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Contemporary Keyboard

October 1977
More On Reverb Unit
Bob Moog

In last month's column I described the operation of the basic wire coil delay line of the sort used in spring reverbs. The frequency response of a single wire coil unit has a series of regularly-spaced peaks and dips. Because of this, a reverb unit using a single spring imparts a metallic, twangy quality to sounds that are fed through it. To overcome this limitation, reverb unit designers employ two or more springs with slightly different delay times, or a single spring with several sections. This results in a composite frequency response with a greater number of irregularly spaced peaks. At least one studio- quality commercially available reverb unit actually uses two identical springs. The secret is that no two springs are exactly alike. The slight differences are enough to increase the randomness and density of the frequency-response peaks, thereby making the reverb sound noticeably smoother and less metallic.
Spring reverbs tend to "flutter," especially when hit with high-amplitude percussive sounds. Fluttering happens when the wire coil vibrates in modes other than the torsion (twisting) mode required for proper delay line action. Flutter sounds like rapid tremolo. Designers control flutter by selecting good springs and minimizing distortion in the drive circuitry. One available unit even goes so far as to incorporate a special circuit that reshapes the attack transients of the audio input.
Selecting a good reverb unit is the same sort of task as selecting any other sound modifier: first eliminate the units with unacceptable shortcomings, then pick from among those that remain the one within your price range that has the best sound. Here are a couple of tests that require nothing but a synthesizer, a monitor, and your ears.
1. Feed steady white or pink noise through the reverb unit you're testing. Listen for excessively metallic quality or obviously oversize frequency-response peaks. A perfect reverb unit will not color white noise at all. You shouldn't expect this degree of perfection in the world of gig-worthy reverb units, but you need not tolerate a unit that whistles, honks, or twangs in response to a steady noise input.
2. Feed a fast-attack, fast-decay shot of white noise through the unit. Listen for flutter-rapidly flapping echoes following the test signal itself. You shouldn't expect complete absence of flutter, but on the other hand the flutter shouldn't dominate the normal smooth sound decay.
3. Listen for driver distortion. This is more elusive to identify than straight clipping distortion. You should feed several simultaneous middle-register tones into the unit; a diminished-seventh chord from a polyphonic instrument will work fine. Listen for muddiness in the reverbed output. If the muddiness goes away when you lower the input level, you probably have driver distortion.
4. Listen for hum and noise produced by the unit. If the drive circuitry has fairly high power and low distortion, reverb hum and noise should be barely audible. Also, the tendency of the unit to howl from acoustic feedback should be minimal. Low drive power makes a high signal-recovery gain necessary, and this is an invitation to hum, noise, and acoustic feedback.
5. Finally, listen for overall reverb quality. Play through the unit for a while. If listening to the reverb sound makes you feel good, you've got what you need.

 

All-Electronic Reverb Units

All-electronic reverb units use electronic devices in place of springs to create time-delay. Two types of all-electronic reverbs are currently available-analog delay lines, which use the same sort of bucket-brigade delay line used in flangers (see my column for Aug.'77), and digital delay lines, which use digital shift-register circuits for the delay function. Both types sample the audio input, that is, they divide the input waveform into a succession of narrow time slices. The average amplitudes of these slices are then sent down the delay line. The succession of closely spaced echoes necessary for reverberation is generated by a feedback path from the output of the delay line back to its input. The loop gain (the amount of amplification through the delay line and the feedback circuits) is critical for proper reverberation time. Also, the noise introduced by the delay line and the feedback circuits is multiplied in the feedback process. In analog delay lines, noise and feedback gain variations limit the overall performance. The state-of-the-art is rapidly improving as quieter, more stable bucket-brigade devices become available. Analog delay line reverb units have the potential for providing musically useful reverb quality with acceptable signal-to-noise ratios and the flexibility of continuously variable echo spacing achieved by means of a programmed sampling rate.
In digital delay line reverb units, the audio input signal is sampled by an analog-to-digital (A/D) converter circuit. The A/D converter turns each time slice of the waveform into a digital number proportional to its average voltage. The succession of numbers thus produced is the digital representation of the waveform. The numbers travel down a shift register, and are then fed back to the shift register input by a conventional digital adder circuit to produce the succession of echoes. The analog-to-digital conversion at the unit's input and the complementary digital-to-analog process at the output may introduce some noise. However, the digital processing that goes on between the two has no inherent noise or gain stability problems. Because of this, it is practical to build elaborate digital reverb units that rival or exceed the performance characteristics of the best professional plate and spring reverbs. Professional digital reverb units, however, are not cheap. Accurate A/D and D/A conversion requires precision circuitry, and many shift register circuits of varying length are needed to simulate the multiple acoustic reflections found in a good performance hall. On the positive side, digital reverb units have the potential for providing genuine studio-quality reverberation with a minimum of equipment space and weight and a minimum of susceptibility to external vibration pickup.