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Topic: How many bits should be given to LFE in 5.1? (Read 6906 times) previous topic - next topic
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How many bits should be given to LFE in 5.1?

Hi all,
I am trying to split the overall bitrate among the 6 channels in 5.1 surround setup. I am missing the factor that would tell me how much bitrate should be given to the Low Frequency channel in this setup.

5.1 bitrate = STEREO + STEREO + MONO + LFE

Factor to convert between mono and stereo bitrate (as we agreed here at HA) in AAC is around 1.6 since joint stereo saves 20% to 30% of the bitrate (STEREO+25%)/2=MONO.

So,
STEREO/MONO=1.6

But,
MONO/LFE = ?

Any other ideas related to this issue are also welcome.

Thanks,
Daniel

How many bits should be given to LFE in 5.1?

Reply #1 –
In order to find out the bit-distribution among channels in surround configuration, I encoded some 5.1 audio using FAAC, NERO and COMPAACT.

Pretty surprising that NERO gives as much bits to LFE channel as to plain mono channel. So it seems and I don't see a reson for this. FAAC performs more reasonably, but it varies bitrate for LFE too much even though there shouln't be any psychoacoustics for LFE... or should it? COMPAACT gives the same amount of bits to all 6 channels, makes no difference between mono, stereo or LFE - completely unacceptable.

Here is a graph that illustrates this case for 5.1 castanet.wav encoded at 320 kb/s (front pair is castaners.wav, back pair is also castanets.wav, center is castanets.wav but only left channel and LFE is also castanets.wav left channel).

NERO:


FAAC:


COMPAACT:




I still remain confused about how many bits should be given to LFE channel in this configuration. Since the channel is band-limited to around 150 Hz (only 3 scalefactor bands, around 10 frequency components out of 1024), it shouldn't need more than 10 kb/s!?

Daniel

How many bits should be given to LFE in 5.1?

Reply #2 –
Hypothesis: Nero AAC expects a regular LFE channel (ie already lowpassed) as LFE and not a full frequency channel. It might have wrongly estimated the channel complexity because of this.

Perhaps if you lowpass to 200Hz your "LFE" channel, Nero will produce a different bits allocation.

btw: what did you used to generate those bit usage graphs?

How many bits should be given to LFE in 5.1?

Reply #3 –
Quote
Hypothesis: Nero AAC expects a regular LFE channel (ie already lowpassed) as LFE and not a full frequency channel. It might have wrongly estimated the channel complexity because of this.

Perhaps if you lowpass to 200Hz your "LFE" channel, Nero will produce a different bits allocation.

btw: what did you used to generate those bit usage graphs?
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I'll check that out later (after the weekend :-) )... How do you handle 5.1 bit-distribution in LAME?

(I added few lines of code to FAAD to write bits spent on each channel element to text file, and Matlab to read from file and 'plot' the graph...)

How many bits should be given to LFE in 5.1?

Reply #4 –
Quote
How do you handle 5.1 bit-distribution in LAME?

Are you serious?
We simply do not handle multichannel.

How many bits should be given to LFE in 5.1?

Reply #5 –
Quote
Are you serious?
We simply do not handle multichannel.
[a href="index.php?act=findpost&pid=283318"][{POST_SNAPBACK}][/a]


I thought I heard somewhere about mp3 extension to 5.1...

How many bits should be given to LFE in 5.1?

Reply #6 –
>>I thought I heard somewhere about mp3 extension to 5.1...
As I understand things, it is defined by standart, but not implemented in any encoder.

How many bits should be given to LFE in 5.1?

Reply #7 –
Speaking of 5.1 - we seem to have the first ever surround release of the official music album:
"Beck's Guero is the first-ever global release of a surround sound album in simultaneous release of the standard commercial CD...". See Beck's web page for details.