group'' (212 bytes) giving an audio compression ratio of 4.83:1. Here is the math:
44100 samples/sec (incoming single channel rate)
/ 512 samples/soundgroup (giving soundgroups/sec/channel)
* 2 channels (giving stereo soundgroups/sec)
* 212 bytes/soundgroup (giving stereo bytes/sec)
* 8 bits/byte (giving stereo bits/sec)
= 292162.5 bits/sec.
ATRAC is 292kbit/sec, giving ``CD like'' audio fidelity. MPEG Layer 1 (i.e. PASC) gives transparent CD fidelity audio at 384kbit/sec, Layer 2 (i.e. Musicam) and Layer 3 give ``CD like'' fidelity at 224kbit/sec and 128kbit/s respectively. A user has compared ATRAC and MPEG Layer 3 and rates ATRAC far better.
The compression of linear (PCM) digital audio into ATRAC format and expansion back again is an asymmetric process. Even if you are compressing and expanding data that has been through the compression and expansion cycle once before, the result of the operation will yield data that is slightly different from what you started with. This may seem strange, since fundamentally it could be done symmetrically. However, there are a few uncontrollable asymmetries in the process. One is the framing of input data. ATRAC starts by breaking the input signal into blocks of 512 samples. If it were possible to arrange that the sample block boundaries created during decompression be used when the signal was further recompressed, it would improve the symmetry of the process. Another is that ATRAC introduces noise, usually at an inaudible level, but in subsequent compression this noise is indistiguisuable from signal and can alter what ATRAC determines to be masked, thereby changing the number of bits allocated to each band. Also, mathematical round off error in the compression and decompression calculations may introduce noise as well.
The next question then becomes, why not copy the compressed data directly, thereby avoiding the asymmetry of compression and decompression? Unfortunately, consumer grade MD machines do not provide access to their compressed data, nor do they provide a way of directly recording compressed data, even if it were available. The S/PDIF digital interconnect only carries data in the linear (PCM) format. Professional machines however (such as the Sony MDS-B5 with direct ATRAC I/O) do allow exact bit for bit MD copies to be made.
MD: 140MB in Data Mode [~160MB for 74 min. in audio mode]
CD-ROM: 650MB
CD Audio: 740MB
DAT: ~1GB
DCC: 175MB [60 min. tape]
About 2 seconds (4 seconds monoaural). Audio information on MiniDisc is organized in cluster units, which constitute the minimum unit for write or over-write of data to the disc. Each cluster contains 32 sectors of audio data, 3 sectors of link information (next cluster, previous cluster, and ??) and 1 sector of "sub data". Each sector is 2332 bytes, and 2 sectors are grouped together into a 4664 byte block holding 11 sound groups. Each sound group is 424 bytes and contains 11.6 msec of stereo data or 23.2 ms of mono data. A cluster is then 74624 bytes (32*2332), containing 176 sound groups (74624/424), for 2.0416 seconds of stereo sound (176*11.6) or 4.0832 seconds of monoaural.
The sound group (1/86th second) constitutes the finest resolution at which the audio stream can be edited. Older consumer grade MD decks have an edit positioning accuracy of approx. 60 msec (5 sound groups) but many modern machines allow edit positioning in units of a single sound group (11.6ms).
According to MiniDisc Rainbow Book recommendations (a ``remark'', not a mandatory requirement) the virgin space that runs to the end of the disc (if any) is always the first allocated, followed (in ascending disc address order) by the remaining free segments located throughout the disc. Free segments smaller than 6 clusters (one cluster is slightly more than 2 seconds stereo) are not allocated until enough adjacent area is freed to make an open space of at least six whole clusters. (Thanks to David Tamkin for this investigation using a Sony MDS-JE520 MD deck.)
On a frame by frame (11.6 ms) basis, ATRAC is always trying to give as much resolution as it can to each spectral component that it deems important. However it is continually short of bits, and except for the case of zero input, or a very simple signal (e.g. a few pure tones), ATRAC is using all the bits per second it has available. If it had more bits per second available it could use them, and record the signal with better sound quality.
In Sony's AES paper on ATRAC they talk about the bit allocation algorithm, and how it in general allocates more bits than are available, so they have a method for subtracting an offset from the allocations in order fit within their bit budget for a sound frame.
In the zero or simple input case, there can be unused bits in some frames, but since there is an unvarying (i.e. fixed) relationship between 11.6 ms of audio and a 424 byte "sound group" on the disc, the system is not flexible enough to actually take up less disc space when it doesn't need it.
The entire rest of the sound reproduction chain is important. The digital to analog converter (DAC) is key to reproducing the sound from the decompressed data. The audio amplifier circuitry is also critical. And there have been reports of MD playback machines (not Sony) that were made unlistenable by poor-quality attached headphones!
Every component of the chain from the DAC to the eardrum is important to good sound reproduction. All elements other than the compression algorithm must be held constant before A: B comparisons are made, for example.
The encoder (recording) side of ATRAC offers room for improvement from one generation to the next (specifically, in the decision about how to allocate encoding bits so as to best match human psycho-acoustic properties). Thus, MD recordings made on a newer machine with a better ATRAC encoder will sound better than old recordings, even when played back on an old machine.
The decoder (playback) side of ATRAC has a fixed structure, though improvements in the digital signal processing accuracy and quality are possible.
