Delays and Echo

Introduction

We are all familiar with the concept of an echo. The classic case is a valley or a cavern where the listener makes a sound and then a fraction of a second later hears a reflection of the sound. A large auditorium and a telephone line can also provide echoes.

Echo

The principle of echo is very simple. Hard surfaces such as rocks, walls, wood and glass reflect sound. Because the speed of sound is quite slow (approximately 1125 feet per second), the listener hears the original sound plus not quite as loud a reflection of the sound. The sound may bounce around many times and go on for seconds.

The process of an echo is shown in the figure below:

The output is the sum of the input plus a delayed version of the input.

Note that it is the same configuration used for a Finite Impulse Response filter (FIR), but using longer delay times.

C Code Implementation

To implement an echo in C is quite simple. For the delay a large buffer is used. New inputs are put in one end and the oldest removed from the other. The output is generated using a simple sum of input + delayed input.

Delayed Playback

A typical example of where delayed playback is used is a live radio phone-in. It is possible that from time to time callers will say something that would be wiser not to broadcast! In these cases, there is a warning of offensive material before transmission and gives the broadcasters time to substitute a jingle or music.

Musical Applications of Echo

A long delay is sometimes used by street musicians. They play some bass notes on their instrument then play some treble notes. Because of the delay, the treble will occur at the same time as the delayed bass. This gives the effect of more than one instrument being played.

A good music source is a musical “round” such as Frere Jacques or Row Row Row Your Boat.

Additional Equipment Required

Multimedia speakers

CD player with connecting cable

Tutorial 4: Echo and Reverb 7 January, 2002

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