This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
The purchase of a CD recorder often results in what used to be an unusual
situation: a machine with two CD-ROM drives in it. This leads to a
number of interesting phenomena, usually having to do with poorly-written
software that can't figure out which CD-ROM drive it's supposed to use.
CD-ROM drives are typically connected to a sound card via a small cable
(a couple of wires twisted together, ending in small molex connectors).
This allows audio CDs to be placed in the CD-ROM drive and played through
the speakers attached to the sound card. Some people, upon discovering
that they have two CD-ROM drives and can use both simultaneously, want to
connect both drives to the sound card's input.
This is where the trouble starts. Sound cards often only have one input.
The immediate temptation is to buy or construct a Y-cable, but this won't
always work. The trouble is that Y-cables only work when you have a single
signal and more than one listener, like a stereo that sends its output to
two sets of headphones. The situation with two CD-ROM drives is of two
outputs and one listener.
Connecting two outputs together is, in general, a bad idea. Remember that
electricity isn't like water: it does not come out of the output and flow
downhill. The voltage at any point on the wire (ignoring minor
distortions) is going to be exactly the same. So if you have a device
that's trying to set it to one level, and another device that's trying to
set it to another level, the two devices are going to fight, and the
results aren't going to be what you want.
In some cases, if a device is inactive, it will allow its output to
"float". The other device can set the voltage to whatever level it wants.
So long as you only use one device at a time, all is well. Many devices,
however, force the output to ground level when not in use. This generally
manifests as a volume level that is almost inaudibly quiet.
Devices that combine multiple audio inputs into something reasonable are
called "mixers". Buying one and embedding it into your PC case is probably
not the best solution.
One possible option, if you're handy with the soldering iron, is to rig up a
mechanical switch that selects which signal gets passed to the sound card.
So long as you weren't planning to play two audio CDs simultaneously,
this should work well.
Some sound cards have multiple connectors on them, suggesting that the card
itself could handle multiple inputs. More often than not, these connectors
are not electrically isolated, so even though they're not sharing the same
cable they will still cause the devices to compete. If the sound card
isn't advertised as allowing multiple independent inputs, don't assume it
Some of the Sound Blaster cards, e.g. SB Live!, do have two independent
inputs ("CD in" and "AUX"). Stay away from the TAD (Telephone Answering
Device) connector though, it's monaural. You may need to un-mute the
auxiliary input in the volume control panel.
You can get an inexpensive Y-cable with a "passive mixer" from "Cables N
Mor" at http://cablesnmor.com/cdrom.html. If you're the build-it-yourself
type, some instructions for building a similar cable can be found on