Since the ATRAC encoder plays the largest role in how an MD sounds, the implication for making digital copies between two MD units is to use the older unit for playback and the newer one for recording. When making analog copies, the relative quality of the A/D and D/A converters must be born in mind.
Sony, in the process of developing ATRAC, unknowingly employed techniques that were covered in prior Dolby patents. Sony made a licensing agreement with Dolby to avoid (or as the result of) a legal challenge from them. Dolby doesn't own all of ATRAC, just some part(s) of it.
(From the February '96 issue of the German Stereo magazine. The phrase sparkling noise in the text below is an imprecise translation of Spratzeln, which has been said to be typified by the sound of CO2 bubbles bursting on the surface of a beer).
|
ATRAC 1 |
ATRAC 2 |
ATRAC 3 |
ATRAC 3.5 |
|
|
Introduction |
1993 |
1994 |
1995 |
Current |
|
Threshold |
15KHz |
18KHz |
18KHz |
18KHz |
|
Noise |
Big! |
Much lower than ATRAC 1 |
Dynamic filter: no noise in breaks |
==DAT |
|
Sound |
metallic |
close to DAT |
no difference to DAT in "blind listening test" |
~=DAT |
|
Sparkling Noise |
hearable |
not much better |
only hearable in very silent passages |
gone |
|
ATRAC 1 recorder |
ATRAC 2 recorder |
ATRAC 3 recorder |
ATRAC 3.5 recorder |
|
|
ATRAC 1 Player |
(see table above) |
15KHz! Less noise |
15KHz! Less noise |
15KHz! Less noise, "sparkling noise" gone |
|
ATRAC 2 Player |
15KHz threshold! Less noise, still metallic sound |
(see table above) |
No difference from ATRAC 2 recorder |
No "sparkling noise", a bit less noise (as when recorded with 2.0) |
|
ATRAC 3 Player |
No big difference from above |
No difference from ATRAC 2 player |
(see table above) |
No "sparkling noise", a bit less noise (as when recorded with 3.0) |
|
ATRAC 3.5 Player |
No big difference from above |
"sparkling noise" still not better, but less noise than above |
low level "sparkling noise" remains, a bit less noise than above |
(see table above) |
In general, all ATRAC versions are fully compatible with each other. However, if you play or record something with 1.0 the result will be rather poor no matter from which version the source came from or goes to. If you take a higher version the result is generally be good enough, when you use ATRAC 3.5 for either playing or recording it gets even a bit better. For portables and car-players with 3.0, the weakest link is the A/D converter.
The practical result: buy a 3.5 or better for 'home use', record there, and you get better quality in your 3.0 portable or car-player.
|
ATRAC 1 to ATRAC 1 |
After 5 generations unacceptable, after 20 generations awful. |
|
ATRAC 2 to ATRAC 2 |
After 5 generations no hearable difference, after 20 generations tiny distortion. |
|
ATRAC 3 to ATRAC 3 |
Not much better than with ATRAC 2. |
|
ATRAC 3.5 to ATRAC 3.5 |
Slightly better than ATRAC 3, relating to noise. |
Translated by Felix Gers ([email protected]).
There are two sources of distortion. One is the chain of components that brings the sound to the MD's input. If you go analog-to-analog, you introduce the CD's DAC and the MD's ADC chips, each with its own artifacts. However, you can bring the digital data stream directly to the MD; then the only source of differences is the ATRAC compression algorithm.
The ATRAC encoder removes information from the audio material in order to store it on the MD (5: 1 compression with loss). To make better MD recordings from CD, connect the MD recorder to the CD player via a digital connection (if possible). Thus, the ADC (poor in the first generation machines) cannot affect the sound quality. Otherwise, when recording via the analog input, make sure to adjust the manual recording level on the MD machine so that the meter peaks just above -12 dB (on the Sony MZ-1, never enable the AGC for CD recording).
Yes, because the MD's analog to digital conversion circuits are involved, in addition to the ATRAC compression. If MD becomes more popular, look to see up-market MD recording decks with high-end input circuitry. This could make a bigger difference than any amount of tinkering with ATRAC, since the input circuits of the early MD models can't be the best.
Generally, analog recordings sound fine. Do set the recording level manually to avoid the audible effects of the Automatic Gain Control (AGC) circuitry adjusting the level during very loud and very quiet passages.
You will have the correct recording level when the level meter is just between 4 and 5 bars. Verified with an MDS-503 from Sony, between 4 and 5 bars on the MZ-R3 record level meter will give you somewhere between -3 dB and 0 dB.
Regarding the 'R30: A user connected a 303 to the R30 using a POC (optical) cable and compared the two meters. The result: Just consider the top bar to be 'digital over' and try to adjust your level in such a way that the second bar doesn't light up too often when recording from an analogue source (even less when recording live). That should do it. Compared to the MZ-1 the R30 meter is not very useful.
Digital recording provides the most convenience when copying CDs: no recording levels need to be set, track marks are copied from the CD perfectly, and analog to digital conversion artifacts (real or imagined) are completely avoided. The one problem with digital recording is that SCMS will prevent further digital copies to be made from the copied MD.
Crutchfield's Tip of the Week is devoted to analog vs. digital recording to MiniDisc.
