Echo and Reverberation Devices
An echo device, or delay device, is a signal processor used to simulate discrete reflections of sound. Echo devices have taken several forms, including tape delay, analog delay, and digital delay systems.
In tape delay, a direct dry signal is fed to a tape recorder and printed to tape. Due to the separation of the record head from the playback head, the tape recorder output signal will be delayed, and can be sent to an input on the console for mixing. The period of delay can be changed by altering the distance between the heads, or by adjusting the speed of the tape transport. A method of feedback can be exploited to facilitate repeating echoes. This consists in routing the tape recorder output back to the input, together with the direct signal. When the amplitude of the output signal is lower than that of the direct signal, the repeating echoes will taper off to inaudibility.
Electronic systems employing solid-state circuitry were later devised to provide echo. These are the so-called analog delay devices. They have several advantages over tape delay systems. They use no moving parts or tape, which can wear out. They allow much more precise control of the period of delay. And they are relatively compact and inexpensive.
Digital delay systems simulate reflections by converting an analog signal to digital data, retaining the data for a period of time in a shift register until a specified number of clock pulses releases the data to the output stage, at which it is converted back to an analog signal.
Reverberation, or "reverb", devices have at least as diverse a morphology as echo devices. In principle, however, the two are basically the same. Reverberation, like echo, consists in reflections, but the reflections of reverberation are so varied, so numerous and so overlapping as to give the impression of a smooth integral body of sound, resembling the initial unreflected sound in character, and gradually diminishing in loudness over a period of time.
Natural versus Artificial Reverberation
In many "live" situations, especially orchestral performances in concert halls, the physical space is specifically designed to provide certain reverberant qualities, and this is all that is required. Almost all applications of artificial reverberation are intended to simulate the complex reflective activity ensuing in a physical room, such as a concert hall, filled with sound. Although possible in principle, it is impractical for most studio facilities to arrange to have at their disposal an adjustable echoic space, or echo chamber, for the purpose of applying particular reverberations to sound sources and recorded sounds. Therefore, it is chiefly artificial reverb devices that serve this function in today's studio environments.
In a spring reverb device, a signal is fed to an input and sent to one end of a suspended row of light-gague metallic springs. The energy of the signal sets the springs vibrating, producing a convincing simulation of natural reverberation in the output signal, which is tapped at the opposite end of the suspension. The sound can be thin and "springy", or relatively warm and smooth, depending primarily on the number and qualities of the springs used. The device itself is inexpensive to manufacture, and so such units are often built into the design of instrument amplifiers. However, due to the limited elasticity of the spring material, a good deal of amplification is required to boost the output signal to a useful level, and the net result is a noisy sound. This design also provides very limited control, if any, over the quality of the resultant sound.
A plate reverb system employs a large metallic plate suspended vertically in a solid frame. A driver induces motion in the plate, which is picked up by contact microphones. The sound produced by such a system is warm and rich, and the output signals are strong enough to be used in professional applications. The decay parameter can be adjusted by the judicious use of damping mechanisms applied directly to the plate. For a considerable period, plate systems were the electronic reberb of choice in professional studio environments. They provide clear, smooth enhancement to vocals and instruments alike.
Like digital delay, digital reverb converts an analog input signal to digital data. To simulate complex superimposed reflections, the program of such a system employs an algorithm including multiple recursions to effectively send the data to the output stage many times in rapid succession, at various intervals and at various levels. Digital reverb systems can provide practically limitless control of parameters, such as initial delay, high-frequency absorption, smoothness, and decay. They are more compact, more versatile, and increasingly more affordable than plate systems, and are currently the predominant type of system used to produce reverberation in recording studios.