DIY Reverb Pedal Circuit: From Spring- to Room-Like Reverb Using Multiple PT2399 IC Chips

Figure 1. Reverberation In Real Situation

Introduction

Real reverberation happens in a performance chamber when the generated sound is reflected by walls, furniture, people, or any other object in complex three dimensional space. The natural process of reverberation is shown in the Figure 1. In the old days, the only way to reproduce a reverberation effect is by using a real reverberation chamber, where a large room with complex geometry and with carefully selected wall material is built to be fed with loudspeakers at certain location and picked up by microphone at another location inside the chamber. The first attempt to mimic a room reverberation without a real reverberation chamber is done by a spring reverb tank (see reference [1]). The basic construction of spring reverb tank is shown in the Figure 2.

Figure 2. Spring Reverb Tank Construction

The audio signal drive an input coil that vibrate the spring at one end, and then the vibration is transmitted to the other end and bounce back and forth between two ends with decreasing amplitude. The complex waves (both transverse and longitudinal waves) is generated inside the spring since the the high and the low frequency waves move in different speed in the spring, and the spring joints
adds more complex reflections in the middle of its traveling path. Different springs with different characteristics can be arranged to produce rich and unique resonant characteristics. This artificial reverberated sound created by the spring construction is then picked up by the output coil and returned back to the electronics circuit for mixing and amplification.

Digital Modelling of Reverb Effect

Reverberation effect processing has been widely researched, and (in my opinion) can be classified on several mainstream methods:

  • System response reproduction: this method treat the modeled system as a black-box which we don’t care about what happen inside this box, and we just measure the output response when we apply the impulse input, and use it to modify the signal by convolution processing (see reference [2]). Whether the modeled system is a real concert hall or a real spring or plate reverb tank, this method would be very simple to implement but the convolution processing would require very high processing power to run.
  • Physical modelling: this method analyze the physical process of the modeled system simulate every physical process in the computation. This can produce very realistic realistic sound but can be high in computational cost (depending on the optimization or mathematical simplification of the model). For example, you can see one example of spring reverb modelling at reference [3].
  • Synthetic Modelling: this model construct a computation model that produce a (perceived) similar effect on the audio output, no mater if the computation model can be related with the physical model or not. This could be a debatable category whether there is exist (or not) for what I call a pure synthetic model that can’t be related to the physical model in any way. Sometimes I see such model is just an oversimplified model of the physical or just approximation of the system response by trial and error computation experiment. For example, a Schroeder reverb [see ref [4]), can be adjusted to mimic the medium-size hall reverberation by setting up some parameter to certain values.

Electronic Circuit Implementation of Reverb Effect: Delay Line Network and Tuned Analog Resonators?

When we analyze that the reverberation phenomenon as the complex echo pattern then we can intuitively build such reverb effect circuit using some delay line networks. On the other hand, if we analyze the reverberation phenomenon as a continuous resonance then we can think that multiple parallel analog resonators (which is tuned at different frequencies) can be used to generate such effect. I have been thinking of this analog resonators for years since I learned electronics, especially on the oscillator subject, where many of oscillator designs are basically an unstable resonator. Please let me know (on the comment) if there is already analog reverb circuit design which is based on analog resonators, so I won’t be reinventing the wheel someday. For the moment, let’s focus on the delay line circuit solution for the reverb circuit design, since it has been proven in such applications.

PT2399 Digital Delay Line IC Chip, Low Cost Solution for DIY Reverb Pedal Project

With the advanced CMOS technology, PT2399 from Princeton technology is getting popular to replace the bucket-brigade device (BBD) that uses switched capacitors array to store audio samples in analog voltage to implement a delay line function. The block diagram of the internal circuitry of PT2399 IC chip is shown in the Figure 3.

Figure 3. PT2399 Digital Delay Line IC Block Diagram

This digital delay line chip is available in 16-pin DIP package, so it will be easy to be handled for do-it-yourself projects. The minimum delay length is 30 ms and the maximum is 340 ms, and the delay setting is easily adjustable by selecting a proper value of an external resistor.

DIY Reverb Pedal Circuit for Simulating Spring and Room/Hall Reverb

Figure 4. Hamuro Spring-Room-Hall Reverb Circuit Block Diagram

I have created very simple reverb circuit that can simulate spring and room reverb effect. It use 5 delay line integrated circuit chips, and the block diagram is shown in the Figure 4. It has decay time control, room size control, and dry/wet balance control. When the room size control is set to minimum, it would sound similar to spring reverb, and whet it set to maximum then it would produce hall or cathedral-like reverb.

The Complete Circuit’s Schematic Diagram

The complete circuit schematic diagram is under development and testing. The basic block diagram of the reverb circuit has been successfully tested on Deepstomp (DIY digital multi-effect stompbox) platform, and we will publish the complete schematic diagram here if it done (to be continued.. )

References

  1. L. Hammond, “Electical Musical Instrument,” U.S. Patent 2,230,836, Feb. 2, 1941.
  2. Fons Adriaensen, “Acoustical Impulse Response Measurement with ALIKI”, 4th International Linux Audio Conference: LAC2006, http://lac.zkm.de/2006/papers/lac2006_fons_adriaensen_01.pdf
  3. Stefan Bilbao and Julian Parker, “A Virtual Model of Spring Reverberation “, IEEE Transactions on Audio, Speech, and Language Processing, Vol. 18, No.4, May 2010 p.799,https://www.era.lib.ed.ac.uk/bitstream/handle/1842/3718/BilbaoS_A%20Virtual%20model.pdf
  4. M. R. Schroeder (Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey),
    “Natural Sounding Artificial Reverberation”, Journal of The Audio Engineering Society, July 1962

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