CD-ROM audio connectors

Introduction

Audio system in CD-ROM is completely separate from PC sounds. When the computer tells CD-ROM to start playing audio CD then the CD-ROM starts playin git all by itself. The CD-ROM driver works just like normal CD player and does not need any PC CPU cycles to do the playback one it has been started. If CD playback seems to take up processor time in Windows the reason is the playback program which constantly ask from CD-ROM the current playing position and updates it to screen, the audio playback does not take any CPU processing power at all.

CD-ROM drives have typically a headphone connector with volume control in the front panel. The headphone jack uses the CD-ROM directly you don't need to connect any CD-ROM audio cables to make this headphone connector to work. The audio cable from the CD to the sound card allows you to use the sound card amp and speakers to playback the sound from CD-ROM. The audio cable just connect the line level audio output form the CD-ROM drive to one PC soundcard line level audio input dedicated to CD-ROM audio cable.

Why there is problems with CD-ROM audio connections

There is quite much variety in CD-ROM audio connectors used in PC market. Many soundcards and CD-ROM drivers have their own special audio connector. All audio connectors basically carry line level (0.3-2 Vpp) analogue audio signals from CD-ROM to computer sound card, but different connector types and pinouts cause too much problems nowadays.

The situtation is little bit stabilizing because SPA MPC working group has done standardization on multimedia PCs. They have specified a standard interface connector and pinout which should be used computer soundcards and the audio cables which come with them. If your soundcard and CD-ROM (which comes with audio cable) both meet MPC standards then you should not have problems in getting the sound working.

CD-ROM/Sound Card Audio Cable Standard for MPC Components

The following cable standards apply only to MPC components (CD-ROM drives or sound cards sold seperately). Full systems and upgrade kits are not required to observe the following specification:

A Multimedia PC CD-ROM drive component must include a cable to connect the drive's analog audio output connectorto an MPC sound card's analog audio input connector. The cable's open sound card connector must be a female 4 pin Molex 70066-G,70400-G, or 70430-G connector with 2.54 mm pitch, or the equivalent),with the following pin assignments:

pin 1 - left signal
pin 2 - ground
pin 3 - ground
pin 4 - right signal

A Multimedia PC sound card component must be capable of mating with the CD-ROM audio cable by having a 2.54 mm pitch Molex 70553 male connector on the card (or the equivalent), or by including short patch cable. The patch cable must plug into the non-standard sound card connector and have an open male connector (Molex 70107-A,or the equivalent) for attaching to the CD-ROM cable female connector.The pin assignments on the sound card connectors must be complementary to the CD-ROM audio cable connectors.

What to do when you have some non-standard components

If your soundcard and the audio wire coming with CD-ROM are not compatible (or your CD-ROM does not come with audio lead), then you have the following possibilities:

Buy a suitable ready-made cable (sometimes hard to get)
Modify an existing sound cable
Making your own sound cable

If you plan to take the route to modify or build your own cable, then you must have some electronics tools, suitable connectors and knowledge in building electronics projects. Be warned that you might not always get stisfying results and in worst case you might evend damage something inside the computer.

First thing to check is to find out all documentation which you have about the connectors in your soundcard and CD-ROM. Sometimes you can find out the pinout information from the manuals or it is printed to the equipments, but sometimes you just can't find that information. Without knowing the pinouts it is no idea to start building a wire or modifying an existing one.

Audio CD's hardware problem checklist

1) Plug a set of headphones in the CD-ROM while playing a CD to determine if the CD-ROM player is working. You usually have a volume control on the front, adjust it if necessary. If you hear sound correctly, the CD-ROM is okay. If not, the CD-ROM is probably defective. If you only hear the sound from left or right channel then remove the wire which goes to soundcard and test if the CD-ROM work as it should without it (wrongly connected audio cable can sometimes mute one headphone channel).

2) Check the audio cable that runs from the CD-ROM to the sound card. On most CD-ROM/Sound card, it is a small grey round 3 or 4 wire cable with colored wires and a small plastic connector on each end. This is not the large flat grey IDE connector cable! Does it exist? If so, is it connected tightly? No pinches or sharp bends?

