[FFmpeg-user] Why _COPYing_ a variable encoded audio (AAC) channel will produce a variably encoded one?

[Sophoklis Goumas]

Hello.

I have a .3gp file containing a video and an audio channel.

I'm trying to get the audio channel in a separate m4a file but when I issue:
ffmpeg -i foo.3gp -vn -c:a copy foo.m4a
I get a constantly encoded AAC audio channel
while the original was a variably encoded one.

In more detail, from this:
Format                : AAC
Format/Info           : Advanced Audio Codec
Format profile        : LC
Codec ID              : mp4a-40-2
Bit rate mode         : Variable
Bit rate              : 32.0 kb/s
Maximum bit rate      : 34.4 kb/s
Channel(s)            : 1 channel
Channel positions     : Front: C
Sampling rate         : 22.05 kHz
Frame rate            : 21.533 FPS (1024 SPF)
Compression mode      : Lossy
Stream size           : 82.4 MiB (60%)

I end up with this:

Format                : AAC
Format/Info           : Advanced Audio Codec
Format profile        : LC
Codec ID              : mp4a-40-2
Bit rate mode         : Constant
Bit rate              : 32.0 kb/s
Channel(s)            : 2 channels
Channel(s)_Original   : 1 channel
Channel positions     : Front: C
Sampling rate         : 22.05 kHz
Frame rate            : 21.533 FPS (1024 SPF)
Compression mode      : Lossy
Stream size           : 82.4 MiB (98%)
Default               : Yes
Alternate group       : 1

Thanks in advance,
Sophoklis