From reference design to USB sound card

March 14, 2012 By Leave a Comment

[Entropia] decided to try his hand at rolling is own sound card. He picked out a DAC chip, started his prototyping by studying the reference design from the datasheet, then went through several iterations to arrive at this working model.

He chose to base the board around the PCM2706. It’s a digital to analog converter that has built-in USB support; perfect for his needs. It’s got a headphone amplifier, but is also capable of putting out S/PDIF signals for a digital amplifier to pick up and use. Not bad for a part that can be had for right around eight bucks.

The first PCB he designed had a few electrical and footprint errors. But he was able to get it to run by adding some point-to-point jumpers, and bending the legs of his capacitors to fit the board area. With those issued accounted for he ordered a second batch of boards. These went together nicely, but the headphone output was incredibly loud. Turns out the filtering circuit had the wrong resistor and capacitor values. Changing them around, and swapping the audio output so that the correct channels were patched to the audio jack brings it to the first release version seen above.

Filed Under: digital audio hacks Tagged With: , , , ,

Playing MP3s from an FPGA

March 13, 2012 By Leave a Comment

Building an audio player is a fun project. It used to be quite a task to do so, but these days the MP3 decoder chips are full-featured which means that if you know how to talk to other chips with a microcontroller you’ve got all the skills needed to pull off the project. But that must have been too easy for [Ultra-Embedded], he decided just to build an MP3 player out of an FPGA.

It’s not quite as difficult as it first sounds. He didn’t have to figure out how to decode the audio compressions. Instead he rolled the Helix MP3 decoder library into the project. It had already been optimized to run on an ARM processor, and since he’s using a RISC soft processor the translation wasn’t tough at all. He’s using a 24-bit stereo DAC chip to bridge the gap between the audio jack and the FPGA output. Clocking that chip with the FPGA isn’t ideal and causes 44.1 kHz audio to run 3% too slow. He says it’s not noticeable, which we believe. But if you try to play along with a song the pitch shift might end up driving you crazy.

If you’d prefer to just stick to the microcontroller based players this one’s small and inexpensive.

Filed Under: digital audio hacks, Microcontrollers Tagged With: , , ,