Hi,

The attached patch dsdelay is supposed to be a variable delay line with a moving write point. vd~ uses a moving read point which is acceptable for most applications but not what I am working on. I have implemented my moving write point delay line in MATLAB and it works reasonably well. However I am experiencing issues with PD implementation.

The patch uses a table to store the delay line values and is supposed to use a circular delay line method (i.e. the read/write points shift with time instead of the actual elements of the delay line). The intention is that the read point shifts along the delay line by one sample per sample, and the write point does the same whilst the delay is not being altered. The write point uses a simple two point extrapolation to allow writing at fractional table indices.

I'm having two problems:

1. I can't find a satisfactory method of wrapping signal values in an arbitrary range. I want to be able wrap a signal value between 0 and 44100 for example, but using

|
[/~ 44100]
|
[wrap~]
|
[*~ 44100]
|

gives rise to rounding errors which create artifacts in the output. This is why in the current state, the output is intermittent rather than continuous.

2. For some reason the 2 point extrapolation seems to be not working correctly. I'm getting a ring modulation type effect on the output. In MATLAB the addition of the two point extrapolation helped to minimise artifacts when the delay time was being altered. Altering the delay time gives rise to horrible artifacts at the moment though.

Any pointers on how to get this working properly would be greatly appreciated.

Thanks

Ben

http://www.pdpatchrepo.info/hurleur/test.zip

Try [expr~ fmod($v1, 44100)] instead of [wrap~].

I don't have time right now to dig into your patch to see what's going on, but I'm curious as to why you're doing it this way. You should still be able to simulate the doppler effect by modulating the read point. In fact, I would think you'd run into problems with skipping samples when writing.

If you're interested, I'm working on a DSP system for wave field synthesis. WFS is an audio rendering technique where an array of loudspeakers is used to reproduce the sound field within a region.

http://en.wikipedia.org/wiki/Wave_field_synthesis

You can simulate Doppler effects etc using a standard variable delay, but it is not an entirely accurate way to simulate sound emitted from a moving source. For example the effects of a change in delay will be heard instantly for a standard variable delay, whereas for a real moving source a change in position will only be heard after the transit time delay.

You are right of course about the skipping of samples. My MATLAB tests have shown me that this gives rise to a type of aliasing when the sound is slowed down by a lot. Thankfully, with the extrapolation for writing to fractional positions, the effect is rather subtle although by no means inaudible. This could be gotten around by looping through every sample between the position change, but I'm pretty sure that would be impossible in pd.

I am doing this because I want to compare various methods of realising moving sources.

Oh and by the way, thanks for the tip about expr~ it worked perfectly. Problem #1 solved.

I think the problem might be that I am using multiple instances of [poke~] and I don't know if they are properly synchronised.