Instructables

Improve Your Soundcard's S/PDIF Output

Picture of Improve Your Soundcard's S/PDIF Output
Many consumer-grade computer sound cards (including ones built in to the motherboard) have digital S/PDIF outputs, however not the high quality outputs found on professional audio equipment. In my music server I have a SoundBlaster Audigy2 connected to my Sony surround receiver using a RG-59 coaxial cable from the 1/8" mini S/PDIF connector on the sound card to the digital audio input on the receiver.

This works great, however when the cable is connected a hum can be heard through the speakers, not mattering what input is selected on the receiver. In my experience with professional analog audio equipment, I instantly recognized it as a ground-loop hum. For those without electrical engineering degrees or unfamiliar with ground-loop problems, a ground-loop in an undesired effect occurring when there is a grounding potential difference on the main power supply of two separate devices and when a cable connects the two devices, connecting the devices' ground planes which will cause an electrical current to flow over the wire connecting the two grounds. This potential difference, although usually very small (in my case, less than a volt) can be amplified in analog sound amplifiers and heard in the speakers as a 60Hz hum. Even though we're using a completely digital audio connection, a ground-loop is still created because the shield of the coaxial cable used connects the ground plane of the PC with the ground plane of the Sony receiver.

For a while I considered getting some extra equipment to use a Toslink optical connection between the computer and the receiver, but instead I decided to apply some knowledge I had used when working with professional audio equipment, and that is inserting an isolation transformer between the two devices. Obviously, since this is a high-speed serial digital data connection and not an analog audio connection, we're going to have to think about this a little bit.

It turns out that we can fix this problem very easily, and probably with parts you already have lying around.
 
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RG-58 is a poor choice of cable. Since S/PDIF is ha 75 ohms input/output impedance the cable should match that to avoid standing waves on the cable. RG-58 is a 50 ohm cable. RG-59 would be a better choice. Nice solution to your problem though. S/PDIF with a twist of AES/EBU :-)
rjchute (author)  Peter_Karlsen5 years ago
Thank you for the catch -- double checked the cable and it is indeed RG-59 (was a cable tv coaxial cable I had lying around). Must have been a typo on my part :)