| The proliferation of CD standards, including CD-ROM XA
and multisession Photo CD, makes it harder than ever to
decide which drive to buy. Further complicating the
decision is the availability of dual-speed drives that,
for a $200 premium, double throughput. One alternative is
simply to wait a few months: Drive makers are constantly
updating their products, and they say that adding XA or
Photo CD support won't be a big deal. But the definitions
of these features can be slippery. Consider NEC's "XA-ready" drives, which can read mode 2 format. To be fully XA compatible, they will need additional circuitry to decode interleaved channels of audio and perform ADPCM (Adaptive Differential Pulse Code Modulation) decompression. To add these capabilities, you're likely to need a new interface card. The same is true of XA-ready drives made by Sony and others. Currently, the only Sony devices with built-in ADPCM chips are the Data Discman and the MMCD Player. Sony says, however, that it will add the relatively inexpensive ADPCM chips to its CD-ROM drives if XA compatibility becomes more important. So far, the most compelling reason to have XA is Kodak's Photo CD. To read a Photo CD, a CD-ROM drive must be XA ready, although it does not have to be fully XA compatible. However, full XA compatibility with ADPCM will be required for future Photo CD applications that will use interleaved audio. Lacking ADPCM, an XA-ready CD-ROM drive could play sound bites attached to individual images, but it couldn't play a continuous audio track while reading one image after another. For an impressive business presentation or narrated slide show, you'd need full XA. Another desirable feature in a CD-ROM drive is the ability to read multisession Photo CDs. Several drives can read the first batch of images written to a Photo CD disc. But when you add another batch of pictures, it becomes a multisession CD, and only a handful of drives can track the modified directory structure and locate the additional images. "I don't think multisession is a big deal," says Pat Fobes, a CD-ROM hardware manager at NEC Technologies (Wood Dale, IL), "but Kodak is putting a lot of marketing behind the idea of multisession, and the public perception will be that multisession is important." As a result, virtually all major manufacturers will be adding multisession capability to their new drives. Usually it requires a patch to the drive's firmware, plus some modifications to the servo tracking system so the device knows what to do when the laser pickup stumbles on an unrecorded region of the disc. Such modifications add little or nothing to the retail price of a CD-ROM drive. If you opt for a dual-speed drive, though, you will pay a premium. NEC introduced this feature in 1992, and it's catching on fast. To see why, compare the specifications: A typical single-speed drive might have an average seek time of 450 milliseconds and a data transfer rate of 150 Kbps. A dual-speed drive's numbers might be 280 ms and 300 Kbps. Such performance is still anemic by hard drive standards, but it makes a big difference when you're reading high-resolution Photo CD images, which can run to 6 MB in size. The next frontier is likely to be 600-Kbps "quadspeed" drives like Pioneer's new DRM-604X. Here's the bottom line. If you need a CD-ROM drive to access static information — encyclopedias, technical manuals, reference books, and so on — you can get by with an inexpensive single-speed drive without XA or multisession support. For multimedia CD-ROM applications involving sound and animation, consider a dual-speed drive for best throughput. If you anticipate using Photo CD at all, you'll need at least an XA-ready single-session drive. For serious Photo CD work, settle for nothing less than a dual-speed drive with full XA and multisession capability. Tom Halfhill is BYTE's senior news editor in San Francisco. You can reach him on BIX as "thalfhill." Copyright 1994-1997 BYTE |