Standard CD-ROMs hold about 650 megabytes of read-only data in a format called ISO-9660 (formerly ``High Sierra''). All current Unixes now support these devices. In fact, most Unix and Linux software is now distributed on ISO-9660 CD-ROM, a cheaper and better method than the QIC tapes we used to use.
CD-ROM drives may be driven through SCSI, through enhanced IDE (ATAPI), or through a proprietary interface card (like the Mitsumi and Sony interfaces). Unix support for CD-ROMs is usually through SCSI drivers (exception: BSDI/386 supports the Mitsumi interface; Linux supports the Mitsumi and Sony interfaces). I recommend that you avoid the proprietary cards; they will effectively cost you money when you need to upgrade. Besides, street prices for SCSI CD-ROMS have dropped below $200.
(A few external CD-ROMs come with a parallel-port interface. Avoid these; they tend to have very slow transfer rates.)
Any CD-ROM you buy should be at least a ``double-spin'' drive meeting the MPC2 (Multimedia PC) standard of a 300K/sec transfer rate when reading. digital data. The older single-speed drives, which only supported the 150K/sec rate Red Book standard for audio CDs, are obsolete. The lowest speed you can buy these days is 4X (600K/sec). 6X, 8X, 10X, 12X, 24X, 32X, 40X, and 56X are available.
The next level up in CD hardware standards is CD-ROM XA. So far, drives that support XA are few and expensive. It's not yet in wide use in the DOS/Windows world, and I don't know of any Unix support for it, either in closed- or open-source code.
CD-ROM access times about 280ms for high-end double-speed drives (to put this in perspective, it's about 30 times slower than a typical 9ms hard disk, but considerably faster than a tape). Accordingly, modern 32X drives are about half the speed of a hard drive.
Most CD-ROMS will include a headphone jack so you can play audio CDs on them. Better-quality ones will also include two RCA jacks for use with speakers. Another feature to look for is a drive door or seal that protects the drive head from dust.
CD-ROM formats are still an area of some confusion. A slight enhancement of the original ``High Sierra'' CD-ROM filesystem format (designed for use with DOS, and limited to DOS's 8+3 file-naming convention) has been standardized as ISO-9660.
There is a de-facto Unix standard called `Rock Ridge' pioneered by the Sun User's Group shareware CD-ROMs. This is a way of putting an extra layer of indirection on an ISO-9660 layout that preserves Unix's long dual-case filenames. Some Unixes (notably Linux, netBSD, freeBSD and BSD/OS) can mount Rock Ridge filesystems.
More much more detail on CD-ROMs, CD-ROM standards and how to buy drives is available in the alt.cdrom FAQ, available for FTP as cdrom.com:/cdrom/faq. It is also archived in the news.answers tree at rtfm. This FAQ includes comparison tables tables of numerous drive types, CD-ROM sources, and ordering information.
Look for the following features as a minimum in your sound card:
16-bit sampling (for 65536 dynamic levels rather than 256).
Mono and stereo support.
Sampling rates ranging fron 8K/sec (voice-quality) through 11KHz (AM-radio quality), 22KHz (FM-radio quality) and standard audio (44.1KHz).
MIDI interface via a standard 15-pin D-shell connector.
RCA output jacks for headphones or speakers.
A microphone jack for sound input.
The most important feature is your sound card is what type of MIDI synthesis it has. All current sound cards have some sort of MIDI (musical instrument digital interface) compliance, but if nothing else, make sure your card supports the General MIDI standard. MIDI is just a set of commands issued by the application that tells the sound card which instrument to play, at what note, and for what duration.
Older and cheaper cards use FM synthesis. This synthesis uses a combination of sine waves to imitate the sounds of the different instruments. The result is like the sound tracks of most computer games sold a few years ago; imitation music with an arcade-like sound.
The method used by most modern sound boards is called wave table synthesis. In this method, digitized samples of actual instrument sounds are used as templates for the tones generated by the MIDI commands. Wave table cards vary in the quantity and quality of samples; one figure of merit often quoted is the wave table ROM size (often 4MB or 8MB). Also some boards have wavetable RAM that can store samples loaded from a disk.
Soundcards with DSP (Digital Signal Processing) can perform synthesis effects on board, relieving the CPU for other tasks. Some DSP chips are even software-programmable. Some high-end cards even include 3D sound effects. Whether the system used is SRS (Sound Retrieval System), Q-Sound, or Spatializer, it is designed to improve the perceived stereo effect of your speakers. These 3D effects work by delaying the timing of certain portions of the audio signal so that different frequencies hit your ear at slightly different times. The downside is that some of the cards equipped with 3D sound add a noticeable amount of noise to the card's output.
If you play a lot of computer games, you'll need to pay attention to compatibility. DOS games are written almost exclusively for the Creative Labs specification; you will need a card that is 100% Sound Blaster compatible. Many vendors do not license the Creative Labs specification but claim that their cards are 100% game compatible. This means that the sound will work, but not all sounds that you hear will be the ones that the game programmers intended. If you play many DOS games, it would be best to buy a Sound Blaster and save yourself a migraine.
Lastly, try to avoid sound cards with built-in amplifiers that are more powerful than 4 watts/channel. Sound cards that have more powerful amplifiers are said to have the problem of adding noise to the card's output. Use powered speakers with a 4 watt/channel card to solve this problem. Most cards are equipped with 4 watts/channel anyway. Wavetable cards are so inexpensive these days that it's almost worth their additional cost over a regular FM synthesis card. If you decide to settle for an FM card, make sure that there is a daughterboard made for the card that will let you upgrade to wavetable synthesis. In some cases, however, the wavetable card is cheaper than buying an FM card and then deciding that you want the wavetable upgrade. If you do decide on the wavetable as your card of choice, PC Magazine rated the best MIDI wavetables (MIDI being the most important feature in my opinion) the Media Vision Premium 3-D, Media Vision Pro 3-D, Creative Labs Sound Blaster AWE32, and the Turtle Beach Monterey (although there are value editions of the Sound Blaster 32 that have fewer ROM instrument samples but maintain the superior MIDI wavetable synthesis).
Linux, includes drivers for the SoundBlaster series and many other boards including the PAS Adlib, the Gravis Ultrasound, and the ProAudioSpectrum. Many of these are included in the 2.0 and newer kernels. See the OSS/Free home page for details
Warning: some sound cards require a specific CD-ROM type! Avoid these.
In speakers, look for a magnetically-shielded enclosure with volume, bass and treble controls. Some speakers run off the card's 4-watt signal; others are ``self-powered'', using batteries or a separate power supply. Your major buying choice is which one of these options to pursue.
Generally speaking, you'll get better value if you buy your sound card, CD-ROM, and speakers not separately but as a bundled ``multimedia upgrade kit'' (even though Unix users will usually have to throw out the bundled software).
One final, important tip: that audio cable from your CD-ROM back to the sound card is used only when you play audio CD-ROMs through your speakers. Software-generated sound goes through the system bus, so you can play ``Doom'' with sound even if your sound board won't accept the audio cable connector.