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Most of the early CD-ROM drives were sold in so-called multimedia kits, to be added into existing systems that really had no way of interfacing them in a standard way. A typical multimedia kit came with a sound card, a CD-ROM drive, and a pair of small stereo speakers. These kits are still sold today, but they are much less popular since most new PCs now come pre-equipped with what these kits contain.
Since the kits came with a sound card and a CD-ROM drive, the most natural way to interface the CD-ROM to the PC was to connect it to the sound card, and that is exactly what was done. Sound cards, especially the SoundBlaster series from Creative Labs, were designed to incorporate one or more proprietary CD-ROM interfaces that were used with specific brands of CD-ROM drives.
In the early days there were only a few different types of CD-ROMs, and a sound card would typically be sold with the interface required for the specific drive it came with in a multimedia kit, or with multiple interfaces for two or three types of drives. Some of the drives that came out later on were designed to emulate these types of drives so that they could use the same proprietary interfaces found on the sound card. The whole situation became rather confusing and messy, which is part of why these proprietary interfaces were eventually done away with.
These are the main proprietary interfaces that were commonly found on older CD-ROM drives (and sound cards):
There were several problems with these proprietary interfaces: the first is the simple fact that they are proprietary. If you changed the drive in your PC and your sound card didn't have a connector for the new type of drive, it wouldn't work. The second is that when many more brands of drives entered the market, the situation with compatibility and knowing which proprietary interface to use for a specific drive got very confusing. The third is that the interfaces tend to use the same physical connectors, meaning the same number of pins in the same configuration, so it was easy to connect to the wrong interface on a sound card that had more than one.
Proprietary interfaces quickly went the way of the dodo when the new ATAPI standard was created, which allowed CD-ROM drives to be connected to compatible IDE hard disk controllers. Proprietary interfaces are most commonly seen on 1X and 2X speed drives. If you have an older 2X drive and try to connect it to an ATAPI controller and it doesn't work, it's probably because it really requires a proprietary interface of one sort or another.