Dolby Surround

This example is to demonstrate Dolby Surround capability for VCD and SVCD formats. We will rip a clip with a Dolby Digital 5.1 (AC3) sound track and make a VCD and an SVCD with the appropriate Dolby Surround. This example is done from a THX trailer.

1. Dolby Surround
2. Dolby Pro Logic
3. Dolby Digital (AC3)
4. DVD AC3 track ripping with DVD2AVI
5. Making the VCD and the SVCD
6. Testing with a Dolby Pro Logic TV set
7. Example materials

1. Dolby Surround

Dolby Surround is a matrix process that enables any stereo (two-channel) medium, analog or digital, to carry four-channel audio. The encoded audio stream is fully compatible with mono and stereo playback. The four channels are called L, R, C, S (Left, Right, Center, Surround) positioned like in figure 1.

Figure 2 is the block diagram of a Dolby Surround encoder. The encoder adds the center channel input C, attenuated by 3dB, to the L and R input signals. The results are fed to the left and right total outputs, Lt and Rt.

The surround input S is attenuated by 3dB, bandpass filtered from 100Hz to 7kHz, and passed through a Dolby B-type encoder modified to produced 5dB of noise reduction rather than the normal 10dB. The result is 90° phase shifted and added to feed the Rt signal and subtracted to feed Lt signal.

Dolby Surround decoder extracts the four channels from the two signals Lt and Rt. The passive decoder makes the reverse function of encoding by extracting Center and Surround signals with subtracting and adding circuits (figure 3).

2. Dolby Pro Logic

Dolby Pro Logic or Dolby Surround Pro Logic enhances Dolby Surround passive decoder by increasing the separation of Center and Surround channels from Left and Right channels.

The Dolby Prologic decoder analyzes Lt and Rt signals thanks to a directional enhancement adaptive matrix that:

• finds the channel of the dominant sound.
• produces a control signal to reinforce this channel.

This control signal is used by the VCA (Voltage Controlled Amplifier) related to the channel.

To find the dominant channel, the circuit computes the ratio between the average values of every signal L, R, C, S. The decoder can produce a channel separation up to 30dB (this value is very significant), but, dominant channel search depends on the audio content and if no dominant channel is defined the matrix effect must be disabled.

The noise sequencer (figure 4), which generates the calibration used during the speaker system setup, is added to the output of the input level controls and is fed to the input of the matrix steering decoder. The matrix steering decoder routes the Center channel signal and controls the level of each of the four matrix outputs to increase separation between channels whenever possible. The Left, Center and Right signals are then sent to the master control volume control. The Surround channel is first sent to an anti aliasing filter, an adjustable length digital delay, a 7kHz low pass filter and a Dolby modified B-type noise reduction decoder. The output of this signal chain is then sent to the master volume control.

3. Dolby Digital (AC3)

Dolby Digital AC3 (audio compression layer 3) is the digital format succeeding Dolby Surround. Every channel is digitized separately then the data is compressed and multiplexed.