PT2399 Guitar Reverb

Reverberation (also known as reverb for short) is an electronic sound effect similar to echo and used to create a fuller sound duplicating the ambiance of a room. It is the persistence of sound due to the reflections from all the surfaces in a room. It is usually very small in duration and has a fast decay. In contrast echo is the sudden reflection of sound without fast decay as the sound may be reflected several times before decaying. The reverberation period is defined as the time required for the sound energy to decay to 1-millionth (60 dB) of its original intensity.

The PT2399 is an excellent IC for guitar reverberation however most of the circuits I have seen tend to produce echo, hence I had to try and figure out the difference between echo and reverberation since both are based on reflected sound waves.

If I were standing in a small room and clapped my hands, there would be an infinite number of sound reflections projecting from all the surfaces in the room. The delay between the incident wave and reflected wave would be so small my brain would perceive both sounds as one, and that is what I imagine reverberation would be. However, if I were to be standing in a very large room the size of a football stadium and I were to clap, the delay between the incident and reflected waves would be so large that my brain would be able to differentiate between the two and therefore that would be considered as an echo. However, many articles suggest that reverberation is essentially repeating echoes of diminishing amplitude.

I do not currently play an electric guitar; however I do play the acoustic guitar, and have learned over the years that a small delay sounds much better. A very short delay tends to adds more to the initial sound making it fuller giving it that honey-rich bass and treble mix that everyone seems to like.

In the field of psychoacoustics, when two sounds are identical in shape and amplitude and displaced in time by less than 10 ms, the brain integrates them together and perceives them as a single sound. It is that range of time from when the brain is not able to distinguish between them, to when it just begins to distinguish the difference. For guitar reverb to sound good, we need to work with delays within this range.

The nice thing about the PT2399 is that the delay can be manually set with an external variable resistor connected to pin 6, and in the documentation they show a table of the values of resistance and the delays achievable. As the resistance decreases, then so does the delay. With resistance set to 27.6 kΩ a delay of 342 ms (max) is achievable, and when 0.5 Ω a delay of 31.3 ms (min) can be achieved.

Whilst studying the Physics of spring reverbs, I noticed that in this type of apparatus the sound travels through a spring back and forth to give that unique guitar sound. Since the reflected sound through a solid such as a spring will be 180-degrees out of phase, it occurred to me that perhaps what is needed is a simple phase splitter circuit that will provide two signals 180-degrees out of phase with each other. The out of phase signal could then be used to produce the reflected wave, which is then mixed with the in-phase signal. I would anticipate that the out of phase signal would provide a dampening effect, which is that unique missing ingredient. I do not believe anyone has done anything like this before.

Since in reality, a natural reverb consists of an infinite number of reflections (returning almost together) with very short and different delays, in practice, this cannot be achieved with just a single circuit. However a good approximation could be achieved with a large number of PT2399 circuits operating in parallel, and each set with a slightly different delay. Of course, I do not have the big bucks to try out such an experiment but it is something worth considering if Daddy Warbucks was your friend.

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