Some 5.1 notes
In which I transcribe some notes I took at a trade fair about the BBC works with surround sound in their TV productions.
At the Broadcast Video Expo today I heard some useful tidbits from Chris Graver, a dubbing mixer with the BBC. He presented a seminar on 5.1 sound.
– Although you might see multiple speakers along the side walls of some cinemas, the sound usually is still 5.1. The same signal is being sent to most of the speakers.
– The six channels of sound are known as ‘5.1’ because the sixth channel (for low frequency effects) has a tenth of the bandwidth of the first five. L, C, R, Ls and Rs have a frequency range of 20-20,000Hz, LFE has a range of 0-200Hz. This channel isn’t usually included in the stereo downmix
– 5.1 ambience shouldn’t be noticeable enough so that people keep looking over their shoulders – you want them looking at the screen.
– A transportation encoding. Not for consumer use. As some delivery media only have four channels of audio, the first two are for the stereo downmix, 3 and 4 hold an encoded Dolby E soundtrack. This soundtrack delivers the 5.1 channels to the distribution systems of the broadcaster/publisher.
– Dolby E has a 1 frame encoding delay. This means you must advance your Dolby E soundtrack by 1 frame to match picture and the stereo downmix
– Dolby E has a 1 frame decoding delay too, but most decks can be set to account for this delay. A minority of broadcasters requiring Dolby E encoded 5.1 expect it be advanced an extra frame (for a total of two) to sync with picture and stereo
– The BBC and others will fail your programme in tech review if you do not add the metadata tag stating the correct Average Dialogue Level. In worst case scenarios, consumer kit might filter some of the dialogue bandwidth out if it doesn’t know the extents of its range.
– Make sure you use the correct track order before encoding as Dolby E: L, R, C, LFE, Ls, Rs – otherwise you might get your Back Right channel filtered as if it was the LFE channel!