Bose AM-30P Instrukcja serwisowa - Strona 7

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THEORY OF OPERATION

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