3) If connected, it may be on incorrectly or wired incorrectly. Usually one end has a flat 4 hole connector that can be rotated 180 degrees to reverse the wiring. Try that. Or the CD-ROM and sound card were purchased separately and the CD-ROM grey audio cable was the wrong type for the combination of card and CD-ROM.

Solving a typical CD-ROM audio problem

You got a CDROM (ATAPI) to Sound Blaster Audio cable which connectr your CD-ROM to your Sound Blaster 16 card. Your sound effects work fine (because they went through the ISA bus) but CD audio playback only cames out one channel. It turns out that although the sound blaster audio connector is the same as the AUX AUDIO (found on a sound blaster), the signals of the four pins are not the same configuration. If you have a "sound blaster audio" cable, it will have four pin connections:

        none(ground)-white(left)-black(ground)-red(right)

But if you try and plug it into the [fitting] AUX AUDIO port on a sound blaster 16 , you will lose the left channel. This is because the AUX AUDIO port is:

        left-ground-ground-right

How to fix: Pull out the white wire from the connector end of the cable and move it to the empty slot so it's in this order:

        white-empty-black-red

Now you should hear CD playback from both channels.

How to get to know CD-ROM audio output pinout

Typically CD-ROM manufacturers print out the pinout to CD-ROM itself or to the manual. If your CD-ROM does not come with the pinout documentation, you have to quess the pinout and then test if your quess is correct. Some popular pinouts:
3 pin connector

pin 1 - Left signal output
pin 2 - Ground
pin 3 - Right signal output

4 pin connetors

pin 1 - Left signal output
pin 2 - Ground
pin 3 - Ground
pin 4 - Right signal output

When you have quessed the pinout, you can test if your quess was right using the following method:

Turn the power on the CD-ROM
Start playing some music CD
Check from what pins the music comes out

You can listen to the output signal using your amplified multimedia speakers usign the following connection: connect the outer ring of the RCA connector in you multimedia speaker to the computer case and connect the center pin to the CD-ROM audio connector pin you wish to test using short piece of wire. Avoid making any showt-circuit, because they can damage the electronics inside your computer. If you want to test which channel is left and which is right on the connector you must have a CD which has sound where you know whenever you are listening to left or right channel (audio test CD is very good for this because they have typically LEFT/RIGHT channel test signals in them).

How to get to know soundcard CD-ROM audio input pinouts

Typical soundcard manuals are not too helpful in telling about the audio connectors in the card. If you can't get information about the connector in the PC soundcard connector you have to make some tests to find out which connector is which. Before starting it is a good idea to take a look on soundcard to find any clues about the pinout (for example you might be able to see that typically the center pins are connected to card's ground plane and you then know that those pins are the ground pins).

MPC specs say that the CD-ROM audio input in soundcard should have the following pinout on the molex connetor.

pin 1 - left signal
pin 2 - ground
pin 3 - ground
pin 4 - right signal

Whan I looked at my SB16 soundacard I also found a audio connector which seemded to have the following pinout.

pin 1 - left signal
pin 2 - ground
pin 3 - right signal
pin 4 - ground

Those pinout diagrams are good guides when you start to figure out the pinout you don't know.

Ultrasound MAX has 3 CD In connector with following pinouts:

Sony: SggS
Panasonic: Sgs
Mitsumi: SgSg

Information about notation used: S means signal wire (left or right channel) and g means ground. Ultrasound Classic either has a 3 pin connector SgS or a 4 pin connector SggS. Pinouts for the Ultrasound PnP models are labeled on the card itself.

You have to be very careful when doing the tests to determine pinout of soundcard you don't know, because all tests must be performed when computer case is open, soundcard is installed inside the connector and computer is running. You need now a system where you have computer running, you have some sound playback device connected to soundcard (multimedia speakers, headphones etc.) and you must have the CD-ROM audio input enabled (and of cource enough volume in soundcard output and CD-ROM input if those can be adjusted).

