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Topic: Volume of AAC conversion harder to control (Read 5790 times) previous topic - next topic
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Volume of AAC conversion harder to control

Hi guys,

for some target devices I create m4a files with qaac converter. Source files have replaygain tags and are scanned with intersample peaks in mind (true peak scanning).

In Foobars converter, I use "Apply Gain and prevent clipping".

Output files, if they're flac (just for a test), but also LAME mp3, will pretty perfectly be at 0.0db, Replaygain Album Peak at 1.00. But the m4a files cannot be as easily controlled. IDK, I picked the loudest track of an album, and the result is:

Code: [Select]
Source FLAC peak 1.49 (clipping, ~+3.5db)
Target FLAC: Peak 1.00
Target mp3: Peak 1.01
Target m4a: Peak 1.26 (~+2db)
Command line args qaac: --ignorelength -s --no-optimize -V 91 -o %d -

Any reason for this?

Re: Volume of AAC conversion harder to control

Reply #1 –
A first observation is, that it is related to the quality setting. cvbr 256 upwards gives a perfect 1.01 peak, while in tvbr (which I use mostly at -V 91) and lower average bitrates volume is pretty unstable, with peaks easily going towards 1.5.
In comparison to lame mp3, where peaks are stable at 1.0, I tested -v0 and -v2.
My test track was Autechre - Dial (from Gantz Graf EP), it should be a CD rip.

Re: Volume of AAC conversion harder to control

Reply #2 –
It's not volume that changes. Peak is not related to volume at all.
It's just a coincidence if you don't see peak changing with MP3. All processing that affects different frequencies different amount changes the waveform and can alter the peaks.
You can test in an audio editor for example by generating noise or square waves (these have all frequency components present) and use EQ to reduce loudness of a random frequency band. You'll notice that the peaks increase.