Quality, Compression and Storage in Digital Video


Frame Rates and Irrelevancy

When comparing accelerated QuickTime systems, the question is no longer "What framerate can I get with a [fill in the blank]". Radius VideoVision Studio can record at 60 fields per second, without missing a frame, on any configuration, unless it has an element that is incompatible, and causes dropped frames at any settings. The question then really becomes: "At what Quality level can I record and play back video without dropped frames on my [fill in the blank]?"

Those still concerned about framerates can find more information.


Compression Quality and Data Rate

Quality can most accurately and precisely be quantified in terms of data rate. The higher the data rate, the less compression is applied. The less compression is applied, the better the image quality.

Note: QuickTime data rates are not the same as SCSI transfer rates. The storage media will need to support the given data rate after subtracting overheads caused by the system, the SCSI Manager, the NuBus, and QuickTime itself from the drive's Sustained Transfer Rate.

A constant quality of compression (i.e. a particular place on the QuickTime Quality slider) will yield a fluctuating data rate, depending on the content of the material being recorded. The more detail in a scene, the higher the data rate for that frame. If the compression on any segment of the material being recorded results in a data rate that is too high for the storage subsystem to keep up with, frames will be dropped.

For example, if a 640x480 movie at "High" quality (about 16:1) from the given source material (say, a driving scene, with occasional shots of trees) requires 2.8MB/second during the scenes with the most detail (the trees), but the storage subsystem can only support QuickTime data rates of 2.5MB/second, then the QuickTime Quality slider will have to be adjusted down to get the data rate down to 2.5MB/second in order to record without dropping frames. In this case, the other material (1.9MB/sec) suffers needless quality loss.

For that reason, VideoVision Studio employs adaptive compression, which instead keeps the data rate constant, and adjusts the compression quality to maintain that value. In this way, every frame is recorded at the highest quality possible without dropping frames.


Testing a Drive

To test a drive's suitability, the ultimate test is to determine the maximum supportable data rate using VideoVision Studio 2.0 and Premiere 4.0. Be sure that the recording Settings dialog in Premiere's Movie Capture menu is set to Report Dropped Frames, and that the scratch disk (under the File Menu's Preferences submenu) is set to record to the disk being tested. VideoVision Studio's Compression Options dialog includes a drive performance estimation feature (the Find button). This will return a value that can be used as a starting point, and finer adjustments can be made until recordings begin dropping frames. The Movie Analysis tool can be used to determine the data rate of the highest-quality movie you were able to record without dropping frames.

If you drop frames, try recording at a lower data rate, until you can record without dropping frames. Then verify that the movie can play back without dropping frames, using Print to VideoVision Studio (If looping is off, then the Player will report if any frames in the movie were dropped on playback). If this succeeds, then this is the data rate supported by that configuration.

SCSI transfer rates can be tested by several SCSI performance evaluation utilities, such as HDT BenchTest from FWB, or Time Drive from LaCie. These numbers are not indicative of performance through QuickTime.

For a brief description of RAID technology, see the Trillium Research document on Adaptec's Web site.



Compatibility is defined from this perspective:

VideoVision Studio should always be able to record without dropping frames at some data rate or window size. If a product causes a user to be unable to record or play back at any data rate, or substantially below the expected performance of the system (i.e.<1MB/sec on an array), the offending element is considered to be incompatible. Also, any product that causes a reduced set of features to be available to the user is also to be considered incompatible.

T-Cal: Thermal Recalibration

T-Cal is sort of the bugaboo of digital video. Drives, as they heat up, will expand, and the heads need to be periodically realigned to ensure data integrity. In most applications, the mechanism can simply tell the CPU to resend the data: "just a sec, I'm recalibrating." This doesn't work in digital audio or video. We fill the buffers up and then those frames are gone.

It is identifiable this way: A user has set their data rate appropriately and recordings are doing fine, but occasionally, there's a clump of dropped frames. Sometimes this is accompanied by a quiet scraping sound from the drive. This is similar to problems caused by fragmented drives, so be sure to eliminate this as a variable.

