Noise from Voltage-Controlled Delay
I'm building a custom Flanger effect in an AD1701 and keep getting background noise when there's a signal present. It sounds something like crackling but isn't a digital chatter. I seem to have narrowed it down to the VC Delay blocks when the control voltage is changing quickly (on the order of a few Hz)
Is there a maximum speed the delay blocks can change at or an easy way to mitigate this problem?
Thanks.
P.S. I am aware that there already is an ADI Flanger effect but I need one with more end-user control.
Audio Signal Processors
noise
SigmaDSP Audio Processors
delay
adau1701
Hi Austin,
When you use a voltage controlled delay in the analog domain, everything works quite nicely because the analog control signal has infinite resolution and the output signal will basically stretch and compress as the delay is modified.
However, in the digital realm, you have to delay the signal in discrete steps. This basically means that samples end up being skipped or repeated, depending on how much and how quickly the delay is being modified.
I expect that in your case, the relatively long delay time is resulting in "jumps" in the signal due to skipped or duplicated samples.
The only thing I can think of to mitigate the problem would be to increase the sample rate, which would basically double the resolution of your delay times. Another option would be to low-pass filter the output, since most of that noise is going to be high-frequency.
Propagate Sample Rate is only used if you have multiple sample rates in the same project, which is very uncommon. For the most part, you'll just need to use Set System Sample Rate.
You can follow this FAQ to see how it is done:
Brett,
Thanks a lot, that sounds exactly like what's happening. Increasing the sample rate (to 192kHz) seems to have helped, I thought I was doing that before but evidently was clicking the wrong button, can you explain the difference between "Set System Sampling Rate" and "Propagate Sampling Rate"?
The noise isn't completely gone with just the sample rate increase, but I'm sure a little time plaing with the speed of the VC-Delay control signal and some mild filtering will be enough to hide it. Either that or I'll call it a 'feature' and let marketing deal with it.
Thanks.
Also, what data format is the delay expecting the control input in? I'm assuming it's 28.0
Correct, it's the number of samples of delay in 28.0 format.
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