Micropolis's high-capacity AV drives deliver uninterrupted data transfer to match the needs of serious audio and video developers
Stan Miastkowski
The worlds of computers and serious audio/video are fast converging, and the range of applications is wide. Computers are being used to produce corporate videos, audiotapes, and computer-specific multimedia applications and presentations on CD-ROMs. With the emergence and predominance of ever more-powerful RISC and CISC processors, that trend will continue to grow.
But there's a problem on the way to this multimedia nirvana. Many of today's hard drives just aren't designed for the stringent requirements of audio and video processing. Micropolis has an answer to these and other problems: a line of SCSI-2 drives specifically designed for the needs of digital medi
a production.
The Micropolis AV drive line is available in both internal and external versions, in three sizes ranging from 1.69 to 9 GB. The external versions feature a clever stackable design that lets you fabricate arrays of many gigabytes. I tested the LT 1700AV, the external version of the 1.69-GB 2217AV drive.
Certainly high-capacity, quick-access, and fast-throughput SCSI hard drives offer many advantages, but most are designed for the requirements of data processing (e.g., word processing, database management, and networking). Applications of these types read and write small chunks of data that tend to be scattered randomly across a disk. Consequently, most drives are optimized for this type of data handling.
Various housekeeping functions, such as error recovery, thermal calibration, and head degaussing, are going on while data is being transferred to and from the disk. These cause regular pauses in the data flow, ranging anywhere from 10 to nearly 1000 ms. That's no problem for
garden-variety applications, but it won't do for digital multimedia--it causes dropped video frames, audible pauses, or even full-fledged application crashes.
Like Tape
To understand the hard drive requirements of multimedia, it's helpful to use a tape analogy. Files on both audiotape and videotape (either analog or digital) are organized as large and continuous blocks of data. The tape-recording and playback process delivers large amounts of data as a continuous flow. The structure of multimedia files is much the same.
There are three interrelated parts to multimedia hard drive requirements: storage capacity, data throughput, and lack of data-flow interruptions. The first is the reason that the AV drives start at capacities of 1.69 GB and go upward from there. Storage requirements for video vary depending on the compression method used and the resolution. For example, at the bottom end, an AVI (Audio Video Interleave) file uses about 8 MB per minute of running time; MPEG compression uses abo
ut 12 MB per minute. At the other end of the spectrum, full-frame, uncompressed, interlaced NTSC video gobbles 1.6 GB per minute. (This causes other problems: SCSI-2 can't handle that type of throughput.)
The audio side has similar problems, especially since professional audio is usually stored uncompressed. CD-quality stereo sound (16 bits, 44.1-kHz sampling rate) requires about 9 MB per minute of storage. Specialized 24-track hard disk recorders, used by some professional recording studios, use nearly 2 MB of storage space per second.
Data throughput is, of course, directly related to storage requirements. If 1 second of high-quality compressed video requires, for example, 2 MB of storage, it needs to be delivered through the pipeline (either reading or writing) at exactly the same rate. That's not a problem for SCSI-2, which theoretically maxes out at 10 MBps (Fast 8-bit SCSI). But the bus that your SCSI adapter is connected to is also a factor. ISA certainly won't cut it, but PCI (Peripheral
Component Interconnect) and the new Power Macs can hit full SCSI-2 speed.
The final, and crucial, part of making a drive truly multimedia-compatible is making sure the flow of data from the drive is completely uninterrupted. As mentioned above, when laying down a finished video to tape, dropped frames or even short-duration pauses in audio are unforgivable in a professional production environment. It's also a problem in CD-ROM recording, where any data pause makes the disc useless (and costs money).
Psyched Out
Some psychological factors come into play here, too. Your eye can ``forgive'' an occasional dropped frame (common when playing AVI files off a CD-ROM), though pauses in the action or jerky motion are plain annoying. Audio has more severe problems. Human ears can detect even the smallest pause or dropout in audio.
Most hard drives have a cache that buffers the data flow, but it's usually small because of the small files that data-processing drives normally handle and because of
cost. You can help it along with a sizable RAM cache (using Microsoft SmartDrive or a third-party product), but even the largest cache isn't much help with multimedia files that can be hundreds of megabytes in size. To keep the data flowing, Micropolis uses a combination of methods (see the text box ``No Interruptions, Please'').
