Skrasoft Dev Blog

I’ve been making hardware changes to the Pokey module so that it can actually be manufactured. Yesterday evening I took a break to write some more Verilog FPGA code for the Pokey controller. The previous test was to write to the Pokey as often as possible, which came out to 1.79 million times per second. This test added a finished start-up routine and tweaks frequency/volume registers to check for glitches.

Listen Here

It is just a simple test. The volume is modulated by a digital triangle wave, and the pitch is modulated by a slower digital triangle wave. Everything works just as expected. There is a lot more stuff I need to port from the microprocessor into Verilog, but it’s going faster than I originally expected. I probably won’t be using dsPIC chips for audio projects anymore after finding out how much more can be done with an FPGA.

In other news, my company is now official. The new name has a better ring to it than “skrasoft,” but I’ll save it for when this module goes on sale. I’m working as fast as I can to start manufacturing without sacrificing any of the features. There are several as-of-yet-unmentioned new features in the FPGA version of the module.

For those who missed it, I posted a test video of the old microcontroller-based module on Vimeo here:

http://www.vimeo.com/9258219

Most 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:

Blue Picket Fences

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:

Noise Censorship

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.