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2.2 Surround Decoder
Surround channel information is audio that is recorded 180° out of phase (+90° left and -90°
right) on to the left and right channels (in equal amplitude). On movie soundtracks this typically
represents special effect sounds that are meant to be heard behind the listener. Center chan-
nel information (movie dialogue for instance) is audio that is recorded in equal amplitude on to
left and right channels that is in phase (0° phase shift between left and right channels), or
monophonic.
The surround decoder indirectly determines the phase relationship between left and right
channels by detecting the average amplitude of the L (left), R (right), and L-R (left minus right)
audio signals. The relationship between these three quantities is enough to determine whether
in-phase or out-of-phase signal components exist.
A block diagram of the decoder is shown in Figure 9. Mathematically, the operation of the
decoder is essentially defined by the two equations shown in the center and surround multiplier
blocks. The bar over the letters "L", "R", and "L-R" symbolize the average (rectified) level of
these audio signals. How the decoder works is best understood by analyzing the block dia-
gram under certain "pure" signal input conditions, namely: left only; right only; left equals right
(mono) ; and surround (L=-R).
Referring to the center channel equation, it can be seen that for either a left only or right only
signal input, the numerator and denominator quantity will be identical and therefore the ratio
will be equal to one, and when subtracted from one will equal zero. Hence, for this condition,
the center channel signal (L+R) will be multiplied by zero and no audio will be passed on to the
center channel output. If a mono signal is applied to the left and right inputs, the numerator will
be equal to zero, and when subtracted from one will equal one. Hence, the L+R signal is
multiplied by one and allowed to pass.
Referring to the surround channel equation, it can similarly be seen that for a left only or right
only signal input, the equation reduces to zero and thus no output occurs. For a mono (L=R)
input signal, by definition there is no L-R signal, and therefore no output. For the case of L=-R,
the numerator is zero and the denominator is non-zero (2L) so that the equation simplifies to
1-0 = 1. Hence, the L-R signal is multiplied by one and allowed to pass.
The left and right channel outputs are actually the sum of four separate signals. For either a
left only or right only input signal, no surround or center channel information exists (as dis-
cussed above) and therefore L+R sums with L-R, resulting in 2L at the left output, and 2R at
the right output. For a mono signal input (L=R), the center channel L+R output signal subtracts
with the left channel L+R input signal, and no left output occurs. Similar action occurs for the
right channel output. For an L=-R input signal, the surround channel L-R output signal adds to
the right channel R-L signal and subtracts from the left channel L-R signal, resulting again in
no left or right channel output.
To summarize the performance of the decoder:
• Left only input results in left only output
• Right only input results in right only output
• Mono (L=R) input results in center only output
• L=-R (right inverted with left) results in surround only output
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