Making a Reverb Pedal Using PT2399 Chips (Part 1)
Real reverberation occurs in the working chamber when the generated sound is reflected from walls, furniture, people or any other object in a complex three-dimensional space. The natural reverb process is shown in Figure 1.
Figure 1. Reverb in a real situation.
In the good old days, the only way to reproduce the reverb effect is to use a real reverb camera — a large room with complex geometry and carefully selected wall material, with speakers and a microphone installed in certain places inside the camera. The first attempt to simulate reverberation in a room without a real reverberation chamber was carried out using a reverb reservoir with a spring (see link ). The basic design of the spring reverb is shown in Figure 2.
Figure 2. Spring reverb reservoir design The
audio signal excites the input coil, which transmits mechanical vibrations to the nearest end of the spring, and then to its far end, and returns back with decreasing amplitude. Complex waves, both transverse and longitudinal, are generated inside the spring. High-frequency and low-frequency waves move along the spring at different speeds, and spring connections add reflection. To obtain time delays of various durations, springs of various types are used: thickness and type of metal, number of turns, spring diameter. The artificially reverberated sound created by the spring is then captured by the output coil and returned to the electronic circuit for mixing with the input audio signal and amplification.
Digital Reverb Modeling
Reverb effect has been widely studied, and in the opinion of the author may be classified as follows:
1. Playback system response: this method treats the modeled system as a black box, we do not care what's going on inside of him, and we just measure the output response using a "processing convolution ”(see link ). Regardless of whether the simulated system is a real concert hall or a real reverb tank with a spring or a plate, this method will be very simple to implement, but “convolution processing” will require very high computing power.
2. Physical modeling: this method analyzes the physical process of the simulated system, simulates it. This can lead to a very realistic sound, but it can require significant computational costs depending on the optimization or mathematical simplification of the model. One example of spring reverb simulation is given at .
3. Synthetic modeling: sometimes the author sees that such a model is simply a simplified model of approximating the response of the system by trial and error. For example, the Schreder reverb [see Link ) can be configured to simulate the reverberation of a medium-sized hall by setting certain values for a certain parameter.
Implementation of the reverb effect in an electronic circuit: delayed network and tuned analog resonators
When we analyze the reverb phenomenon as a complex echo pattern, we can intuitively construct such a reverb effect pattern using a network of delay lines. On the other hand, if we analyze the reverberation phenomenon as a continuous resonance, we might think that to create such an effect, several parallel analog resonators that are tuned to different frequencies can be used. The author has been thinking about this for many years. Please let the author know in the comments if there is already an analog reverb circuit based on analog resonators so that the author does not invent a bicycle. At the moment, the author has focused on a solution with a chain of delay lines.
PT2399 Digital Delay Chip - A Budget Solution for DIY Reverb Pedal Project
Princeton's advanced CMOS PT2399 technology is gaining in popularity for designing a device with a switchable capacitor bank (BBD) for storing audio samples in “analog” as an implementation of an analog delay line. The block diagram of the PT2399 is shown in Figure 3.
Figure 3. Block diagram of the digital circuit of the digital delay line PT2399 The
digital circuit of the delay line is made in an accessible 16-pin DIP package. The minimum delay time is 30 ms, the maximum is 340 ms, and the delay setting can be easily changed with an external resistor.
Hamuro Spring-Room-Hall reverb block diagram for a small room
Figure 4. Block diagram of the Hamuro Spring-Room-Hall
reverb The author created a very simple reverb scheme using 5 PT2399 circuits, which can simulate the effect of spring reverb in a room. It has the ability to control the delay time, room volume and balance. When the room volume control is set to a minimum, it will sound like a spring reverb, and if it is set to a maximum, it will produce a reverb like in a hall or cathedral.
Full circuit concept
A complete circuit diagram is under development and testing. Basic reverb circuit has been successfully tested on the platform Deepstomp (DIY digital is multi-stompbox effect), and will be published in the second part of the article (approx. Author).
1. L. Hammond, Electical Musical Instrument, U.S. Patent 2,230,836, February 2, 1941.
2. Fons Adriansen, “Measurement of Acoustic Impulse Response with ALIKI,” 4th International Linux Audio Conference: LAC2006
3. Stefan Bilbao and Julian Parker, “Virtual Spring Reverb Model,” IEEE Sound, Speech, and Language Processing Transactions, Vol. 18, No. 4, May 2010, p
. 799 4. M. R. Schroeder (Bell Telephone Laboratories, Incorporated, Murray Hill, NJ),
“The Natural Sound of Artificial Reverb,” Audio Engineering Society, July 1962