[Adapted from the DAT Heads FAQ]
SCMS stands for "Serial Code Management System" and is the way copies of digital music are regulated in the consumer market. It is information that is added to the stream of data that contains the music when one makes a digital copy (a "clone"). When making an analog copy only the music is transferred so there is no SCMS, and copying is totally unrestricted. Decks considered "professional" -- usually more expensive and with pro features, such as balanced XLR input/output -- are exempt from needing SCMS. Different manufacturers' pro decks behave differently: some allow one to set the SCMS code how one wishes, some only if the pro i/o is used, and some ignore it completely.
SCMS Bit Definitions:
Bits Meaning Explanation
---- ------------ ----------------------------
00 Permitted No restrictions at all
11 Restricted Allow 1 generation
10 Prohibited Do not allow copies
SCMS Operation on consumer decks:
Source Recorded on copy
-------------- --------------------------------
Analog input 11
CD 10
Digital, 00 11 or 00 (depending on model)
Digital, 11 10
Digital, 10 Will not record
Dubbing MD to MD with SCMS:
Play Deck Connection Record Deck SCMS Problem?
-------------- ------------- -------------- --------------
Consumer MD Digital Pro MD No
Pro MD Digital Consumer MD No
Pro MD Digital Pro MD No
Consumer MD Digital Consumer MD Yes
any MD Analog any MD No
SCMS does not limit the number of times you can copy a certain CD or MD. For instance, you can make 20 copies of a CD - you just can't copy any of the 20 copies.
It is ironic that if SCMS is to prevent unauthorized duplication of copyrighted information it has the above loopholes that a professional ripoff artist can easily use. And it is unfortunate that supposedly legitimate users, such as musicians recording their own music on cheaper, consumer decks, are restricted in the number of generations they can copy their music. [jfw/rg]
The pregroove wobble is at a higher linear frequency on the 74 minute blanks, causing the disc to spin slower (74 min: 1.2 m/s vs. 60 min: 1.4 m/s) and therefore record more information in the same linear space. There is also information encoded in the read-only TOC near the inner circumference of the disc telling the player how long the disc is. Originally it was difficult to make the 74' blanks, but nowadays the manufacturing costs of 60' and 74' discs are the same.
Sharp and Sony have both said that they will produce high density MD Data Drives with a capacity of about 650MB in 1997. Such capacity would allow a CD's worth of data to be stored uncompressed on an MD. However, new audio MDs made with such capacity would be incompatible with the existing hardware base (see below), and (assuming 16 bit linear recording), would not increase audio fidelity substantially, if at all. The main benefit of uncompressed recording would be the ability for professional users to make perfect MD duplicates. If the high density MD data drives become popular, it is possible that professional MD audio equipment will be made which uses MD data discs to store uncompressed audio.
No. Twice the tracks and density -- implies major changes in the read-write mechanism, the laser optics, etc., plus, the decoder side of ATRAC includes no provisions for linear data.
Physically, a pre-recorded MD is just like CD, using the same optical parameters, material, and production methods. Only the data contents are different (compressed ATRAC audio vs. 16 bit linear PCM audio). Unlike recordable MD, pre-recorded MDs do not have the magneto-optical coating layers or the lubricating layers. They are made with the same plastic-aluminum structure as CDs, so there is no way to record or erase anything on them. Prerecorded MD's are also read exactly like CD's (the player focuses a laser on pits and valleys within a transparent polycarbonate substrate backed by a coating of aluminum thus reflecting or dispersing the beam).
Recordable MD's are similar, but a pre-groove replaces the pits and valleys and an MO coating replaces the aluminum one. When recording, a laser is focused from one side of the disc onto the pre-groove and heats a spot on the MO recording layer to its Curie point while a magnetic field from a head in contact with the other side of the disc aligns magnetic dipoles within (read magnetizes) the heated spot on the MO layer (the N/S orientation corresponding to 0s and 1s in the data). During playback the MD machine focuses the laser on the pre-groove again, but at lower power, and the data is read back by measuring changes in polarization of light reflected from the previously magnetized regions (the Faraday effect).
In order to playback both pre-recorded and recordable media, all MD units have a dual function optical assembly which can, depending upon the disc type, detect changes in either reflectivity or polarization.
In terms of audio quality, pre-recorded MDs are in theory no different than recordable MDs, although, as always, audio quality depends upon which version of ATRAC the discs are encoded with. Some users have noticed certain pre-recorded MDs that sound worse than a homemade copy of the same CD with a modern MD recorder. It's likely that the ATRAC encoders used for prerecorded MDs are improved over time just as they are in the consumer units.
Finally, recordable MDs have an extra hole in their case, which is sensed by a microswitch within the MD unit that connects to the REFLECT pin of the microprocessor. It tells the MD player's microprocessor whether signal reflectivity is high (for pre-recorded) or low (for recordable).
On recordable MDs, 32 out of 36 sectors in every cluster (the smallest recordable unit) are used for storing audio data. Of the remaining 4 sectors, 3 are needed to accomodate the start up and run out of the error correction code, as stated in the MiniDisc IEEE paper: ``Because of the long interleaved ACIRC error correction code, three sectors must be used as "linking sectors." If the user changes or adds new data to the MD disc, two or three sectors for every start and end position of the new data need to be recorded.''. In other words, some space is wasted to accomodate re-recordability. On pre-recorded MDs, however, the data is fixed during mastering as one continuous stream, so these 3 extra sectors can be devoted to audio data. The Sony DADC MD handbook says that the maximum duration of a pre-recorded MD is 78:16.
