Analog Delay Lines to Produce Special Audio Effects Such As Echo, Reverb, Room Expansion, and Many Others

You can use analog delay lines to produce special audio effects such as echo, reverb, room expansion, and many others. Find out how here. RAY MARSTON SOLID-STATE DELAY LINES ARE WIDELY 2. If the ears perceive two sounds that image of environmental conditions. For used in modern music and audio systems. are identical in form but time-displaced example, if a sound reverberates for a total They can be used to produce effects such by IS50 ms, the brain perceives them as of two seconds, and an echo occurs every as echo, reverb, chorus, phasing, and two independent sounds, but integrates 50 ms, the brain may interpret its environ- flanging (sweepable filtering) for per- their information content into a single ment as being a 50-foot cave or a similar forming artists. In addition, delay lines easily recognizable pattern, with no loss hard-faced structure. But if the reverbera- can produce ambience synthesis and of information fidelity. tion time is only 150 ms, the brain may room expansion for home hi-fi systems. 3. If the ears perceive two sounds that interpret the environment as being a soft- Further, a delay line can be used as the are identical in form but time-displaced ly-furnished 50-foot room. Those sorts of heart of a clicWscratch filter for 2 record by more than 50 ms, the brain perceives facts can be used to trick the brain into player, and as a sound-activated switch. them as two independent sounds, but may drawing false conclusions concerning its There are two basic types of solid-state be unable to integrate them into a recog- environment; in fact, it is those tricks that delay systems: analog and digital. Digital nizable pattern. are used in ambience synthesizers and delay systems tend to be more expensive 4. If the ears perceive two sounds that room expanders. and more complex than analog types, ex- are identical in shape but not in magni- 6. The brain is highly sensitive to sud- cept when delay times exceed 250 ms, so tude, and which are time-displaced by den increases in sound intensity, such as we'll confine this discussion to analog more than 10 ms, the brain interprets them clicks and scratches on record albums, systems only. But before we get into the as two sound sources (primary and sec- but it is insensitive to sudden decreases in . hardware, let's talk about psycho-acous- ondary), and draws conclusions con- intensity. We're speaking now of tran- tics. In that way we'll learn why the cerning both the location of the primary sients that last only a few milliseconds. effects produced by delay lines work. sound source and the distance between the Those facts are what make clicWscratch two sources. filters possible. We'll discuss how to do Regarding location identification, the that later, but first let's discuss some of the Many of the special effects obtainable brain identifies the first perceived signal. basics of how delay lines work. with delay lines depend heavily on how as the primary sound source, even if its the human brain interprets sounds. The magnitude is substantially lower than that Delay-line basics brain does not always perceive sounds as of the second perceived signal. That is Modern analog delay lines come in in- they truly are, but it interprets those known as the Hass effect. So a delay line tegrated-circuit form and are commonly sounds so that they conform to a precon- can be used to trick the brain into wrongly known as CTD (Charge Transfer Device) ceived pattern. Therefore, sometimes the identifying the location of a sound source. or bucket-brigade delay lines. Those de- brain can be tricked into misinterpreting Regarding distance identification, the vices contain a number of analog memory those sounds. Scientists have discovered brain correlates distance and time-delay at cells (buckets). Each cell is actually a several psycho-acoustic laws that explain roughly 0.3 meters (about 13 inches) per sample-and-hold circuit and, usually, how the brain interprets various sounds. millisecond of delay. So a delay line can 512, 1024, or 4096 cells are wired in se- O 1. If the ears perceive two sounds that be used to trick the brain about how far ries. Analog input signals are applied at 3 are identical in form (waveshape and am- apart two sound sources are. the first cell and the delayed output is plitude) and time-displaced by less than 5. The brain uses echo and reverbera- taken from the last cell. 31 10 ms, the brain integrates them and per- tion (repeating echoes of diminishing am- Figure 1-a illustrates the basic compo- ceives them as a single sound. plitude) information to construct an nents of an analog delay line. Each bucket % analog delay lines have clock dividers so that only a single-phase clock input is needed. We'll discuss specific IC's later in this article. FIG. 1-AN ANALOG DELAY LINE is composed of a bucket-brigade delay line, input and output lowpass filters, and a two-phase clock (a).The circuit produces a time-delayed replica of the input signal (b). consists of a small-valued capacitor and a S1. On the second half-cycle, all charges FIG. 3-A TWO-PHASE CLOCK and several MOSFET which function together as a (including the most recently sampled one) biasing resistors are required to drive a bucket- sample-and-hold stage. As shown in Fig. are shifted to the next bucket, but the input brigade delay line. l-b, a signal fed to the input of a delay line is not sampled. That double-shifting pro- will appear at the output at a later time; as cess repeats on each clock cycle, and in- How much delay? we'll see below, that time depends on both put samples are repeatedly taken and then The cells of an analog delay line are the clock frequency and the number of shifted. alternately empty and charged. Each buckets. Near the end of the delay line two sec- complete clock cycle shifts a charge two The input signal is applied to the delay tions of buckets are wired in parallel. stages along the delay line. Therefore the line via a lowpass filter that has a cutoff Those sections differ in that one section maximum number of samples that a line frequency one third (or less) of the clock has an extra bucket. There is also a phase can take is equal to half the number of frequency. The lowpass filter is used to difference between the clock signals ap- stages. For example, a 1024-stage line can overcome aliasing (intermodulation) plied to the parallel sections. So the cir- take only 512 samples. The actual time problems. The output of the delay line is cuit has two outputs which, when added delay available from a line is given by t = passed through a second lowpass filter, together, effectively fill in the "gaps" in STI2, where t is the time in seconds, S is which also has a cutoff frequency one the main delay line. The outputs can be the number of stages, and T is the period third (or less) of that of the clock. That summed either by shorting them together of the clock. In terms of clock frequency, filter also rejects clock-breakthrough sig- or, preferably, by connecting them to a that equation is t = Si2f. nals and integrates the output of the delay BALANCE potentiometer, as shown in Fig. For example, a 1024-stage delay line line into a faithful (but time-delayed) rep- 2. The final output of the delay line is thus using a 10-kHz (100 ~s)clock gives a lica of the original input signal. a time-delayed replica of the original in- delay of 1024 I (2.10,000) = 51.2 ms. A As shown in Fig. 2, an electronic put signal. 4096-stage line gives a 204.8-ms delay at switch is placed at the input of the delay Figure 3 shows the essential compo- the same clock frequency. line; that switch is externally biased to a nents of an analog delay circuit. The de- It's worth pointing out that the band- pre-set voltage. Charges can then be shift- lay-line MOSFET's have a tetrode width of an analog delay line is limited to ed from cell to cell, one cell at a time, at a structure, so the IC needs two supply volt- about one third of the clock frequency. rate determined bjl an external two-phase ages (V,, and V,), plus a ground con- Hence a delay line clocked at 10 kHz has a clock. One phase of the clock controls nection. The input terminal must be useful bandwidth of only about 3.3 kHz. shifting, and the other phase drives the biased into the linear mode by resistors R1 input-sampling switch. The operating se- and R2. The two outputs of the device are Simple effects quence is as follows. added together, as discussed earlier. Last, Figures 4-15 illustrate a variety of ap- On the first half of the clock cycle, the the IC must be provided with a two-phase plications for analog delay lines. In these contents of each bucket are shifted to the clock signal; usually that signal is a pair of block diagrams we will, for the sake of next bucket in line and a sample of the out-of-phase squarewaves that switch simplicity, not show the usual input and input signal is fed to the first bucket via fully between V,, and ground.

Load more