Digital Delay Effects Unit (Part 1)
Introducing a low cost project, but with a specification equivalent to studio units costing much more
Many musical effects such as echo boxes, flanging pedals etc. use a time delay as part of their circuitry. The cheaper units, aimed at the stage musician, offer only one or two effects per box; in addition, they use analogue delay components whose sound quality deteriorates considerably as the delay increases. High quality delay units for studio applications, in contrast, use digital techniques offering theoretically unlimited delay times; however, they are very expensive, often with four figure price tags.
Now, the E&MM Digital Delay Effects Unit offers you the best of both worlds; it gives all the time delay effects, with digital quality, all for the price of a high quality analogue unit (but with much superior specifications), the most popular effects are shown in Figure 1, along with the various ways of producing them. The E&MM Delay is shown in block diagram form in Figure 2, and by manipulating its variables all the effects in Figure 1 may be obtained. These are introduced here; the project will be concluded next month, with full circuit and construction details.
Phasing is produced by mixing an audio signal with a delayed version of itself. The frequency response this produces is known as a comb filter. Feedback is sometimes used to make the frequency response more peaky, which in turn produces a more noticeable colouration of the sound. By slowly modulating the time delay, the notches in the comb filter expand and contract, producing an interesting musical effect. Phasing effect pedals use a phase shift filter rather than a time delay line, although the effect is the same. Phasing is characterised by having very few notches within the audio band, typically 2 to 5. This is equivalent to time delays between 0.2ms and 0.5ms.
Phasing and flanging are often confused, which is not surprising as the two effects are produced in a similar way. To obtain a flanging effect use a time delay varying between 1 and 10ms. A 10ms delay will produce a comb filter with 100 notches (over a 10kHz bandwidth). Flanging often uses strong feedback which produces a heavy colouration of the sound.
ADT (Automatic Double Tracking) and chorus are both very similar effects. The chorus effect uses a time delay that is slowly modulated, and the original and the delayed signal are mixed together producing a 'spacey' effect. ADT uses a longer time delay to simulate a very short echo, short enough to give the impression of two sound sources.
Time delays greater than 30 or 40ms become noticeable as distinct echoes. Time delays of around one second are very useful for building up melodies with several repeats. Also, it is possible to freeze the sound in the digital memory and have it continuously recirculate without degeneration. This repeating sound may then be used as a sequencer-like backing, or transposed using the delay time controls.
Vibrato can be produced on any time delay setting, but best results are obtained on the 40ms delay with 10kHz bandwidth. A modulation speed of 3 to 7Hz with a small modulation depth should do it.
The E&MM Digital Delay Line is obtainable as a complete kit of parts from Powertran Electronics, (Contact Details). With ¼ memory, i.e. 400ms maximum delay, the kit costs £130 + VAT. Extra memory parts are £9.50 + VAT per 400ms, so the full 1.6s delay would cost £158.50 + VAT.
E&MM Cassette #6 digitised and provided by Christian Farrow.
Feature by Tim Orr
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