Sony claims a blank MD can handle up to 1 million recordings... Time will tell! In practice, the user Table of Contents (user TOC or UTOC) of an MD could become messed up by accident (defective MD recorder or physical damage) and render the MD unusable. The Sony portable MD recorder (MZ-1) allows to recover some unusable MD's in test mode (machine open and test jumper in place) via the TOC erase function: the TOC gets obliterated and that results in a "like-new" blank MD. Physical damage to the TOC cannot be repaired, but similar defects in other parts of the MD should be removed from the available space by creating short quiet tracks that include the problem and recording after labeling these as "bad".
Sony claims in their Magneto Optical FAQs that data may be stored with magneto optical technology for more than thirty years without loss or degradation. Once written to the disk, data are safe from the magnetic fields and heat found in normal environments. However, strong magnets placed directly against the MD can destroy data, as evidenced by informal experiments conducted by users.
Rarely. A read-ahead buffer stores a few seconds of the audio material in memory during playback. The ATRAC decoder takes the data from the buffer, rather than directly from the medium. Thus, if the mechanism mistracks because of shock or vibration, the data continues to flow from memory while the MD machine recovers. If no more data remains available (when the disturbance lasts a long time), the audio material gets interrupted. Also note that the read-ahead buffer exists on every MD machine as part of the MD format.
Yes. Most MD machines feature a 10-second read-ahead buffer, but some only offer 3 seconds of memory, such as the first production run of Sony MZ-1 portable MD recorders and the Aiwa AMD-100.
Yes. The MD format stores data like hard-disk or floppy-disk drives in computers. The TOC contains a list of starting/ending positions and names for each track, like the directory in computers. Tracks can be erased, divided, combined, moved [or just ``swapped'' on some early models] and named like the files in computers. For example, after recording 11 5-minute tracks on a 60-minute MD, 55 minutes are used and 5 minutes remain. If the user decides to erase track #8, the TOC gets updated, and now 50 minutes are used by 10 tracks and 10 minutes remain. If the user now decides to make a 7 minute recording, no problem. After pressing the rec. button, the MD recorder seeks the next "empty" spot on the medium (according to the TOC) and starts. The resulting track consists of two separate segments of audio material: five minutes at the end of the MD and two minutes out of the space where track #8 used to be. During playback of the 7 minute track, the read-ahead buffer keeps the audio material seamless while the mechanism jumps between the first chunk of the track and the second one... Now try that on any sequential medium (DAT, DCC, analog cassette)!
Many users have reported problems if the MD recorder experiences shock and vibration while recording. Apparently the read-ahead buffer also works for recording, but strong disturbances could cause the laser to erase other spots on the MD, like the TOC or existing tracks, thus damaging previous recordings. To remain on the safe side, the MD recorder should not be subjected to shock or vibration while recording.
MD drives certainly buffer during recording, otherwise MDs could not make recordings over discontiguous free blocks on the disc due to the dead time during interblock seeking. The bigger problem is, what happens when the recorder is jarred and the hot laser skids across already recorded material? One of the brochures for the professional MD units mentioned that they had a special circuit to cut the laser power when any shock occured, thereby avoiding overwriting [much] already written material. On the normal consumer portables, you could probably lose some material when a shock occurs during recording. In any case, they're able to recover and get back on track.
The user-interface for title entry on many portable MD recorders is limited, requiring a button push to cycle past every letter. Most current home decks have a remote with a non-querty character input method. Two notable exceptions are the Aiwa portable that uses the volume thumbwheel on the remote to select each character, and Sony's modern MD decks that allow titling with a separately available keyboard-like remote that eases characten entry. There is a limit of of 255 tracks as defined in the MD format. There also appears to be an upper bound in the title and track names of 1700 characters total. Finally, certain units cannot display long titles, for example Sony's MDX-U1 car player only displays the first 32 characters of a title.
Sony wants MD-Data to replace 3.5" floppy-disks (1.4 Mb, 2.8 Mb and 21 Mb floptical), not CD-ROM (a well implanted medium in the computer industry by now, and currently cheap). MD-Data, has been released and provides 140 MB of data storage on an MD but is not compatible with the Audio MD format (Data drive cannot read or write Audio data except in "play" mode.)
No, there is a read-only label track on each MD that distinguishes an MD Audio and MD Data disc, MD equipment looks at this label to determine what kind of disc it is accessing. An MD audio disc inserted in an MD Data drive shows up as having 1k in disc, 0k available. The ATRAC encoded music information is not accessible. An MD Data drive user has written some information about the MD Data drive and cross compatability.
Sony makes an adaptor for that bizarre headphone connection. It's $13.34 (US). Call Sony at 1-800-488-SONY to order one. The part number is: in the US: PC-MP1HG, in Asia: PC-MP1S.
