(lib)faac imposes limits on bitrate?

[lightdee]

I'm trying to encode some audio on Ubuntu 12.04 Linux. I need to use libfaac (using version 1.28) to encode and I want to encode at a very low bitrate - the total including transport stream overhead must be below 64kbps.
The input audio is: PCM, 48kHz, 2 channel, s16, 1536 kb/s

Using the faac command-line tool, I get the output below, encoding 5 minutes of audio - so there's plenty of time for the bitrate to average out. I cannot get it to go below 64kbps, or above 165kbps for that matter. I have tried setting some of the other parameters, but to no avail. When I set the bitrate > 64, it works fine.

$ ./faac -b 40 -w -o /tmp/yellow-audio-40k.mp4 /tmp/yellow-audio.wavFreeware Advanced Audio Coder
FAAC 1.28

Average bitrate: 64 kbps
Quantization quality: 100
Bandwidth: 5442 Hz
Object type: Low Complexity(MPEG-4) + M/S
Container format: MPEG-4 File Format (MP4)
Encoding /tmp/yellow-audio.wav to /tmp/yellow-audio-40k.mp4
frame | bitrate | elapsed/estim | play/CPU | ETA
14063/14063 (100%)| 64.2 | 10.7/10.7 | 27.91x | 0.0

$ mediainfo /tmp/yellow-audio-40k.mp4
General
Complete name : /tmp/yellow-audio-40k.mp4
Format : MPEG-4
Format profile : Base Media / Version 2
Codec ID : mp42
File size : 2.35 MiB
Duration : 5mn 0s
Overall bit rate mode : Variable
Overall bit rate : 65.7 Kbps
Encoded date : UTC 2012-12-05 00:03:38
Tagged date : UTC 2012-12-05 00:03:49
Writing application : FAAC 1.28

Audio
ID : 1
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 5mn 0s
Bit rate mode : Variable
Bit rate : 64.2 Kbps
Maximum bit rate : 73.7 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 2.30 MiB (98%)
Language : English
Encoded date : UTC 2012-12-05 00:03:38
Tagged date : UTC 2012-12-05 00:03:49

[El Heggunte]

Hmmm, why do you "have to" use faac only?
Can't you use NeroAacEnc under WINE for example?

Keep in mind that lower bitrates sound better/acceptable only with the High-Efficiency mode
(read: Spectral Band Replication)

[poisondeathray]

I tried it with ffmpeg libvo-aacenc instead of libfaac (on windows) and it was able to reach the targets of 40kb/s, 48kb/s AAC . Some AAC encoders have unavailable bitrate options, and only specific numbers work (e.g. you might not be able to use 45kb/s)

Another option might be to pipe ffmpeg into another aac encoder like neroaac


see this thread:
http://forum.videohelp.com/threads/322328-libfaac-encoding-with-ffmpeg

bat99 is on linux , so either option should work for you on linux

[poisondeathray]

I agree with El Heggunte,

And even to my non-audiophile ears , there is a world of difference between something like neroaac HE-AAC and libfaac in terms of quality, especially at low bitrates

[lightdee]

i'm working on an application which already is using libfaac as it's chosen aac encoding library. It would be a fair bit of work to switch libraries. I do wish libfaac supported HE-AAC... maybe I could use the built-in ffmpeg aac encoder

[lightdee]

Finally figured it out. In order to encode at bitrates < 64kbps, you must also set the bandwidth:
-c <freq> Set the bandwidth in Hz. (default=automatic)
the bitrate and bandwidth parameters work in tandem to generate an output bitrate. It's automatically set to
about 1/2 of the sample rate, but you must set it much lower (in my case <5000) to allow libfaac to
encode at 40kbps or similar.

[bat999]

Finally figured it out... you must also set the bandwidth
...you must set it much lower (in my case <5000) to allow libfaac to
encode at 40kbps or similar.

Yes, that's done the job.
I suppose this low bandwidth is OK for speech.

@ubuntu:~$ faac -b 40 -c 4000 -o foo.m4a foo.wav
Freeware Advanced Audio Coder
FAAC 1.28

Average bitrate: 40 kbps
Quantization quality: 100
Bandwidth: 4000 Hz
Object type: Low Complexity(MPEG-4) + M/S
Container format: MPEG-4 File Format (MP4)
Encoding foo.wav to foo.m4a
   frame          | bitrate | elapsed/estim | play/CPU | ETA
 1297/1297  (100%)|   42.1  |    2.1/2.1    |   14.68x | 0.0