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:
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.
Thought it was interesting to see and hear. Carry on about your business now!