A digital delay line (or simply delay line, also called delay filter) is a discrete element in a digital filter, which allows a signal to be delayed by a number of samples. Delay lines are commonly used to delay audio signals feeding loudspeakers to compensate for the speed of sound in air, and to align video signals with accompanying audio, called audio-to-video synchronization. Delay lines may compensate for electronic processing latency so that multiple signals leave a device simultaneously despite having different pathways. 

Digital delay lines are widely used building blocks in methods to simulate room acoustics, musical instruments and effects units. Digital waveguide synthesis shows how digital delay lines can be used as sound synthesis methods for various musical instruments such as string instruments and wind instruments. 

If a delay line holds a non-integer value smaller than one, it results in a fractional delay line (also called interpolated delay line or fractional delay filter). A series of an integer delay line and a fractional delay filter is commonly used for modelling arbitrary delay filters in digital signal processing. The Dattorro scheme is an industry standard implementation of digital filters using fractional delay lines.

Theory 

The standard delay line with integer delay is derived from the Z-transform of a discrete-time signal  delayed by  samples:        
In this case,  is the integer delay filter with:

The discrete-time domain filter for integer delay  as the inverse zeta transform of  is trivial, since it is an impulse shifted by :
Working in the discrete-time domain with fractional delays is less trivial. In its most general theoretical form, a delay line with arbitrary fractional delay is defined as a standard delay line with delay , which can be modelled as the sum of an integer component  and a fractional component  which is smaller than one sample:This is the  domain representation of a non-trivial digital filter design problem: the solution is an any time-domain filter that represents or approximates the inverse Z-transform of .

Filter design solutions

Naive solution 
The conceptually easiest solution is obtained by sampling the continuous-time domain solution, which is trivial for any delay value. Given a continuous-time signal  delayed by  samples, or   seconds:                 

In this case,  is the continuous-time domain fractional delay filter with:

The naive solution for the sampled filter  is the sampled inverse Fourier transform of , which produces a non-causal IIR filter shaped as a Cardinal Sine  shifted by :The continuous-time domain  is shifted by the fractional delay while the sampling is always aligned to the cartesian plane, therefore:
 when the delay is an integer number of samples , the sampled shifted  degenerates to a shifted impulse just like in the theoretical solution.
 when the delay is a fractional number of samples , the sampled shifted  produces a non-causal IIR filter, which is not implementable in practice.

Truncated causal FIR solution 
The conceptually easiest implementable solution is the causal truncation of the naive solution above. Truncating the impulse response might however cause instability, which can be mitigated in a few ways:

 Windowing the truncated impulse response, therefore smoothing it. Note that in this case we have to add a further shift  in order to align the window and the  and provide symmetric filtering.
 General Least Square (GLS) Method: iteratively adjusts the frequency response by windowing a Least Square Integral Error design, which minimises the square integral error between ideal and truncated frequency responses of the filter, defined as:

 Lagrange Interpolator (Maximally Flat Fractional Delay Filter): adds "flatness" constraints to the first N derivatives of the Least Square Integral Error. This method is of particular interest because it has a closed form solution:

What follows is an expansion of the formula above displaying the resulting filters of order up to :

All-pass IIR phase-approximated solution 
Another approach is designing an IIR filter of order  with a Z-transform structure that forces it to be an all-pass while still approximating a  delay:The reciprocally placed zeros and poles of  respectively flatten the frequency  response, while the phase is function of the phase of . Therefore, the problem becomes designing the FIR filter , that is finding its coefficients  as a function of D (note that  always), so that the phase approximates best the desired value . 

The main solutions are:

 Iterative minimization of Least Square Phase Error, which is defined as:

 Iterative minimization of Least Square Phase Delay Error, which is defined as:

 Thiran All-Pole Low-Pass Filter with Maximally Flat Group Delay. This yields a closed solution for finding the coefficients  for positive delay :

What follows is an expansion of the formula above displaying the resulting coefficients of order up to :

Commercial history 

Digital delay lines were first used to compensate for the speed of sound in air in 1973 to provide appropriate delay times for the distant speaker towers at the Summer Jam at Watkins Glen rock festival in New York, with 600,000 people in the audience. New York City–based company Eventide Clock Works provided digital delay devices each capable of 200 milliseconds of delay. Four speaker towers were placed  from the stage, their signal delayed 175 ms to compensate for the speed of sound between the main stage speakers and the delay towers. Six more speaker towers were placed 400 feet from the stage, requiring 350 ms of delay, and a further six towers were placed 600 feet away from the stage, fed with 525 ms of delay. Each Eventide DDL 1745 module contained one hundred 1000-bit shift register chips and a bespoke digital-to-analog converter, and cost $3,800 ().

See also 
 Analog delay line
 Delay-line memory
 Interpolation

References

Further reading

External links 
 Introduction to Digital Filters by Julius Smith
 Spectral Audio Signal Processing by Julius Smith
 Physical Audio Signal Processing by Julius Smith
 Discrete-Time Modeling of Acoustic Tubes Using Fractional Delay Filters by Valimaki Vesa

Digital signal processing
Audio effects