Problem converting to mono in FDKAAC

[WallyWalters]

Converting mono WAV files (1-channel/44.1KHz/16-bit) to aac with the dBpoweramp FDKAAC encoder results in m4a files which MediaInfo reports as "1-channel/44.1KHz-22.05KHz/16-bit," but which open as 2-channel/44.1 KHz/16-bit files in foobar2000, Adobe Audition, Audacity and others. Furthermore, converting the output back to WAV in dBpoweramp using the "as source" settings outputs a new WAV file which Mediainfo reports as 2-channel/44.1KHz/16-bit, and which is double the size of the original WAV file. My command line is:

FDKAAC -p 29 -b 24K -m 0 test.wav

How should it be changed to guarantee mono output?

[mville]

Re: Problem converting to mono in FDKAAC

Right click the m4a file >> Properties >> Audio Properties tab. Is the Channels property = 1 (mono)?

[WallyWalters]

Re: Problem converting to mono in FDKAAC

Sorry, should have included that. No, the Audio Properties tab lists Channels 2 (stereo)

P.S. Just noticed, there's a typo in my original post. My command line should read

FDKAAC -p 5 -b 24K -m 0 test.wav

. That's significant because HE-AAC-v2 (Profile 29) isn't designed for mono and won't work with it, where as HE-AAC-v1 (Profile 5) will. But the result is the same -- stereo output from a mono source file.

[mville]

Re: Problem converting to mono in FDKAAC

Try adding the Channel Count DSP on conversion and set it to Fixed 1 mono.

[WallyWalters]

Re: Problem converting to mono in FDKAAC

Same result. Besides which, I'd really like to get this thing working right entirely from within the command line, in case I want to incorporate it into a batch file later.

[mville]

Re: Problem converting to mono in FDKAAC

Just did some testing and I can confirm that converting a mono wav (16-bit, 44.1kHz) using the m4aFDK (AAC) encoder gives:

1. Encoder >> LC AAC >> produces an m4a file with 1 channel
2. Encoder >> HE AAC (below 96 kbps) >> produces an m4a file with 2 channels
3. Encoder >> HE v2 AAC (below 56 kbps) >> produces an error

I assume this is by design.

Although the HE AAC (below 96 kbps) v1 encoder produces a 2 channel file, both channels are identical so is effectively mono (on playback). Further, my mono CDs ripped to flac are also 2 channel, but exhibit the same behaviour, where both channels are identical.

[WallyWalters]

Re: Problem converting to mono in FDKAAC

Found an explanation that helped greatly in my understanding. Thanks, everyone.

"Your HE AAC file IS mono, the decoder just outputs upmixed stereo. This is because of parametric stereo, which is always signaled implicitly. Because of that, the decoder upmixes ALL mono files to stereo, even if there is no PS in the file."
https://hydrogenaud.io/index.php/top...html*msg258934