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Winamp Fraunhofer MPEG-4 AAC Encoder?
HA2013
post Dec 23 2012, 00:28
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Is FhG AAC encoder bundled with Winamp 5.63 a decent one, or there are better free alternative?.This encoder can achieve a maximum bitrate of 448 kbps on CBR mode and LC profile, but I wonder if it's more efficient than other encoders, as I'm considering switching fro MP3 insane preset encodec with LAME 3.99.5 and re-ripping lossless source into AAC-LC (.m4a) files.Thanks in advance.

This post has been edited by HA2013: Dec 23 2012, 00:31
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Hex144
post Feb 13 2013, 22:19
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I noticed a weird behaviour with the FhG AAC encoder (bundled with Winamp). If I apply a lowpass filter to the audio source before encoding with VBR 5 or 6 (of 6), the bitrate goes up (contrary to what's expected).
LPF was applied using:
CODE
qaac --decode -b 24 --lowpass xxxxx input.wav output.wav

The more aggressive the lowpass, the greater the bitrate swell, i.e. bitrate increases with decreasing lowpass cutoff frequency.
Maximum bitrate is achieved at 15-16 kHz LPF, with a gain of up to 5% (depending on the music). Going below 15, the file size starts to fall.

Lowpass & VBR 4 doesn't lead to (large) increases (nor decreases) with the music I've tested so far.

Can someone with in-depth technical knowledge shed some light on this seemingly anomalous behaviour?
Are those extra bits put to good use, at least? Or maybe they're just wasted in inefficient programming?
I'm trying to make a decision whether I will continue using the lowpass.
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C.R.Helmrich
post Feb 14 2013, 14:50
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In order to be fast, the Fraunhofer AAC encoder has a simplified psychoacoustic model. It tries to achieve the target bit-rate regardless of whether the input is low-passed, but due to some small inaccuracies/assumptions, it might be off by a few %, as you noticed. It does, however, realize that (filtered-out) content above the lowpass is masked, so yes, the saved bits are put to good use below the low-pass frequency, and are not wasted.

I also see from your qaac command-line that your low-pass input to the AAC encoder is 24-bit. This might also increase the AAC bit-rate somewhat over inputting 16-bit.

Lastly, why are you using a low-pass at all? The encoder applies its own low-pass (VBR 4 e.g. has one at 16.5 kHz), and anyway, a very small percentage of the bit-rate is spent on (nearly) inaudible frequency regions.

Chris


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