This information came in part from a Japanese magazine "MJ" and the 9/96 issue of the German Stereo magazine:
|
IC Generation |
IC Part Number |
MD Deck |
Introduction Date |
|
ATRAC 1 |
CXD-2527 |
MDS-101 |
2/93 |
|
ATRAC 2 |
CXD-2531 |
MDS-102 |
11/93 |
|
ATRAC 2 |
CXD-2531R |
MDM111 |
|
|
ATRAC 3 |
CXD-2536 |
MDS-S30/S35/S1 |
11/94 |
|
ATRAC 3 |
CXD-2536R |
MZ-R3 |
5/95 |
|
ATRAC 3.5 |
CXD-2536A |
MDS-JA3ES |
6/95 |
|
ATRAC 3.5 |
CXD-2536B |
MDS-503 |
10/95 |
|
ATRAC 4.0 |
CXD-2650R |
MDS-JE500/S37 |
8/96 |
|
ATRAC 4.0 |
CXD-2652AR |
MZ-R50 |
8/96 |
|
ATRAC 4.0 |
CXD-2652R |
MZ-R30 |
8/96 |
|
ATRAC 4.5 |
CXD-2537R |
MDS-JA50ES |
12/96 |
|
ATRAC 4.5 |
CXD-2654R |
MDS-JE520 |
9/98 |
See also the ATRAC version table.
In a word: No. The perceived problems with optical interconnects relate to an optical cable's greater theoretical potential to distort the digital signal, particularly to create small inaccuracies in the arrival time of data bits ("jitter"). However, in MiniDisc recording jitter is not an issue since the digital input signal's sample values are recovered and passed directly into a memory buffer or into a sampling rate converter that is clocked with the clock embedded in the input signal. The sampling rate converter and/or memory buffer allow the audio samples to be subsequently read and passed to the ATRAC converter with an accuracy determined by the MD unit's internal quartz clock. Even if jitter was an issue however, it is doubtful that the short cable lengths involved in home HiFi systems could produce audible differences between optical and digital cables.
Regarding the occurance of outright bit errors due to a marginal cable: S/PDIF contains only parity information, there is no error correction capability. If the errors are bad enough to cause bits to arrive with incorrect values, the likely result is that the digital audio receiver will not be able to lock on to the signal.
A short paper by DJ Greaves goes into further detail about S/PDIF, and has some comments about why jitter is not a problem even in equipment without buffers. Another paper by Tomi Engdahl goes into great detail about S/PDIF, even giving schematics for AES/EBU <-> S/PDIF conversion. Finally, Digtial Domain has written a very comprehensive paper on jitter in digital audio systems.
The short answer: If you are trying to make a digital connection between devices with differing signal types, you will need a converter. Core Sound sells something called a "Digital Format Translator" for $95 which will convert between coaxial SPDIF and TOS-link optical. There is more information on Core Sound's DFT page.
A cheaper route, if you're willing to do a little electronics work, is to follow Shawn Lin's instructions for making a converter from parts.
What follows is a discussion of audio digital interfaces. These interfaces come in 2 classes, optical and electrical.
The optical format has two connector types: the small, squarish "TOS-link" connector and the optical miniplug, which has the same connector dimensions as a normal (electrical) mini-plug. You can buy optical cables with any combination of these two [male] connectors at the ends. TOS-link is usually limited to maximum cable lengths of 10 to 15 meters. The Sony part numbers for the optical cables are as follows: miniplug/miniplug: POC152HG, miniplug/TOS-link: POC151HG, TOS-link/TOS-link: POC-15HG. These cables can be ordered from Sony Parts (see below).
There are two electrical formats. [the following excerpted from the DAT-link manual]
SPDIF: (Sony/Philips Digital InterFace): This is the interconnect that is most often used on consumer DAT machines. The connectors are standard RCA phono connectors. This type of connector may also be lableled "IEC Type II" or simply "Digital I/O". Standard analog phono cables can usually be used for the digital data, however some cables that are designed for analog may not be able to carry the high rates needed for the digital data, especially over long distances. Many high-end audio stores carry special digital phono cables that solve this problem. [The pro-audio FAQ says not to use audio cables, but that video cables will work].
AES/EBU: This type of cabling is most often found on professional equipment. It uses three-pin XLR connectors. Cables designed for analog applications work fine for AES/EBU connections as well. However, note that shielded cables (most cables are shielded) must be used, otherwise unacceptable levels of radio or TV interference may be generated. This type of cabling is the prefered choice for long distance runs between digital audio equipment.
It is important to realize that there are subtle differences in the control information that is sent along with the audio data on these different connectors. The two main formats of this information can be broadly categorized into Consumer and Professional. For most applications, if you are using the SPDIF or fiber-optic connections, the consumer format applies. For AES/EBU connections, the professional format applies. Some DAT machines will not operate at all unless the correct format is used.
You can call Sony Direct Response at 800/222-7669, there are about 10 Sony Factory Outlet stores around the country and they can give you the location of the one nearest you.
You can also try here:
SONY
Gallery of Consumer Electronics
663 N Michigan Ave
Chicago IL 60611
(312) 943-3334
(312) 943-0817 (fax)
In California, Sony has a "factory outlet" store:
Tracy Outlet Center
1005 Pescadero Ave., Ste. 183
Tracy, CA 95376
Tel: 209-832-3440
Fax: 209-832-4435
There is another Sony outlet store in the Grove City Outlet Mall, Grove City PA, three miles south of I-80 on I-79.
You can get Sony service manual and parts/accessories at:
Sony Electronics Inc.
