Skrasoft Dev Blog

Like Kicking a Slinky

Several weeks ago, I ported a 2nd order delta sigma D/A converter block diagram from “Principles of Digital Audio” into some Verilog for an FPGA.

The output is a 1-bit logic signal that, when smoothed, matches the input. The human ear does this smoothing naturally, so that slinky-looking wave will actually sound just like a sine wave. Notice that the 1-bit output looks more like an actual sound wave in air: variations in pressure following amplitude.

Delta sigma generates a stream of pulses that have a density corresponding to input amplitude. There is a lot of noise in addition to the input signal, but as long as it’s supersonic it does not audibly matter. I did learn, however, that it can really screw with a microphone preamp. Saying “2nd order” refers to the feedback system the converter uses. Higher orders push more converter noise past the range of hearing.

I ran a digital sine wave generator into my D/A and tied the FPGA pins straight into my soundcard. It was a just a quick sanity check.

Click Here to listen , but be aware it’s just a 1k sine wave

The converter I built was for a 16-bit input, and that recording ended up with a noise floor on par with 14-bit performance. The distortion is pretty high, but that could be clock jitter, a lack of proper filtering, or even the sine wave generator itself. I was just happy that it was making intelligible sound in the very first test on actual hardware.

This entry was posted on Saturday, December 12th, 2009 at 8:17 pm and is filed under Experimental, Light Technical. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.