![]() With (say) a 1024 stage delay line this gives a delay of just over 100 milliseconds (1024/2 (2x50,000) = 0.01024 seconds, or 10.24 milliseconds). This is the main limiting factor on performance, since it necessitates a clock frequency in the region of 50kHz in order to realise the full 20kHz audio bandwidth. The clock frequency must be at least twice the maximum input frequency, and should preferably be at least three times the maximum input frequency. The delay is equal to the number of stages divided by double the clock frequency. This system of passing electrical charges along a series of capacitors is analogous to buckets of water being passed along a line of people, and it is from this that the term 'bucket brigade' delay lines is derived.Ī clock oscillator is used to control the rate at which samples are taken and passed along the line, and the delay obtained depends on both the number of switch/capacitor stages, and the clock frequency. In a practical circuit there would be typically between about 2 capacitors/switches, and the input samples are passed along all of these until they eventually reach the last one (and the output). Capacitor 1 then passes on its charge to capacitor 2, while capacitor 3 passes on its charge to a fourth capacitor. While the second capacitor passes on the charge to a third one, the input is again sampled by the first capacitor. Looking at this system in a highly simplified way, it involves sampling the input signal with a capacitor, which is charged to the input voltage, and then passing on the charge from this capacitor to a second one. The analogue delay chips currently available all use the same technique, which is based on capacitors and electronic switches. In the mean time analogue delay lines remain the only option for applications where a small, low cost delay line is required. In due course the cost of digital delay lines will probably fall to the point where they can be used in low cost effects units and not just in the more sophisticated and expensive items of equipment. Also, a 14 or 16 bit system is needed for optimum audio quality. Apart from the cost of two high quality converters (one A/D and one D/A), long delays require a large amount of RAM due to the high sampling rate that must be used (typically about 50kHz). While this system can give excellent results in practice, even with very long delays, it is still rather expensive. By continuously reading the RAM and appropriate number of locations behind the current storage point, any desired delay can be obtained. ![]() In practice the digitised samples are stored in successive RAM locations until the RAM is full, whereupon the system cycles back to the beginning and starts working through the RAM again. ![]() After the required delay the RAM is read and the signal is converted back into analogue form. ![]() ![]() The most sophisticated type uses a digital technique which differs slightly from one system to another, but the basic idea is to digitise the input signal and then store it in RAM. Although there are several types of delay line, only two of these are in common use in musical applications. Probably most readers will at some time or other have used a delay line, since these are used in a number of popular effects units, including flangers, phasers, solid-state echo and chorus units. Robert Penfold takes a look at delay line chips for echo, reverb and flanging ![]()
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