National Parts Center
8281 N.W. 107th Terrace
Kansas City, Missouri 64153
phone: 800-488-7669 (parts)
816-891-7550 ext 33 (publications/manuals)
In Europe, you can order Original Sony Parts here:
Gehado Electronic Service GMBH
Freigrafenweg 29
44357 Dortmund
germany, Tel: +49-231-937-000-21,22,23...28
and Sony service manuals can be ordered here:
Schaltungsdienst Heinz Lange
Zehrensdorfer strasse 11
12277 Berlin
Germany, Tel: +49-30-72-38-13
Try here for Sony equipment:
Sony Electronics Inc.
Factory Service Center
Sony Service Center
390 University Avenue
Westwood, MA 02090
Phone: +1 800 282 2848
Fax: +1 617 329 1345
Sony also has a National Direct Response Center at 800-222-7669, representatives there can help you troubleshoot problems with Sony equipment.
Try here for Sharp equipment:
Video Electronics & Parts
5260 Northwest 167 St.
Miami Lakes, FL 33014
305-623-8875
A $35 diagnostic fee is charged, but will be subtracted from the repair charges.
Or here for direct factory service:
Sharp Engineering SEK
2-chome, 13-17
Kitaku, Higashi Tabata
Tokyo, JAPAN
Enclose a note describing the problem in simple and clear English.
Any storage case for audio cassette tapes will work just fine for MDs, as long as it does NOT have individual compartments into which the cassettes are to inserted. Any open shelf for cassettes will be just the right depth for MDs, with about an inch of extra headroom which is actually fairly convenient (leaves room for your hand to grab a disc).
I did find the Napa Valley "Cassette Crate 12", a single-shelf crate that (obviously) holds 12 cassettes, and it clearly would hold about twice that number of MDs in their little sleeves. So presumably the 2-shelf model that I got might be called "Cassette Crate 24". And any other brand would be just as good.
If it helps, the Napa Valley outfit is:
Napa Valley Box Co. Inc.
11995 El Camino Real
San Diego, CA 92130
(619) 259-3000
but their stuff is in practically all good music stores. (Oddly, in spite of their name, they are nowhere near the Napa Valley, and their products are made in Mexico.) -Gary Milliken ([email protected])
I am using a plastic rack that is actually meant for DAT-tapes. You know the type that c-cassettes used in the history. You can hang the rack on the wall and attach several of them together... Two MDs fit into a space for one DAT. It might not be very nice looking furniture in the living room, but for example in a studio it is actually quite handy as when You hang the racks so that You can store the disks horizontally you can also easily read the labels... -Janne Auvinen Turku, Finland([email protected])
DAT cases work the best for storage. I bought mine in San Fran for under $35. It will hold a total of 80 MD's. It is heavily padded and comes with a durable shoulder strap and casing. -[email protected]
Thomas Halasz ([email protected]) reports that the hard plastic box that comes with Ferrero Roche Chocolates (Italy) is the perfect size for holding 25 MDs, sans jackets. The box is transparent with an articulated lid. The chocolate is good too.
Other users report: ``I just stick my MD's Sans case in a plastic 3X5 index card box. It holds 25, fits like a glove, and cost $2.'' and ``Here's a really cool MD storage idea (at least I think so); the wood crates that hold three bottles of wine with sliding wooden covers and wooden dividers. Fits perfectly, one box holds lots of MD's. Just watch out for slivers.''
Yet another simple suggestion: Just glue a stack of MD cases together. ([email protected])
You can also check the MD page accessories table for suppliers of cases.
A standard Avery label fits almost perfectly onto a minidisc sleeve with no cutting. It is Avery Label J8666 (perhaps also known as J7666). It is their 3.5" disk face only label. Word can automatically format for this label in it envelopes and labels section.
Sony MD decks have an UNDO button as well as a hidden super UNDO function. A user describes how to cancel a pending TOC update on modern Sony MD decks.
If your machine doesn't have an ``UNDO'' function, there are two approaches. If you've got an MDS-302, 303, S35, or S37, try this first:
This machine doesn't have a specific undo function, but it can be made to forget about the edits in a simple manner, without having to open the unit. When you have deleted a track by accident for example, you just unplug the unit. Make sure that the unit isn't playing or recording before you unplug it (press the Stop button first). Now wait for about 30 secs for the power supply capacitors to discharge. Press the AMS knob, and while holding it, plug the unit back in. If you're lucky the recorder will enter in Test Mode (it always did this correctly in my case). Now you can simply press the Eject button to remove the minidisc without the new TOC being written to disc! The reason that you must enter Test Mode is because when you just plug the unit back in, it still remembers that the TOC isn't written yet. The moment you plug it back in the new TOC is written to the disc and you have lost the deleted track forever! After you have removed the disc press the Repeat button to leave Test Mode. You have to unplug the unit and plug it back in to enter the normal user mode.
-Steven Scholte ([email protected])
The following is a method that will work for all machines, but it requires more work:
I'm sure everyone who has a Minidisc deck has done this. You're editing down a disc, splitting blank spots away from other tracks, then deleting the blank spots - and all of a sudden, your quick fingers just deleted the track, rather than the space. This is usually followed immediately with loud obscenities. :) I hear that some of the new units have an "undelete" feature, but my MDS-302 does not have one.
