While I was working on another post, I made a few recordings of the signal straight off the Due DAC and then recorded the same through the output circuit. I just thought I might share the sounds and screeenshots of the waveforms.
The DAC output circuit is shown above. It's basically a bandpass filter. But really it's a single order lowpass and a single order highpass filter. The lowpass filter is used on DACs to smooth out the jaggies - they're called reconstruction filters. Note that this is not the same thing as an anti-alias filter which is a high-pass before an ADC stage. Your reconstruction filter is typically half your sampling rate. This filter is designed for 10kHz which would be good for a 20kHz signal. That may be a little optimistic for our Due that has a lot of stuff to do, but since we're talking modular synths, you will most likely have a big fat resonant low pass filter. Or even a wimpy one.
The highpass filter is used to block DC. You have to block DC when your DAC stage is a 0-5V signal and you want a -2.5 - 2.5V signal. The downside of blocking DC is that you have to also take some of the other low frequency signal so you need to decide how big of a capacitor you want (read pay for) versus how much of the low end you can stand losing. In most cases, you can manage up to 10-15Hz. That's right where we the nw2s::b is designed.
So without these filters, you'll get some DC offset which limits your headroom and you'll get a lot of high frequency information that really doesn't belong. Here's what it looks like and how it sounds:
[soundcloud]https://soundcloud.com/scottwilson/nw2s-b-noise-before-output[/soundcloud]
And with the filters, you can see that the corners are considerably smoother and the DC is being filter out since the squareness it more trapezoidal.
[soundcloud]https://soundcloud.com/scottwilson/nw2s-b-noise-after-output[/soundcloud]
Thought it was interesting to see and hear. Carry on about your business now!
