LAME lowpass: stereo vs. mono, Lowpass differs with same bitrate per channel Options

[dutch109]

I am encoding a mono file to mp3 with Lame and a bitrate of 80, and a stereo file with a bitrate of 160.
Since the bitrate per channel is the same in both cases, I'm curious why is the lowpass different?

Here are the command lines I use and the output I get:

CODE

lame --nohist --noreplaygain --preset cbr 80 mono.wav mono.mp3

CODE

LAME 3.98.4 64bits (http://www.mp3dev.org/)
Using polyphase lowpass filter, transition band: 20094 Hz - 20627 Hz
Encoding mono.wav to mono.mp3
Encoding as 44.1 kHz single-ch MPEG-1 Layer III (8.8x)  80 kbps qval=3
    Frame          |  CPU time/estim | REAL time/estim | play/CPU |    ETA
  2298/2298  (100%)|    0:00/    0:00|    0:00/    0:00|   88.279x|    0:00
Writing LAME Tag...done

CODE

lame --nohist --noreplaygain --preset cbr 160 stereo.wav stereo.mp3

CODE

LAME 3.98.4 64bits (http://www.mp3dev.org/)
Using polyphase lowpass filter, transition band: 17249 Hz - 17782 Hz
Encoding stereo.wav to stereo.mp3
Encoding as 44.1 kHz j-stereo MPEG-1 Layer III (8.8x) 160 kbps qval=3
    Frame          |  CPU time/estim | REAL time/estim | play/CPU |    ETA
  2298/2298  (100%)|    0:01/    0:01|    0:01/    0:01|   35.732x|    0:00
Writing LAME Tag...done

I get the same lowpass difference using different settings (forcing dual mono mode, using cbr without the preset switch, etc.).

Thanks

This post has been edited by dutch109: Mar 6 2012, 21:48

[dutch109]

I agree with you, that is why I am posting this here, hoping a lame dev will either enlighten me or confirm this is a bug.