An S/PDIF Sound Card Using The PCM2906




When S/PDIF became available in the Teensy Audio Library I thought this might be the solution to ground loop problems I’d been having when interfacing projects to my PC. However, I quickly realized I didn’t have any sound cards with an S/PDIF interface.

In the belief that I’d rather build than buy I decided to update one of my previous projects, a PCM2904 based sound card, to include an S/PDIF interface. The update was a cinch because TI has a pin-for-pin replacement for the PCM2904 (the PCM2906) with an S/PDIF interface. All I had to do was replace the audio jacks with fiber-optic transmitter/receivers connected to the appropriate pins.

The resulting schematic and prototype are shown below. (If you’re interested, the boards are available through OSH Park)

An S PDIF Sound Card Using The PCM2906

There’s not a lot to it. It’s simply the reference circuit from the PCM2906 data sheet with portions I don’t expect to use eliminated.

Testing The Interface

To test the interface I looped the transmitted fiber optic signal back to the receiver. When plugged into my Linux laptop it registered as a Texas Instruments PCM2906C Audio Codec as shown below.

Bus 004 Device 003: ID 058f:6366 Alcor Micro Corp. Multi Flash Reader
Bus 004 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 004 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 004: ID 1bcf:2c02 Sunplus Innovation Technology Inc. 
Bus 003 Device 003: ID 413c:3016 Dell Computer Corp. Optical 5-Button Wheel Mouse
Bus 003 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 013: ID 08bb:29c6 Texas Instruments PCM2906C Audio CODEC
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Starting up the PulseAudio Volume Control and playing back a file using VLC, I could see the input and output audio levels varying with the signal from VLC.

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If you want to actually hear the looped back audio you can use the PulseAudio loopback module.

For more detail:   An S/PDIF Sound Card Using The PCM2906




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