Spike at 39khz?

I noticed my Shruthi has a huge spike at 39khz, even when silent. Is this normal? Or is it possible that I set up the filter wrong somehow? I know that’s way beyond what I can hear, so would it even cause any problems? I’m using the SMR-4 filter by the way.

I can’t figure out how to add an image to the post, but here’s a link to a screen shot:

The Shruthi-1 uses PWM modulation rather than DACs to generate the audio and control signals. This means that all the raw signals output by the digital board have huge spikes at 39kHz. In fact, all the signals out of the digital board look like 39kHz square / rectangular wave. The “actual signal” is encoded as its PWM modulation.

However, the 39kHz PWM carrier is filtered on the filter board. If you look at the SMR-4 schematics:

  • C11 filters the cutoff CV.
  • C6 filters the resonance CV.
  • C14 filters the VCA CV.
  • The PWM carrier is filtered out of the audio signal by… The 4-pole filter itself ; and C5 at the output stage.

As a result of all these filtering stages, the 39kHz is -12dB below signal level with the filter wide opened (very worst case) ; and 80dB below signal level with the filter closed (cutoff set to 40). If your readings indicate something larger, it is probably because one of those caps is loose. It has already happened to me twice :slight_smile: Depending on the way you have set the filter range trimmer (lowwww) “wide open” will mean “cutoff at 40kHz” or “cutoff at 16kHz” and this will have an impact on the level of the residue.

The only situation where the residual 39kHz might be a problem is when the signal is fed into an old-school digital device (sampler, FX) which doesn’t have brickwall filters on its inputs. Then it will be mirrored (if the device’s sample rate is 48kHz, nyquist is at 24kHz, 39kHz is folded back to 24 - (39 - 24) = 9kHz). The way modern AD conv. work, they really need to have good brickwall filters so it won’t go in. It doesn’t get in any decent sound card sampling at less than 96kHz ; it doesn’t get in speakers ; it doesn’t get in any audio encoding/bounce to a file with a sample rate below 78kHz.

Ok, so perfectly normal :slight_smile:
I don’t think it ever got above -12dB, even when the filter was wide open, but I’ll double check when I get home.