First thing to do do is to look at the soundcard itself. Usually the ground pins in the audio input are connected together in the soundcard and connected to the computer power supply ground which is permantly connected to computer case (you can use multimeter to check those ground connections if you have one). Be caseful not to cause electrostatic discharge damages to the soundcard and other components inside the computern when you look at the soundcard or make measurements on it.

To perform further testing you need a sound source which gives out line level audio signals and does not get damaged if the output is accidentally short circuited to ground. Normal CD-ROM playing audio CD qualifies quite well to that (no CD-ROM I know have not been damaged if the output is shorted to ground for short time, but be warned that your CD-ROM might be that exception).

Anyway you connect the audio signal output to the CD audio input of the computer and see if it works as expected. When you connect only one audio channel at the time you can easily see what pin is for left channel and what is for right channel. When you know the pinout you can start building the suitable cable.

Tips for building the cable

Do everythign carefully, because mistakes can become costly. Make sure that you attach the wires correctly, do not make short circuit and secure the wire carefully to the connectors. Do not let any base wire to be visible near the connector (it is good idea to shield the exposed wiring with shrinking insulation tubing or electrical tape). Remeber that it is possible that the audio wire gets loose someday and if this can cause any short circuits inside your computer can easily cause lots of damage to the electronics inside your PC.

For sake of audio quality use only shielded wiring. Inside of your computer is electrically very noisy place and you don't want your audio wire to pick upt that interference.

What to do if you can't get audio cable between CD-ROM and soundcard to work

If you just can't get the audio cable from CD-ROM to the soundcard to work properly and you still want to get the audio from CD-ROM to be played back through your soundcard then you have only one option: external audio cable. You cn wire an audio cable from CD-ROM headphone output to line level inout of the soundcard. The solution is not very compact and the sound quality might not be the best (the headphone amplifier in CD-ROMs are typically not very good), but you get the audio throuhg soundcard as you wanted.

Other possibility is that if you know the pinout of the CD-ROM audio output, you can wire the audio signals from it to separate audio connector on the back of your computer.

Connecting audio from more than one CD-ROM to one soundcard

Soundcards are typicallu designed to accept the audio signal only from one CD-ROM drive. Even if the soundcards have many types of CD-ROM audio input connectors those are just wired together and are designed to be used so that there is only one sound input in use at one time.

The first idea which might come to mind is to splice together the audio wires from those two CD-ROM drives. This might seem to work but this is not the correct way to do it. Splising two audio signal wires toether will lead to decreased audio level, incleased noise, increased distortion and in some cases damages to the equipments. So don't splice those audio signal wires directly together.

The correct way to do the connection is to use an audio mixer circuit. A professional mixing desk is very good in mixing multiple signals together. Because you might not want to spend a small fortune to get a good mixing desk and you want somethign which fits nicely inside your PC and is also easy to use you might be interrested in the following circuit.

For left channel:

  CDROM audio 1 -------\/\/\/\---+
        ground  ---+   R1  10K   +------------ OUTPUT
                   |             |       +---- ground
  CDROM audio 2 ---)---\/\/\/\---+       |
        ground  ---+   R2  10K           |
                   |                     |
                   +---------------------+

And for right channel: CDROM audio 1 -------\/\/\/\---+ ground ---+ R1 10K +------------ OUTPUT | | +---- ground CDROM audio 2 ---)---\/\/\/\---+ | ground ---+ R2 10K | | | +---------------------+
This simple circuit just mixes the audio signal from two CD-ROM drives. Because of the simplicity of the mixer circuit the signal level is reduced but it otherwise works nicely.

References

Gravis Ultrasound FAQ
Minimum Multimedia PC Level 1 System Requirements
Minimum Multimedia PC Level 2 System Requirements
Minimum Multimedia PC Level 3 System Requirements
Various own exeriences on CD-ROM drive installing
Information from e-mail discussions with people who I have helped to selve this kind of problems

Tomi Engdahl <[email protected]>