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Moving From Stereo to Surround Sound Audio

on Jan 8, 2013

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Audio workflow diagram.
Audio workflow diagram.
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Surround sound audio is getting more elaborate as it moves beyond 5.1 six channel mixes. 7.1 mixes add left and right rear channels, and mixes greater than this, such as 9.1 or 11.1, add height information into the audio surround sound mix.

The addition of height information into the mix is commonly referred to as 3D audio. The focus of this discussion will be moving to a 5.1 surround sound mix from a current stereo audio system.

The move from a workflow that handles stereo audio to one that handles surround sound presents challenges. There are of course more audio channels to contend with, and a need to make sure that these additional channels can pass through a system.

However, there are additional items such as surround sound monitoring, using audio bit rate reduction to fit the additional channels, the use of audio metadata when using bit rate reduction technologies, maintaining audio quality and the video to audio timing relationship (lip sync) and the additional advanced audio processing that may be required.

Definitions
Upmixing is the process of converting stereo to a surround sound mix. This can be compared to the up conversion of SD video (Standard Definition) to HD (High Definition). In fact, the transition from SD to HD video typically drives the transition from stereo to surround sound audio mixes.

Downmixing converts surround sound to a stereo mix. This can be compared to the downconversion of High Definition (HD) video to Standard Definition (SD) . Audio downmixing and video downconversion are important to support current and legacy SD and stereo audio systems as HD and surround sound roll out.

As well, the up and downmix process are also referred to as matrix decoding and matrix encoding. Matrix techniques use phase and intensity differences to carry surround signaling in a stereo signal.

For example, Dolby ProLogic II and DTS Neural Surround use this technique (and are compatible with each other). Stereo audio that carries this “matrixed” audio information is known as LtRt (Left Total, Right Total). When a stereo audio signal does not contain matrix-ed information, it is known as LoRo (Left Only, Right Only). Stereo audio is also referred to as 2.0.

This audio matrixed technique should not be confused with audio processing techniques such as bit rate reduction (or compression) using Dolby E, Dolby Digital, Dolby Digital PlusTM or DTS technologies.

The bit rate-reduced audio that is carried in the space of a stereo audio channel cannot be monitored unless it is uncompressed (or decoded), whereas, LtRt can be monitored as a stereo audio signal. And, audio “compression and limiting” processing used to affect the dynamic range of the audio signal should not be confused with bit rate reduction compression.

Surround sound audio is referred to as L/R/C/LFE/Ls/Rs (Left, Right, Centre, Low Frequency Effects, Left Surround, Right Surround) and 5.1.

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