Micropolis claims that, thanks to these performance enhancements, its 2217AV drive can deliver a guaranteed 2.9 MBps of uninterrupted data flow and, from the outer zones of the drive, sustained throughput as high as 4.3 MBps. A 2.9-MBps throughput rate is enough to smoothly supply 24-bit, 640- by 480-pixel video compressed at a relatively conservative 10-to-1 ratio.
Measuring Results
BYTE's Avitest benchmark was written specifically to test guaranteed data rate. It reads a SCSI drive sequentially from the first data block to the last, measuring how long it takes to deliver each chunk of data and keeping track of the maximum response time. Avitest controls the SCSI d
rive fairly directly using ASPI (advanced SCSI programming interface) calls. You can set the block size for the reads. Dividing that read block size by the maximum response time yields the maxiumum guaranteed data rate. Most modern drives use a ZBR (zone-bit recording) scheme, where outer data tracks hold more data sectors than inner tracks; this is also true of the Micropolis AV drives. As Avitest moves from the outer tracks, where more data is accessed with each revolution, to the inner tracks, the maximum response time increases.
BYTE tested the 2217AV in a Zeos 60-MHz Pentium system using Adaptec's AHA-2940, a PCI-bus SCSI-2 card with a maximum SCSI throughput of 10 MBps (Fast 8-bit SCSI). The drive did not meet the expectations created by Micropolis's marketing material. With a large block size of 230 KB, the maximum guaranteed throughput after a single read-through of the drive was around 2.1 MBps. During that test, maximum response time rose from 98 ms at the start to 112 ms at the end--a surpri
singly modest climb. Tested throughput dropped, as expected, with smaller block sizes. A block size of 87 KB reduced guaranteed throughput to 1.8 MBps.
To see how well a drive handles soft errors, Avitest can run repeatedly for several hours. Extending the test time to 5 hours with the 2217AV turned up occasional response times that decreased guaranteed throughput to 1.6 MBps with 230-KB blocks. In other words, a 230-KB block would occasionally take as long as 145 ms to read, probably due to soft errors.
Avitest also gives an average throughput for reading the whole drive. It measured average sustained throughput for the 2217AV at a respectable 2.7 MBps. This is a conservative measure derived by dividing the amount of storage on the drive by the time it takes to read the drive from beginning to end. Talks with the vendor indicated that Micropolis's testing worked more closely with the hardware than did BYTE's testing. The Avitest approach is, however, reasonable from an application's point of vi
ew.
While the 2217AV didn't meet its maker's claims, according to BYTE's throughput testing, it should come close with the drive caching provided by most operating systems and by many multimedia applications. Even the drive's longest delays are not very long. With reasonable caching, the 2.7-MBps average throughput measured by Avitest is probably a better measure of throughput for multimedia applications.
The Facts
Micropolis AV Drives
Internal: 4110AV (3 1/2-inch, 1-inch high, 1.01-GB), $1110;
2217AV (3 1/2-inch, 1.69-GB), $1590;
4221AV (3 1/2-inch, 2.1-GB), $2410;
3243AV (3 1/2-inch, 4.3-GB), $3895;
1936AV (5 1/4-inch, 3-GB), $3410;
1991AV (5 1/4-inch, 9-GB), $5845.
External: LT 1700AV (1.69-GB), $2075;
LT 2100AV (2.1-GB), $2865;
LS 3020AV (3.02-GB), $3955;
LT 4300AV (4.3-GB), $4350;
LS 9100AV (9-GB), $6295.
Micropolis Corp.
21211 Nordhoff St.
Chatsworth, CA 91311
(800) 395-3748
(818) 709
-3300
fax: (818) 701-2809
Photograph: Micropolis's AV drives come in both internal and external versions, with capacities ranging from 1.69 to 9 GB. External modules can be stacked to create larger arrays.
Stan Miastkowski is a BYTE consulting editor based in Peterborough, New Hampshire. He has been writing about computer technology for over 16 years. You can contact him on MCI Mail at 530-9979 or on the Internet or BIX at
stanm@bix.com
.
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