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Mike Hudson
12 Reverberation effect
12.1 A basic reverb
The basic idea of a reverb was first created using a variable delay with its signal fed
back to create a series of delays that fade out over time (feedback gain must be less than
one). Equation 17.1 shows how the gain parameter is calculated to control the amount
of feedback and to determine the amount of reverb reflections.
Figure 12-1 shows this structure (comb filter).
Figure 12-1: Comb filter
𝑔 = 0.001𝛕/𝐑𝐕𝐓
(17.1)
Where τ is the delay time in ms and RVT is the total reverb time of the comb filter.
Reverb time is defined as the time it takes for the output to fall to zero (-60dB) when an
impulse is applied.
This form of reverb was implemented on the DE2 board but did not give very
satisfactory results. The sounds seemed to resonate at the high frequencies and could
completely alter the tone depending on the delay. Further techniques were explored.
12.2 The all-pass filter
The all-pass filter is effectively the same as the comb filter but with a feed-forward path
in addition to the feedback path (Figure 12-2). The all-pass filter has a flat frequency
response unlike the comb filter and allows for a frequency independent delay. Figure
12-3 shows how the all-pass filter was realised in practice- in Quartus as a block
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