Shruti-1 internal signals
  • I bought a second-hand scope today to help me debug some stuff in the new project. I had some fun looking at the Shruti-1 signals. They are quite ugly indeed – some very low subharmonics ; and PWM meanies at around 30kHz. It takes some mental effort to realize that we shouldn’t really care, because there’s some stuff downstream in the processing chain that efficiently lowpass / hipass that. What struck me is that if I had a scope at the time I started working on the project, I would have never done it this way because the ugly waveforms would have driven me crazy.

  • i dunno, looks kind of pretty to me :)
  • Come on, this is supposed to be the sine wave! (on a side note, soundcards have pretty sharp brickwall filters before their ADC, so when you record this on a computer, it indeed looks like a sinewave, because the sharp undulating ugly monster is at 31kHz).

    My hears have a brickwall filter at 14kHz because I’m getting older – and it has alwyas sounded to me like a pure sine wave…

  • oh, dont get so scientific about it. If it sounds good, who cares. Did Roland obsess about the 303 square actually being a square (not even close)? ‘round here, that’s what we call “character”

  • So, what does this mean? Do you think you'll be reviewing the waves and making them more pure in future firmware releases? (I'm more of a "results-oriented" guy, so if it sounds good, it doesn't matter what it looks like.) Maybe the skewed waves have their own idiosyncratic sound, exclusive to the Shruti-1?

    EDIT: Altitude said exactly the same thing as I did, as I was typing it.
  • I won’t revise this because this is a hardware thing, not a software thing (the wave is pure, it’s just the way it is converted from digital to analog which is awesomely dirty). I had actually played with proper DACs and the difference (measured on a sound card) was barely noticeable. The point is that if I had seen this earlier I would have been scared and would have not gone in this direction at all (= no DAC, plain 1-bit PWM, relying on a hi-pass/low-pass filter somewhere downstream to kill the nasty harmonics). As said, on any device with a limited bandwidth (soundcard, human ears…), this doesn’t show.

    Almost all the projects I have seen which are based on AVRs for generating audio are indeed using DACs – but I think I’ll stay faithful to high-speed bit banging and filtering for the moment :)

  • So is this sample taken before the CEM, after the RC filter?

  • This is the sample taken before the CEM, after the active RC filter (which is a high-pass removing the 2.5V DC offset), and after another passive RC filter out of the circuit. Without this RC filter it looks even more ugly, since it's really pure bit-banging...
  • i think it's just cool that way!

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