I did this once (twice actually, within a half hour) while editing down a disc on which I had made a one-time live recording of a choral performance, that could not be re-recorded. Desparate, I remembered that the MD unit only wrote the TOC (Table of Contents) to the disc when the disc was ejected. When you do edits, it's actually only changing pointers in memory, which are then all written at once to the disc when it's ejected.
With this in mind, I unplugged the unit while it was still turned on. I removed the case, and examined the drive mechanism. There is a large gear at the back, and I found that by turning it by hand, it worked the eject mechanism, and the disc was slowly ejected as I turned it. Once I had the disc in my hand, I plugged the unit back in.
I figured it would be confused, so I took a blank MD, slid the write-protect tab open so that it wouldn't write to it, then plugged it into my recorder. Once it figured out that it had a new, fresh disc, I then ejected it and re-inserted the original disc with the live recording on it.
As I had hoped, the disc had been restored, and all the edits I had made (including the one wiping out an entire track) had been forgotten.
Now, whenever I do any editing, I periodically eject and re-insert the disc (just to write the edits to disc). This way, if I DO mess up and have to go through the manual eject sequence again, I won't lose ALL of my edits.
-Scott MacLean ([email protected])
The MDS-501 can me made to "forget" about editing simply by unplugging it. Even if you turn it back on and eject the same disc (without removing it the hard way) your edits will be lost.
For the MZ-R30 portable, this will work:
The MZ-R30 writes the TOC-Area after pressing the Stop-button or, if batteries are in use, after disconnecting the AC power adapter. After deleting anything, the R30 begins playing the succeeding piece automatically. As long as the R30 is playing, the TOC-Area is not written. To Undo a delete, take away all power sources while the R30 is playing; the TOC-Area will be the same as before the deletion took place. But note, if you use the AC power adapter and LIP-12 or LR6 AA in the supplied battery case, you must first remove all batteries before disconnecting the power adapter, otherwise the TOC-Area will be written right after disconnecting the power adaptor.
The MDS-503 has an "undo" function for just this purpose.
Since MDs store digital data, there will be no qualitative difference between the sound of various discs so long as the disc's raw bit error falls within the bounds of what can be handled by the MD ACIRC error correction system.
Presented here are the results of a small experiment I conducted to compare the raw bit error rate of various brands.
Eric Woudenberg
The service manual for the Sony MDS-503 MiniDisc Deck tells how to put the unit into test mode, and from there how to display the RBER (raw bit error rate [per second]) as it reads the disk. Using this feature I compared the bit error rates of TDK, Idemitsu, Maxell, Panasonic, Sony and Keep blanks.
In Andy Poggio's CD paper he says: "The CD specification allows for discs to have up to 220 raw errors per second. Every one of these errors is (almost always) perfectly corrected by the CIRC scheme for a net error rate of zero."
I have reason to believe that 220 is the threshold for MD as well, since MD uses [modified] CD logic for encoding data on the disc, and since 220 is actually one of the error thresholds used for setting the MDS-503 focus bias.
The un-surprising result: No discs had anywhere near an RBER of 220.
The procedure is to use "continuous recording" in diagnostic mode to make a test disk, then look at the error rate while reading back from the disk.
The table below shows the average and maximum RBERs for each brand. "In", "mid", and "out" refer to the three areas on the disc which are tested by default, they begin at cluster 40, 300, and 700 respectively.
|
Brand |
in |
mid |
out |
|||
|
|
avg |
max |
avg |
max |
avg |
max |
|
TDK |
4.97 |
20 |
5.08 |
15 |
4.16 |
19 |
|
TDK (1) |
4.05 |
22 |
5.00 |
15 |
3.66 |
18 |
|
TDK (2) |
5.96 |
20 |
2.47 |
6 |
3.84 |
18 |
|
IDEMITSU (3) |
2.11 |
15 |
1.15 |
5 |
1.22 |
5 |
|
MAXELL |
1.18 |
4 |
3.05 |
15 |
2.24 |
8 |
|
PANASONIC |
6.00 |
13 |
5.45 |
10 |
4.13 |
8 |
|
SONY (4) |
43.03 |
88 |
3.93 |
14 |
2.00 |
7 |
|
KEEP |
2.10 |
5 |
1.93 |
5 |
3.23 |
13 |
|
Sony ES |
1.60 |
5 |
1.00 |
3 |
1.59 |
5 |
|
Canfield Audio 60/GL-16658 |
7.05 |
13 |
8.50 |
19 |
5.82 |
12 |
|
Canfield Audio 74/GL-16658 |
11.76 |
31 |
10.13 |
25 |
9.36 |
20 |
|
Fuji 60/ 5Z20M536 |
0.87 |
5 |
0.76 |
3 |
0.83 |
3 |
(Original Data)
Notes:
Some discs had been recorded on previously in normal audio recording ("non-continuous") mode. The RBER of these areas was typically 20-50, and always higher than the areas made with "continuous-recording" in diagnostic mode.
The Sony disc was the only surprise, perhaps it had a weak/bad spot. In any case, it was still well within the presumed threshold for CIRC.
I am not sure if comparative quality judgements can be made from these results since the tests were done on three short (approx. 10 second) intervals of each disc. I think the main conclusion is that all the discs are well within the threshold of "identical" from a post error correction point of view.
