BBD Reverbs.
First there were plate and spring line reverbs, then along came an electronic “chip” called a Bucket Brigade Delay (BBD). This used high impedance CMOS devices and capacitors to pass the electric charge (audio) along the line, each stage in the line adding a small delay. These worked well, and eliminated the problems of external noise entering the signal chain. However the way they worked introduced their own artefacts such as aliasing at high frequencies, filtering was used filtering to restrict this, and they also introduced a certain amount of distortion, which gave them their own unique low-fi sound. By using multiple chips with different delay times you could get some quite complex reverbs.
Delay2.
Creates a delay (echo or reverb) effect on audio signals very similar to BBD chips.
Note: If you intend to modulate the delay time, you should enable interpolation, this reduces clicks and “zipper” noise.
Plugs.
Left Hand Side:
Signal In:- (Voltage) Input Signal
Modulation:- (Voltage) Varies the delay time dynamically (0 to 10V)
Feedback:- (Voltage) Controls the amount of feedback of the delayed signal.
Right Hand Side:
Signal Out: – (Voltage) Audio output signal
Parameters:
Delay Time (secs): – Maximum delay time in Seconds. The delay time is limited to a maximum of 10 seconds.
Interpolate Output: – Provides smoother modulation of delay time, but with an increase in CPU load.
Basis BBD style, single delay reverb effect.
We can emulate this in a SynthEdit module, which takes the audio input, and much like the old BBD chips, slices up the audio into samples and passes them down the delay line.
Shown below is our very basic reverb effect.
Delay Time:- Set the delay time to 0.001 Seconds (10mS) in properties
Delay control:– Set the maximum voltage for the delay slider to 0.06 volts, this gives us a delay time range of 10mS to 70mS.
Feedback:– Set the maximum voltage for the Feedback slider to 8 volts, as we don’t want the absolute maximum feedback level as it will just give uncontrolled oscillation.
Wet/Dry:- This control will default to a range of -5 volts to +5 volts. Leave this as it is to give the full range of wet to dry effect.
There’s one important point to note, if this reverb is being included as part of a Synthesizer VST we need to think about polyphony. It really isn’t needed for a reverb effect so we need to containerise our reverb, put a Voice Combiner module in place in the input- the voice combiner forces then the Modules in this container to stay Monophonic. Otherwise every time you play more than one note on your Synth plug in it will create an un-necessary clone of the reverb module wasting CPU and memory.
Making the Reverb Stereo.
Converting to Stereo is easy, we just create two identical Reverb chains, one for the left channel, and one for the right channel. Keep the two delay and feedback levels linked otherwise some very peculiar (and unwanted) phase and stereo imaging effects can be introduced.
Limitations of basic Delay reverbs.
This is structure is OK for a very basic reverb effect, 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 so good. So what’s the answer?
Fortunately there is one that was devised by Manfred Schroeder, known as the Schroeder Model of reverb.
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