What AAC 5.1 bitrate is reasonable

[justinhow]

For some time I have been encoding 5.1 surround sound in movies as AAC (FDK) at 640kbps.
I am trying to get good quality results without being too wasteful. Is this bitrate a reasonable bitrate or overkill?
I am conscious that as we move towards HEVC this bitrate becomes a notable proportion of the overall file.

Any help would be gratefully received.

[JackNF]

Well I look at it this way: 5.1 surround has 6 channels. That would be comparable to 3 pairs of 2 channel audio.

What bitrate would you be happy with using for simple 2-channel stereo? Triple it and use that for 5.1 surround, or the closest you can get.

Happy with 128kpbs stereo? 384kbps surround should do. 160kbps stereo? 448kbps surround is close enough to 480.

Conversely, 640kbps surround would be comparable to ~213kbps stereo.

That's probably over-simplifying it quite a bit, I'm not sure how joint-stereo might apply to more then 2 channels (if it does at all) and in a lot of movies the surround channels have much less going on in them so they don't need nearly as many bits as the main channels, but I've generally been quite happy with the results I get when using this particular quick n' dirty way of thinking.

[moneymatt4life]

FDK@5.1-384kbps has a lowpass of ~15.5kHz
FDK@5.1-448kbps has a lowpass of ~16kHz
FDK@5.1-640kbps has a lowpass of ~17kHz
FDK@5.1-768kbps has a lowpass of ~17kHz, so that means that bitrates higher than 640 will have a lowpass of ~17kHz unless you specify a lowpass(which isn't currently possible in handbrake)

i remember reading somewhere that with a good AAC encoder(which FDK is, despite its technical quirks), 64kpbs/channel was a good starting guideline and then going up from there if the quality isn't suitable to you...

[justinhow]

Thanks and interesting and exactly the sort of things I was trying to get my head around.

Presumably there is no "joint" in 5.1 and they are encoded as 6 equal and distinct channels - but this is just my guess and may well be wrong. So how does it carve up the bitrate per channel - does the .1 get as much as the 5. ?

Anyone?

[moneymatt4life]

the AAC wiki says "The MPEG-2 audio tests showed that AAC meets the requirements referred to as "transparent" for the ITU at 128 kbit/s for stereo, and 320 kbit/s for 5.1 audio." pretty sure the same should apply to FDK, you'll have to test that out for yourself...

well, i'm not so sure about your question though... 5.1ch is regarded as 6ch audio (5 full audio channels with a LFEch), which you know, so it should receive as much bitrate as the other channels, but there really isn't much audible sound in the LFE channel since it's on the lower spectrum of human hearing... so it could be ignored in bitrate per channel configurations if you really wanted, i dunno... *EDIT: "audible" probably isn't the word to best describe it, i dunno*

normally i would say use VBR and let the encoder decide the bitrate but since handbrake can't do VBR FDK yet, we're both stuck with CBR...

[justinhow]

Thanks for all the info
So it would seem I could probably reduce my rate to 448 without any noticeable reduction in quality.
I guess we are stuck with each channel getting equal bitrate (1/6th) even though in an ideal world you might have the front 3 having a higher bitrate than the surrounds and the LFE lower than all.

[Deleted User 13735]

Many DVD movies come in at 384 Kbps AC3 5.1
Efficiency debates notwithstanding, your should keep your AAC conversions the same 384, since the goal it to retain all the high frequencies from the source. 640 Kbps seems a bit of overkill considering the source.

[justinhow]

I am only talking about High Def stuff really where the sources can be up to 1.5 Mbps DTS etc.