Using a single Delay2 for a very basic reverb effect is all well and good, but has its limitations, one of which is that at the short delay times we are using there can be noticeable peaks and troughs in the frequency response, as what we are setting up is basically a comb filter, so we can get some ringing effects on transients at certain frequencies, and also a “drainpipe” like effect on other sounds. Not good. So what’s the answer?
The Schroeder Model of Reverb.
If we put a sharp transient sound such as a handclap into a delay line then
exponentially decaying impulses follow the first impulse. Though this
is similar to an exponentially decaying reverb, in the frequency
response peaks occur at equally spaced frequencies like the teeth of a
comb, hence the name comb filter.
This will result in a ringing metallic sound, especially once feedback is added into the mix. Digital audio pioneer Manfred Schroeder proposed using parallel delays with differing delay times, in combination with all-pass filters for digital reverb. His idea proposed a structure with four parallel delays and two serial all-pass filters. The delays create the reflections, and the all-pass filters “smear” any transients thus making the reverb more diffuse and reducing any resonances. Schroeder’s design is shown below in block form. Each comb is a Delay2 module.
6 x Delay container:
A set of six Delay2 modules pre-set to differing delay times within the range of 10 to 60 mS which is the ideal range for reverb, any longer and we start to get an echo effect, and shorter and there’s no noticeable reverb effect.
All the outputs from the Delay2 modules are fed into the Input 1 of a Divide module, this is because if we just feed all the signals into a normal module they will all be added together, more than likely exceeding the input range of the module causing clipping. Input 2 of the divide module is set to 6 (the number of delay modules) to restore the normal signal range. There have been various calculations made for the “ideal” delay times, but in practice a natural reverb depends on the shape and size of the area, and the things in that area.
There has been a calculation made of the “ideal” delay times which are
50, 53, 61, 68, 72, and 78 ms
All Pass Container:
The All Pass container uses two more Delay two modules in series one set to 17mS, and another to 74mS these two delays give an added “blurring” effect to any transients, along with a 1pole Low Pass filter for variable HF damping on the reverb “tail”
Feedback Container:
The feedback container has a Feedback – Volts module to allow feedback. A combination of the inherent time delay in a feedback loop in SynthEdit, and the two All Pass filter modules adds further frequency dependant phase shifting to further reduce ringing and metallic sounding transients. Again the frequencies of the All Pass filters are fairly random, but spaced out to try and give maximum effectiveness in reducing unwanted audio artefacts.
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