Building a USB Audio Interface for Digital Modes – Part 1

Being a member of the DigitalDX group in SARTS:

http://www.sarts.org.sg

my preferred activity in this area of ham radio is with radio interfacing.

I use a Tigertronics Signalink USB as my audio interface for digital modes.

When I opened up the Signalink, I noticed that the main USB codec function was carried out by a PCM2904 device. This device costs about $10 Singapore dollars in single piece quantity from RS or Farnell.  The design of an application circuit for the device is fairly straightforward.

Here is my LPKF milled prototype that works rather well.

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1KHz sine wave generated using Cool Edit

A04_6276

10 KHz.

A04_6295

At 20 KHz, the waveform looks relatively good.

A04_6296

At this point, I started getting curious about the PCM2904 device. It was a rather easy device to deploy as a USB sound interface. With a clean response up to 20KHz, it was clearly a proud flag bearer of the Burr Brown name. My previous experiences with Burr Brown D/A devices were equally positive.

Thus encouraged by the performance of the BB device, I started on the design of the supporting audio as well as VOX circuits which would have, as its final form, an improved device for interfacing between Ham radio Deluxe and my rigs.

At a chance visit to Sim Lim Square one Saturday, I bought one of these for $10.

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One thought came to my mind as I was driving home. If this cheap USB audio dongle is as good as it claims on the bubble package, then my effort at building the PCM2904 board would have been in vain.  Here you have a ready made product, all nicely built for the price of the bare BB chip.

The lab tests were to prove otherwise, as you shall see.

3KHz sine wave.

A04_6279

10KHz

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20KHz

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As you can see, the cheap USB audio dongle is definitely not up to the job for audio interfacing to my rig for Digital Modes.

End of Part 1.

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One thought on “Building a USB Audio Interface for Digital Modes – Part 1

  1. Very interesting to see the test results on the cheap USB sound dongle! I will keep this in mind when surveying ‘cheap’ product of this type.

    I came to your site by way of a web search on the TI PCM2904 as I have been looking at the internals of my Tigertronics USB Signal Link as well. I am planning on making use of the 2nd audio input, that is, the “Left” channel on the TI chip.

    73, Jim, W B 5 W P A (active lately on 160m and 40m WSPR using tuned loops)

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