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Topic: Apple AAC CVBR conversion - bitrates above 320 kbps (Read 3211 times) previous topic - next topic
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Apple AAC CVBR conversion - bitrates above 320 kbps

I have an observation regarding foobar's Converter. With Apple AAC encoder (qaac), in Constrained VBR mode, there are currently options in foobar for bitrates up to 512 kbps. However, qaac can only encode CVBR up to 320kbps. So I believe the 384, 448 and 512 kbps options are redundant, as they cannot be achieved with qaac. I would suggest that they should be removed.

To verify this, I have tried encoding a FLAC file in both CVBR 320 and CVBR 512, and those two files turned out to be the same (same average bitrate, same size).
 

Apple AAC CVBR conversion - bitrates above 320 kbps

Reply #1 –
Quote from: tahaa7 on
I have an observation regarding foobar's Converter. With Apple AAC encoder (qaac), in Constrained VBR mode, there are currently options in foobar for bitrates up to 512 kbps. However, qaac can only encode CVBR up to 320kbps.

Available bitrate is dependent on number of channels and sample rate.
For 48000Hz + 8ch, highest bitrate becomes 1280kbps.
See qaac --formats.

As an alternative way to specify bitrate indepentent of number of channels and sample rate, you can set bits per sample as bitrate value.
When bitrate given is less than 8.0, qaac takes it as bits per sample. Acutal bitrate is computed as :
Code: [Select]
bitrate = bits per sample * number of channels * sample rate

For example, when you say "--cvbr 1.5", it becomes 1.5 * 2 * 44100 = 132300 bps for 44.1kHz stereo.
(To be precise, LFE channel is not counted here. So, 5.1ch is treated as 5ch).

Apple AAC CVBR conversion - bitrates above 320 kbps

Reply #2 –
OK, but when I try to convert a 16-bit/44.1 kHz stereo file to CVBR AAC, I get the option of 512 kbps anyway, whereas with such a file foobar should only offer up to 320 kbps, since this would be the equivalent of using "qaac -v 0" command.

Apple AAC CVBR conversion - bitrates above 320 kbps

Reply #3 –
encoder settings are used for all files, not just stereo.

Apple AAC CVBR conversion - bitrates above 320 kbps

Reply #4 –
Quote from: lvqcl on
encoder settings are used for all files, not just stereo.


OK, but for multichannel files, bitrate can go higher than 512 kbps anyway.

Apple AAC CVBR conversion - bitrates above 320 kbps

Reply #5 –
Foobar converter allows CVBR up to 512kbs, which can be used for multichannel audio. It is not channel-specific, its generic. It does not change settings dialog according to specified or detected output channel count. If you set CVBR to 512kbs but output is stereo file will be encoded with 320kbs. Is is a limitation of QAAC or more likely Apple AAC encoder itself. For multichannel output bitrate will go up to 512kbs.