The little USB sound card interfaces I ordered from China on eBay came in today.  These are based on the CM108 chip that is necessary for the usbradio channel driver in Asterisk.  The plan is to build a cheap radio interface.

Once I opened them up I found these are “blob” units.  Rather than a real chip on the board with solder pins, the board has an expoy blob over the circuit.  So you can’t get to the GPIO3 (13) or VOLDN pins (48) that would normally be used for PTT and COR respectively.  However, it turns out that this isn’t the big problem it might seem at first.  In hindsight I do wish I had known this up front and had known to ask the seller if it was a real chip on the board.

But it turns out that the usbradio chan driver does support the use of serial and/or parallel port interfaces.  This gives you full control of lots of things (channel steering, fans, lights, etc.) if you like.  It also allows you to do PTT and/or COR.  I’ve got enough CPU horsepower that I’ll just let the DSP work out COR.  So all I need isn an output pin to drive a transistor for PTT.

Otherwise these interfaces appear to register so the audio portion of the interface should work fine.

Lessons learned about the cheap USB sound fob’s:

  • Ask the seller to confirm that they contain a chip with pins, not a blob.  If they won’t confirm and the price is right, order a single sample only.
  • The URI sold by DMK Engineering at only $69.95 is a great deal.  It offers tons of control, amplification, is well built and reliable.  Just spend the money and be done.