The other consideration is longevity, and I quote two paragraphs from the excellent book by John Watkinson The Art of Digital Audio where he is discussing the magnetic layer of a Magneto-Optical disc:
Magnetic layers with practical Curie temperatures are made from proprietary alloys of iron, cobalt, platinum, terbium, gadolinium and various other rare earths. These are all highly susceptible to corrosion in air and are also incompatible with the plastics used for moulded substrates. The magnetic layer must be protected by sandwiching it between layers of material which require to be impervious to corrosive ions but which must be optically transmissive. Thus only dielectrics such as silicon dioxide or alumnium nitride can be used.
...
The master is developed and electroplated as normal in order to make stampers. The stampers make pre-grooved disks which are then coated by vacuum deposition with the MO layer, sandwiched between dielectric layers. The MO layer can be made less susceptible to corrosion if it is smooth and homogeneous. Layers which contain voids, asperities or residual gases from the coating process present a larger surface area for attack. The life of an MO disk is affected more by the manufacturing process than by the precise composition of the alloy.
So, perhaps, more expensive discs might last longer (rather than have a lower initial error rate). I don't know the real situation though, maybe all the manufacturers have excellent quality control.
My comments are with reference to modern MD machines (ca. 1996), I am unfamiliar with the behavior of the older units. I am also speaking based upon my observation of simple experiments on an MDS-503, not on any intimate knowledge of MD technical specifications.
There are two potential problems with fragmentation, loss of free space, and interruption of music due to excessive seeking.
Concerning interruption of music, fragmentation alone will not cause a problem. A typical MD player can buffer 10 seconds of music and read the disc at 150 Kbytes/sec. (roughly the 1x CD data rate) which is over 4 times the MD audio rate. You will only have problems if it takes the player more time over any 10 second period to access and read segments of music than it does to play them.
My simple tests with the '503 showed that it could not keep up with an arbitrarily long string of 2 second segments located at alternate ends of the disc (on a 74' MD), but that it could with 4 second segments. Assuming a full stroke seek takes about 2.3 seconds (measured by listening to the unit seek) and 3 seconds of music can be read in about 0.7 seconds, it would seem that the smallest segments a player could keep up with on a continual basis would be about 3 seconds long if they were located at worst case locations on the disc, and would cause the player to be constantly seeking and reading. You would need to have a string of segments this size or smaller, for at least 10 seconds duration, at opposite ends of the disc, to cause a problem.
This simply cannot happen through fragmentation alone since all free list segments are at least 12 seconds long (see below). A worst case seeking condition that caused intermittent muting could still be created if 3 second or shorter segments from opposite ends of the disk were catenated together, but I assume free list segments are kept sorted, so that unless you are trying to create such a pattern, you are rather unlikely to make one in the normal course of editing.
The free space problem is caused by unused segments of disc space less than 12 seconds long that are not available for reuse. My simple tests showed that whenever the MDS-503 could coalesce space it did, so that lost space is only a problem when many small unused segments less than 12 seconds long are scattered throughout the disc, not adjacent to any existing free space. It is possible that in a normal editing operation of deleting dead space between tracks, up to 12 seconds per track could be lost, and on a disc with 25 tracks that would be 5 minutes lost to fragmentation. Though this does not seem troublesome, one real problem may occur in a sound effects application where all the tracks are tiny (< 12s), since if you were to delete every other track, nothing could be coalesced and there would be no change in the amount of free space.
Though the degree of fragmentation depends upon your recording and editing patterns, I cannot see it causing problems in any but extreme circumstances. It should also be noted that all fragmentation is eliminated when an "Erase All" operation is performed. This restores the disc to a single TOC entry containing all the free space.
I have done extensive testing with this, and have yet to find a single instance where the MD encoding/decoding affected the Pro-Logic encoding in any way. In every test I did, the surround steering information remained intact, and the result sounded precisely like the original.
Scott MacLean ([email protected])
Two users have reported that their car units began skipping badly, but that after cleaning the laser lens by hand the problem disappeared (though both noted that the lens did not appear to have any dust or dirt on it before cleaning). Subsequently one user now uses a TDK lens cleaning disc when problems develop, which appears to be as effective as hand cleaning. No reports yet on the need for or effectiveness of the head cleaning discs. (Thanks to Jon Long ([email protected]) and Shawn Lin ([email protected])
In their IEEE paper on the MD system, Sony writes:
A very high durability of the [magnetic head] contact cycle can be achieved, e.g., more than a million passes.
Taking the worse case scenario, an MD machine left in record-pause mode at the lead in (i.e. innermost) area of the disc [diameter: 32mm] and running at the highest linear velocity (1.4m/sec) would spin at [1.4/(.032*3.14159) * 60] = 836 rpm. At this speed, 1 million revolutions [passes] would take approx. 20 hours. So, record-pause for several hours is okay. A day or two is probably not.
Here's a tip when recording a CD unattended: put the CD player in repeat-all, this will cause the MD player to record to the end and stop (rather than go into record-pause). You can easily delete the extra tracks later.
The switch has no electrical connection and is only provided to help you identify which batteries are charged. If you carry many batteries in the same bag, the ones showing a red dot can indicate empty, while those with the switch in the other position can indicate charged. You have to remember to move the switch after changing or charging batteries.