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4. Stereo-Signals

Stereophony intends to give the listener a life-like impression of a sound event. This requires the transmission of at least two information signals.
With commonly used intensity stereophony replay happens with the help of two separate loudspeakers. The "L-signal" is led to the left loudspeaker and the "R-signal". to the right one. The directional impression depends on the intensity relation between the two signals. In principle there are the following voltage relations in both channels during replay of stereophonic impressions:

Voltage relations during replay of stereophonic impressions
Fig. 8

To get the "middle" impression, the signals L and R have to be identical. If the voltage in the L-channel decreases andincreases in the R-channel, the sound source seems to move more and more to the right side.
If stereophonic productions are broadcast by a transmitter, it must be guarantied that both the stereo-listener and the mono-listener receive a sound impression artistically seen satisfactory. (compatible system).
For example, if the mono-listener receives only the left channel, he lacks in the information from the right channel and vice versa. Consequently, the mono-listener needs both information, L+R. Such a signal is called mono or M-signal.

M = L+R

In fig. 9 we see the M-signal in its correct and wrong polarity:

Correct and wrong polarity of the M-signal
Fig. 9

  1. The wrong polarity of the M-signal during a transmission of the left impression will not be noticed by the listener.
    Also a change of the wires - a - and - b - in the mono-technique cannot be noticed.

  2. During the transmission of a middle impression, the M-signal has the desired amplitude if the polarity is correct.

If there is a wrong polarity in one channel the M-signal becomes 0.

This means complete suppression of information for the mono-listener.

A wrong polarity of the R channel produces a negative R signal.

M = L+(-R) = L-R

This means that the amplitude of the M signal is too small, if not 0. The M-signal is no longer the sum, but the difference of both. But the difference is smaller then the sum (if R is different from 0).

For stereophonic reproduction the M-signal alone is insufficient because the two loudspeakers need the signals L and R. But M is the sum of both.

Besides the M-signal a broadcasting transmitter needs a further information, the so-called S signal (side signal).
From both signals M and S, the signals L and R can be reconverted:

S = L-R

A broadcasting transmitter transmits M and S.
Stereophonic replay occurs at L and R.

During a monophonic replay a wrong polarity in the L or R channel causes a bigger or smaller decrease of volume. It can lead to a complete suppression of a sound source.

With stereophonic replay a wrong polarity (for example in the right channel) leads to a signal -R instead of +R in the loudspeaker. This will not lead to volume (level) differences, but a localization of different sound sources is no longer possible; the result is a sound impression which is completely different compared to the original sound event.

4.1 Stereo-Signals Summary

Direction L R M S



too loud for mono



Amplitude S="0" Localisation in centre spot
Amplitude S=M Localisation at a side
Polarity + Localisation left
Polarity - Localisation right

M-Signal: Left 100%
sound source
with full level)
Right 100%
Middle 200% (not compatible)


PDF Stereo Recording Techniques (570 KB)
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BackNextUp Stereo recording techniques