Crestron
Surround Sound
Primer
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The surround signal is split into two identical signals; one
signal is phase shifted by +90 degrees relative to the fronts and
the other signal by -90 degrees, creating a 180-degree phase
difference between the surround signal components.
To recapture the matrix signals:
1. The center channel is extracted by summing the left-total and
right-total signals.
2. The surround signal is extracted by taking the difference
between them.
3. The identical center channel components in the left-total and
right-total signals cancel out one another in the surround
output.
4. The equal and opposite surround channel components cancel
out one another in the center output.
5. Signals that are different in left-total and right-total are applied
to the front left and right speakers.
6. Signals that are identical and in-phase are applied to the center
channel.
7. Signals that are identical but out of phase are applied to the
rear surround speaker.
Dolby Pro Logic
Dolby Pro Logic is an enhanced type of Dolby Surround decoder
introduced in 1987. Pro Logic uses directional enhancement circuitry to
actively steer sound toward the dominant position. Dolby Surround Pro
Logic decoding yields: left, center, right, and a monaural surround
channel.
Dolby Pro Logic Decoder Block Diagram
It determines the dominant signal of the four outputs by comparing the
signal level of the left-total and right-total signals while simultaneously
comparing the level of the sum and difference signals. An active
matrixing circuit uses this information to output the dominant signal to
Primer – DOC. 6122
Surround Sound
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