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Digital Audio/Video => Movie/Multichannel audio => Topic started by: sauvage78 on 2018-01-11 05:40:04

Title: What is the correct bitrate distribution per channel for 5.1 ?
Post by: sauvage78 on 2018-01-11 05:40:04
Hello HA, can someone tell me what is the exact correct bitrate distribution per channel for 5.1 ?

Is it 5 full channels + 1/3 bitrate channel like in 384Kbps for 5.1 = 72Kbps x5 + (72/3=24Kbps), which is the equivalent to 144Kbps for 2.0 (72 x 2 channels) ?

or 5 full channels + a 0 bitrate channel like in 640Kbps for 5.1 = 128Kbps x5, which is the equivalent to 256Kbps for 2.0 (128 x 2 channels) ? (the subwoofer sound would then be some sort of duplication from the data of the other channels)

 I find contradicting answers when I google it ...

Thks ;)
Title: Re: What is the correct bitrate distribution per channel for 5.1 ?
Post by: polemon on 2018-01-11 10:53:14
The LFE channel is an effects channel, and is often calculated from the signals from the other available channels.

It depends on the actual format though, Dolby Pro Logic (with it's later implementations, like IIx and IIz) is able to kinda shift the channels such that the surround channels are encoded with a more efficient encoding. The older (first version) of Pro Logic, used a single rear channel which had a significantly lower fidelity than the stereo channels, and the center channel.

Dolby Atmos uses sound objects, where the overall efficiency is unrelated to the channel per se, but the sound object being played.

Dolby Digital and Dolby Digital EX use similar ways to bandpass channels and encode them differently. Formats like AC-3 (ATSC A/52) allow various encodings of each channel individually. In AC-3 the LFE channel is encoded as an individual channel.

Similar things are true for AAC, although as I understand the constraints are different.

I believe it is a case-by-case basis. What format etc. are we looking at?

Oh, and there's also SDDS, which puts these constraints in place differently still.
Title: Re: What is the correct bitrate distribution per channel for 5.1 ?
Post by: sauvage78 on 2018-01-11 12:34:41
Well I am interested to know for all 5.1 formats found in 2K & 4K blurays (so all Dolby & DTS formats, both lossy & lossless as source) as well as Opus & Apple AAC (as target) ... I was hoping that there would be a general answer ;) It would ease my life !

I am trying to make sense of all the 5.1 bitrates communly found like 448Kbps, 504Kbps, 640Kbps, 768Kbps,1536Kbps in order to build myself a quick reference table of equivalence between 2.0 & 5.1 so that I know when to keep the master (if around 64 to 96Kbps per channel) & when to re-encode (if twice that bitrate or more per channel).

Pio2001 (a very old user here with good knowledge) once said on hardware.fr forums that you could just do 2.0 bitrate x2.5 but I found several other guys talking about a channel at 1/3 the bitrate so I am confused now.

When you go backward & divide some of these communly found bitrates by /2.5 you often end up with a non-integer number which ... is suspicious.
Title: Re: What is the correct bitrate distribution per channel for 5.1 ?
Post by: polemon on 2018-01-11 12:54:11
Well, the channel order alone is varying between formats.

The Dolby Surround formats like AC3 have different constraints, but technically each channel, or channel pair can be encoded at a somewhat wide range of bitrates. It kinda depends on a per-case basis. With Dolby Atmos, where you have up to 24.1.10 channels in a home setup and sound objects, this entire thing kinda works differently in general. The smallest Atmos setup is 7.1.2, and any audio files mixed for more, are mixed down to the setup installed. This also works in the opposite way, where something mixed for 7.1.2 will be extrapolated to a larger number of channels, or from data which only contains surround, but non-atmos data, like regular 7.1 audio tracks. How and if containers like MKV is able to contain Atmos, I have no idea.

When you have audio mixed for 2.0 and you want to extrapolate that to 5.1, then this can be done (and is done all the time) with a decent sound processor, this is more or less what Dolby Pro Logic was intended for. However this is more a function of keeping media backwards compatible, but also allow for an upgraded experience for users which have the ability to playback the signals.

FFmpeg should be able to at least extract each channel, and display its bitrate individually, unless there is some inter-channel coding going on, like joint-stereo. SDDS supports something like parametric 5.1. where a single channel has a displacement information, and the "position" of a virtual sound source can be adjusted between the output channels.
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