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From the DC 30 Mailing List 22/10/96: Ultra SCSI and Capture Rates Explained |
The question was whether there was any
point to purchasing an UltraWide SCSI hard drive that
transfers at 20MB/sec This question reflects a common
misconception that people often have about SCSI hard
drives that I think is important The following list shows the maximum theoretical transfer rates achievable by several SCSI formats that are currently available:
What must be considered in determining
the actual performance of a hard drive for Digital Video
is the actual media Make no mistake, the 6 megabyte per second peak transfer rate of the DC30 is an impressive target. I suspect few consumers will ever come close to achieving it. For reference, high-end avid systems and Media 100 systems usually aren't run at transfer rates this high. Last year, rates this high were a $5000 option (called the HDR, High Data Rate) on the Media 100, and weren't possible on Avids! To get this kind of performance, first Miro will have to finish their latest drivers for the board that compensate for some of the bottlenecks inherent to the AVI format. Second, you will have to have a damn fast hard drive or disk array to keep up with the data. Remember, in addition to the 6 megabytes per second your machine will be tearing through for video capture, you will need another 150k/sec for audio, and usually a little extra speed as a latency buffer, particularly on slower systems (I hope your PCI bus is operating at 33Mhz! 120mhz and 150mhz pentiums are 30Mhz!) To choose the fastest drives available, there are a couple of things to look for. First, make sure you are getting a recent model drive. Recent improvements like MagnetoResistive (MR) heads and PRML channels offer significant improvements in transfer rates. A 5400 RPM drive with MR heads and PRML will be faster than a 7200RPM barracuda without! Most decent hard drives today 2 gigabytes and up will transfer at least 4 megabytes per second. The 9 gig drives from seagate are pretty consistent in their performance (avid likes them) but they are not stellar performers. Expect about 4.5-5 megs/second from these. Barracudas with MR heads transfer about 7 megs a second from what I have heard, but I don't use them so I'm not sure. Micropolis drives have crapped out on me too many times, but they have transfer rates higher than seagates sometimes. The drives I use now are IBM Ultrastar series, the 5400 RPM drives transfer at 5 megs/second, and are very quiet and reliable. The 7200 RPM Ultrastar XP's will do 8 megs/second, and the Ultrastar XP2's will sustain 10 megs per second. They are a bit pricey, about $1500 for a 4 gig, but these are the fastest and most reliable drives I have used. I have read good things about HP drives as well, which are also more expensive than most, but they make an 8 gig drive that looks particularly impressive. I hope people who read this take a moment before getting up in arms about the fact that they aren't reaching the 6 meg/sec rate that miro claims. I trust that the DC30 will turn out to be a winner in the budget DTV market, even if most people only get about 4 megs/second or so through the board. Daniel Summer consultant/digital video producer, |
Wavelet Captureing is low loss, so at 20:1 wavelet compressed video looks as good as a 10:1 MJPEG video. It also takes up a whole lot less space, and requires less then 2 megs per second transfer rate. All this means you can get nearly 30 minutes of SVHS quality video on a 1 Gig EIDE Hard Drive.
The downside is that most users want video that
looks as good as the original video. MJPEG 8:1 to 12:1 is
considered "SVHS" quality.
7:1 and below is considered "near broadcast". To reach
7:1 MJPEG quality, a wavelet board looses its advantage and
requires the same 7:1 compression. Same disk size, same thruput,
same need for SCSI drives.
In addition, it has yet to be demonstrated that a 7:1 wavelet looks as good as a 7:1 MJPEG video. So at least for the first generation of wavelet boards, expect very good consumer quality svhs video, but not prosumer quality video.
As an AV Master user who has gotten through all of the start-up headaches, etc..., I am puzzled by the debate over which board to use. If I was doing this for a living, yeah, I'd go for the DPS. It's a professional level card used by professionals. But to imply that the AV Master is not capable of producing professional results is absurd. I am a serious amateur and can produce professional results with the AV Master. As far as long rendering times in Premiere, etc..., it's not true. I can produce 5-10 minute clips at full 640 x 480, with numerous complicated transitions, 44khz audio, voiceovers, etc... in 10-15 minutes. That's not slow in my mind. Try that with Media Studio Pro 2.5, and yes, you'll wait... and wait...
As far as quality of the video, I don't care what card you're using. Garbage in - garbage out. Get a real high quality Hi8 or SVHS input stream and you'll be in great shape. I saw video from a DPS and an AV Master at the Spring Comdex. The difference was zero. If anything, the AV Master video looked better.
The bottom line - if you're a professional and do this for a living (and have unlimited resources), go for the DPS. If you're not (or even if you are a "semi-pro"), get the AV Master. It produces quality that's good enough for me to make a few thousand dollars here and there. My customers are not complaining. And when you do get the AV master, support it with a "kick-ass" subsystem - fast and wide (or better) SCSI drives, plently of RAM, good adapter card, and fast processor (I'm using a Dell PPRO200 with 64 meg, Micropolis 3243WAV).
Let's stop argueing about which board is better. They both have their place and they both work well. I'm making money with the AV Master and so are others! If I ever get good enough to quit my "day job", I'll worry about which card is the "best".
From: Stuart [email protected]
Date: Wednesday, 13 November 1996 7:26
Interleave Option
You can set the amount of audio to be stored in the movie between blocks of video, called interleaving audio and video. You can specify amounts in frames, seconds, or minutes. In most cases, the default amount (1 second) works best, but if you notice delays in your movie and choppy audio, you may want to experiment with different amounts. For movies that will be played from CD-ROM, choose 1 frame.
Note: For the smoothest playback, you can load all of the audio into RAM first, which allows the video frames to be retrieved from the hard disk without interruption. To load all the audio into RAM first, choose a value for the Interleave field that is longer than the duration of the entire movie. For this method to work properly, you must have enough RAM available to load the entire audio portion of the movie and the audio portion must be five minutes or less in duration.
Setting your audio options to Interleave = 1 Frame will allow you to playback and cature 2GB file. As stated above, it will sync Audio and Video better than 1 second, and the playback will be smoother.
Try it and let me know!