A special package came today.

## Archive for the ‘DSP’ Category

### Swimming with Sharcs

Friday, September 24th, 2010### 1-bit FPGA D/A Update

Monday, February 8th, 2010Most of my hobby time lately has been spent sourcing parts and doing circuit layout for the Pokey module, but yesterday I spent a little more time with the 1-bit D/A I was working on. Spectral plots are in order!

Running the output of the FPGA directly into my soundcard with a couple wires, this is the output spectrum for a 1k sine wave:

Alright, so that looks pretty terrible. The sine wave data is 16-bit, so the noise floor should be around -96 dB (the bottom line of the graph), yet it’s in the -86 range. There’s also a lot of distortion: both even and odd harmonics.

One potential reason for these problems is that my soundcard probably does not like 25 MHz input frequencies. To help that a little bit, I added an RC filter between the FPGA and soundcard. Using 16 kHz as a cutoff, the new result looks like this:

That’s much better. True CD quality would just have one spike at 1000 (for 1 kHz), as 2k, 3k, 4k… are distortions. The worst case offender is just under -40dB, giving this converter noise performance greater than 16-bit, but distortion performance that’s around 7-bit. I do not have an obvious solution for the distortion problem, nor can I prove with my current setup that it *isn’t* just a result of my soundcard not being able to handle the high frequency energy. If anyone has any insights, please add a comment or two.