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Thread: Can i modulate the delay time?

  1. #1

    Can i modulate the delay time?

    Hi, was hoping to implement a crude emulation of the old oil-can delay effect using a digital delay line with two taps and a configurable bandpass filter feedback loop.

    I was wondering if I can modulate the delay time with an LFO to simulate the 'wobbliness' of the original oil can effect or whether that's likely to introduce zipper artifacts. I guess the other question is how do i introduce wow into a signal (i.e. a time-varying playback-speed effect). I mean it's easy with an oscillator- just add LFO FM but how do you do it in a delay?

    Suggestions anyone? I'd love to have something like this to play with.

    Dirk

  2. #2
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    The short answer (which is not helpful) is that you can do anything you want...but it could take a lot of development and effort.

    The longer answer is that you could modify the delay class to allow for better modulation of the delay time. It would have a cool sound. But, it would sound like a non-professional late-80s digital effects processor (for good or for ill). I don't think that it would sound like an oil can (or tape) wow and flutter.

    To actually do what you want, you need to model the fact that the audio is read from the can (or tape) at slightly different speed than the speed when it was written. The only way that I can think to do this is by resampling in real time...and sampling at smoothly changing non-integer ratios so as to model the smoothly changing wow and flutter. This seems hard to do.

    Since people have implemented this wobbly sound in delay pedals before (strymon), there must be a simpler way. I'd love to hear it!

    Chip

  3. #3
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    If you use a regular delay with a fixed dealt time (like is currently in the library) you would get a more 'analog-style' delay if you could smoothly vary / modulate the sample rate of the whole system. That gets you the effect of the playback speed being different than the speed when the audio was recorded.

    But, the default teensy library is set for a constant sample rate. There are definitely posts here on how to change the sample rate (I've used those posts myself). You could use that info to try this.

    Note that those posts are aimed at big changes in sample rate. You only need small changes. A few percent up and down? You'll have to dial those examples way back.

    It's possible that you won't be able to change the sample rate in fine enough increments to sound smooth.

  4. #4
    Senior Member PaulStoffregen's Avatar
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    Signal controlled delay (or voltage controlled delay if you like to think in modular synth terms) is on a long list of audio library features I hope to implement this summer. The reality is I've not managed to do much on the audio library for the last 2 years while making Teensy 4.0 & 4.1. But once we have 1.53 released, the bootloader chip for sale, and a start on 1.54 (mainly better file & filesystem support), I really want to build many of these long-requested features like signal modulated delay.

  5. #5
    Yea, i had a look at the delay object in the audio library and the delay was configurable for each tap, but not modulatable. We're also short of a pitch shifter/playback speed object. And I'm no whiz at DSP. The design of a modulated delay is rather beyond me.

    Now filesystems I can help with because i've ported the whole of the ESP8266 FS and File objects to work with SDFat-beta and LittleFS for a drum synth with automatic caching in flash RAM.

  6. #6
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by dirkenstein View Post
    We're also short of a pitch shifter/playback speed object.
    The wavetable synth has pitch shifting of its samples.

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