Multi-Channel Formats

Here is a rundown of some common multi-channel formats. There are many more, and you must consult any records that may be attached to the source material and read the manu-als for your encoding software to know how to map these to the output encoded media.

Table 7-2 Audio Track Arrangements

Tracks Speakers Description

Mono 1 Old legacy program material will only have a single audio track. You must duplicate this across both tracks of a stereo pair for most applications. It is very likely that home movie audio from magnetic striped film will only have mono audio.

Dual mono 2 This is the duplicated mono on two stereo channels.

Two-sided 2 The radio news-gathering format uses 2-track tapes mono but records one track in each direction, requiring

that the tape be turned over to record the other track.

Stereo 2 True stereo recorded through a pair of microphones or mixed from multi-channel master tapes. Be careful to preserve the relative phasing between channels or sound will appear to move around.

Pseudo stereo 2 Sometimes old mono recordings will be reprocessed with filters to separate out various instruments that are then panned from one side to the other to create an artificial stereo effect.

Mix minus 2 Sometimes, the stereo signals are not a stereo pair in the true sense. One could be a backing track while the other is the same but with perhaps the vocals missing. In news gathering, one channel is probably ambient background noise while the other is the presenter. The two channels must be mixed carefully to turn them into a presentable stereo sound track.

Coding Technologies: http://www.codingtechnologies.com/

Continued

Table 7-2 Audio Track Arrangements (Continued)

Tracks Speakers Description

Dolby 3 front and An analog-matrixed signal is decoded to create the Surround 1 rear extra surround channels. Too much compression

will destroy the matrix information. Separate them out and then encode each independently.

Quadraphonic 2 front and An experimental analog surround format used for 2 rear music in the 1970s. Not much content survives in

this format.

5.0 surround 3 front and A modern surround format used for music

2 rear applications and lacking the low frequency channel.

Dolby Digital 3 front, Surround systems constructed according to the (AC-3) 2 rear, and 1 Dolby model are configured in a variety of ways

low-frequency with anything from 1 to 6 channels being used.

channel There are also complex matrix-encoding signals that must be preserved so the older Dolby Surround systems will work. This is critical, and you must have some knowledge of how this system works to be able to encode it competently. Most important, the sample rate must be high enough to preserve the surround information. There are many versions and variants of this system, and it is only Dolby Digital 5.1 if it says so on the box.

DTS Digital 3 front, An alternative to the Dolby system is the Digital Surround 5.1 2 rear, and 1 Theatre Sound system. This is often presented on

low-frequency major DVDs containing Hollywood blockbuster channel movies as an alternative to the Dolby sound track. It is

not unusual for DVD video files to have 10 or more audio tracks associated with them. Every one has to be carefully encoded and precisely synchronized.

THX 3 front, This extends the DTS system from 5.1 channels Surround ES™ 3 rear, and 1 to 6.1. All the earlier caveats apply. You must be

low-frequency confident about what you are doing when encoding this sort of channel complexity. Also known as DTS-ES 6.1.

THX Surround 3 front, 3 rear, and This is also known as Dolby Digital EX and EX™ 1 low-frequency supports 6.1 audio channels.

channel

Sony SDDS 5 front, 2 rear, and This is used in large cinema auditoriums and has 1 low-frequency 2 additional speakers on either side of the screen.

channel

Continued

Table 7-2 Audio Track Arrangements (Continued)

Tracks Speakers Description

THX 3 front floor, This is a system proposed by LucasFilm/THX that Periphonic 3 front ceiling, embodies dual 5.1 surround systems, one at floor

2 rear floor, 2 rear level and the other at ceiling level, in order to ceiling; a low- convey height and surround information to the ear.

frequency driver It became known as Periphonic.

at floor level and another at the ceiling level.

A lot of useful information about surround-sound systems can be found at the Time for DVD Web site.

There are some enhancements to the Mac OS at version 10.4 (known as the Tiger release) that will be useful when managing complex audio projects. The system has an audio-processing environment built in called CoreAudio. This manages the audio unit plug-ins on a systemwide basis; the audio processing is available to any application that cares to take advantage of it.

With the 10.4 release, the CoreAudio support provides much help with managing multi-channel surround-sound mapping, and a lot of the headaches to do with keeping tracks organized are now taken over by the OS and done automatically. This makes the Mac OS platform a good choice for some production work at the high end.

Be sure to properly license any DTS Digital Surround decoding software you use from DTS. The open-source VLC player has had to remove DTS decod-ing support due to a threat of legal action over patent violations.

Specialist audio processing tools such as Sound-Soap, E-Magic Logic, and ProTools are worth purchasing and keeping available nearby to run a quick audio-processing job when necessary.

Time for DVD: http://timefordvd.com/tutorial/SurroundSound.shtml DTS: http://www.dtsonline.com/

7.8.1 Caveats Regarding Surround Sound

Over-compression of the audio with normal compression tools will lose any embedded surround-sound information. The consequence is that sound will come randomly from any speaker and may jump around as the compression factor changes. You may also hear strange bumps and whining noises from speakers driven by the surround circuits. This is due to the severe corruption of the surround-sound signal. In some formats, this is an ana-log-coded frequency or phase modulation that is outside of the normal range of human hearing and therefore is outside the range that some audio codecs are optimized for.

7.8.2 HE-AAC Surround

The Fraunhofer Institute has developed some very innovative approaches to compression, especially in the area of MPEG-4 and the related coding techniques for audio and video.

At the IBC 2004 conference, they showed how the coding of surround-sound audio could be accomplished at 48 Kbps. This has profound implications.

The quality delivered is superior to that available from a matrixed analog audio sig-nal. This announcement is innovative because it broadcasts a compatible signal that can be played back on both stereo and mono systems.

It is based on coding a down-mix as a mono audio signal and then adding difference information that can be cascadedonto that signal when it is decoded. You simply stop decod-ing when you have extracted the number of channels you need. It’s a very clever solution.

Fraunhofer has also developed an MP3-based surround audio solution but while this is interesting, the AAC-based techniques will likely deliver a longer-term solution.