For this reason alone, most old drives are incompatible with VVS. We are working with Seagate, Conner, Micropolis, Quantum and DEC, and each vendor is releasing so-called "A/V" drives which have ways of dealing with it. We expect that this will be less and less of a problem as the industry matures.

Drive vendors are now coming out with mechanisms that minimize this effect, through caching, control, an extra tracking platter, or whatever.

It's not usually a problem on DEC mechanisms, and it's manageable on Seagate mechanisms. The 221xAV's from Micropolis have been the most successful so far. Conner is releasing Embedded Servo drives, which do not require recalibration. Quantum offers Intelligent Thermal Update in its Empire and Grand Prix series. Fujitsu is releasing popular A/V mechanisms as well.

Software control was offered for the Maxtor MXT series, but it voided the warranty. These mechs have been discontinued, and there is currently no replacement from Maxtor.



VideoVision Studio features audio recording at up to 8 bit, 22kHz recording quality. Playback is always done through the motherboard audio. When the VideoVision card was originally designed, prior to the A/V Quadras and Sound Manager 3.0, this made sense. With the advent of 16-bit motherboard audio, and redirectable audio output with Sound Manager 3.0, this became less useful.

The current approach to taking advantage of the high-quality audio available on the motherboard of most Macs in which VideoVision is used, is to completely bypass the audio connections on the VideoVision Breakout Box entirely. This requires stereo 1/8" mini-jack-to dual-RCA cables in the speaker and microphone ports of the Macintosh motherboard. (Careful -- the audio input port on the Power Macs has a connector that is not compatible with all sereo-mini-to-RCA adapters!)

Be sure to use version 2.5.1 or later of the VideoVision Studio software, particularly on Power Macintoshes, because with earlier version, using onboard audio input caused dropped frames.

Recording with 16-bit audio does cause a performance hit that exceeds that of the increased bandwidth of high quality audio. This has to do with the way Sound Manager uses the bus during recording. For example, the current maximum on an 840 AV when using 16-bit audio is: 4.9MB/sec on a StudioArray, although we have heard that the most recent ATTO card drivers can hit 5.5.

Version 1.2 or later of the Digidesign I/O drivers allows recording of video and audio simultaneously with an AudioMedia II at up to 3MB/sec. Spectral Innovations had released an improved driver for the NuMedia board, but we never tested it, and they are out of business. MediaVision's Pro Audio Spectrum 16 Mac is pretty much incompatible, with no attempts having been made to resolve open issues.

Playback is fine with all solutions, so audio can be added "in post," or separately, in Premiere or whatever (yes, we know it's a pain) if you require higher data rates or incompatible audio products.

It is a NuBus problem, not a SCSI problem. The data rate hit lies in the way sound drivers use the NuBus -- they take control of it for too long, and our interrupts don't get serviced, and we drop frames. This occurs at any data rate, sample rate, or bit depth, and is completely independent of SCSI and drives. This is evidenced by the fact that playback is fine. (Audio is PEANUTS in terms of bandwidth; CD-Quality audio runs at about 166K/second. What data rate did you say you were recording your video at..?) However, some card manufacturers have found ways to ameliorate the situation.

AudioMedia II and Power Macintosh

There is an incompatibility between VideoVision Studio, Digidesign AudioMedia II, and the Power Macintosh 8100/80, 7100/66, and 7100/80. It is characterized by pink and green blocks in the playback window of a movie, and is usually followed by a system freeze. The only workaround for this incompatibility is a complete logic board upgrade to a BART21 Power Macintosh.

Details are available by selecting this link.


DIY: Do-It-Yourself Arrays and Drives

Using software like Remus, RAID Toolkit, or Anubis with mail-order mechs to make arrays is a popular alternative to buying expensive prepackaged ones. The problem is that there are many (like about sixty) settings within the drives themselves that dramatically affect performance, and there will never be a DIY array that performs as well as a prepackaged one from a company that tunes their arrays with VVS, like ProMax, Hammer by Streamlogic, and Micronet.

Termination and cabling can be a nightmare, causing anomalous and intermittent problems. Headache avoidance, support and maximum performance are what you are paying for in the arrays.

Compatibility Among Drives/Arrays in the Same Machine

(Coming Soon!)