7 min read
5.1 surround sound explained: channels, order and downmixing
Six channels, two rival orderings and one channel everybody misunderstands. A working guide to 5.1 for delivery.
5.1 means five full range channels plus one low frequency effects channel. It has been the baseline cinema and broadcast format for three decades, and it still causes delivery failures, almost always because of channel order.
The six channels
- L: front left.
- R: front right.
- C: centre, which carries nearly all the dialogue.
- LFE: low frequency effects, band limited to roughly 120 Hz.
- Ls: left surround.
- Rs: right surround.
Two orders, endless confusion
The SMPTE order is L, R, C, LFE, Ls, Rs and is what film delivery expects. The ITU or film consumer order is L, R, Ls, Rs, C, LFE. A file authored in one and interpreted as the other puts dialogue in the surrounds and effects in the centre. It is instantly audible and instantly rejected by QC.
Always confirm the order written in the file's channel mask or metadata rather than trusting a filename. The inspector on this site reads the channel layout directly from the header.
The LFE is not the subwoofer
The LFE is an effects channel with its own headroom, traditionally 10 dB of in band gain, not a bass management feed. The playback system decides what goes to the subwoofer. Putting general bass into the LFE means it disappears on systems that fold it differently.
Downmixing to stereo without wrecking dialogue
A LoRo downmix sums centre into both sides and folds the surrounds in, each at a reduced level. The standard coefficients apply about minus 3 dB to the centre and surrounds and usually discard the LFE entirely, because summing it into stereo produces boom without benefit.
If the resulting stereo sounds muddy or the dialogue sits too far back, adjust the centre gain rather than compressing the result. The surround downmixer here exposes a gain control per channel so you can hear each decision before exporting.