#14 possible channel# issue?

[Stefan Erkens]

Hey there. First of all: HUGE thanks for this work!!! I think at this time this is the only useful tool to deal with aax files.

My problem is the following:
If I convert a aax file, which has 64kbits (simply calculated by Filesize/Runtime), to m4a (also 64kbits), it sounds not as good as the original file played in AudibleManager or anywhere else. Granted that it is the same codec (AAC/MPEG4), this should not be the case, right?
I suspect that the original AAX file is only mono but the converter outputs a stereo file.
I am not 100% sure, because I can't get any info on the number of channels from the original file.
Could this be the case?

[Stefan Erkens]

I did some more investigation and found out the following things:
The AAX file is actually stereo, there is a little intro music, wich has different data for both channels. But apart from that, the entire Audiobook is dual mono (100% flat line on the XY-Diagram).

Still, that does not really make sense.
If I convert the file to stereo 64kbits, it sounds crappy.
If I convert it to stereo 128kbits, it sounds good.
If I convert the stereo 128kbits file to 64kbits mono, it still sounds good.

There are two possible reasons:
1st: In the original file only the first chapter (with the intro music) is encoded in real stereo, the rest is dual mono. The converter only checks only the first chapter and sets stereo for the whole audiobook, resulting in either bad quality or doubled bitrate or
2nd: The original encoder is "smarter" when encoding a stereo file with (almost) identical streams for both channels than the one you are using.

I'm sorry to bother you with such "small" details, but if I figure this out, you tool is simply perfect.

[Stefan Erkens]

After talking with some other ppl, who are more familiar with audio encoding, I am sure it is just about the encoder.
The encoder in the ffmpeg.exe ist just not the best one, especially not for MPEG4/AAC.
When converting to AAC, you should use twice the input bitrate as output bitrate.
The MP3 encoder however is much better. But still at 64kbit, its pretty bad. For me MP3 and 128kbit/s ist die best solution, granted the Input file has 128kbit or less. In that case I can't hear the slightest difference between a file encoded with 128kbit and a file encodede with higher bitrate (192-320kbit). I am using an Asus Xonar STX soundcard either on AKG headphones or on my HiFi System and still there is simply no difference hearable.

So my advice:
If the AAX file is 64kbit or 128kbit --> go for MP3 and 128kbit
If the AAX file is 192kbit --> go for MP3 and 192kbit

(To find out, what bitrate you AAX file is: just divide the Filesize by runtime in seconds and multiply by 8)

The Ticket can be closed.