It doesn't make any sense to me either.
The default Mp3 transcoding rules began using CBR, then it was 'switched
to using ABR' (http://forums.slimdevices.com/showth...MP3-CBR-vs-VBR),
and then 'changed to -V4 VBR with this bitrate cap'
(http://svn.slimdevices.com/slim?view=revision&revision=23010). I'm not
sure what the logic was behind the last switch, but it does seem
flawed.
I'm not sure how much better using something like "-V0 -B xxx" would be
with lower bitrates such as 160. For a given bitrate/quality level, you
end up needlessly limiting the bitrate of frames that need it more and
you somewhat increase the bitrate of frames that would be given less at
that level.
This is from the LAME documentation. Using -B with ABR/VBR potentially
gives you lower quality than even CBR.
>
> -B n Max bitrate
>
> Maximum allowed bitrate when using VBR/ABR.
>
> *Using -B is NOT RECOMMENDED*. A 128 kbps CBR bitstream, because of the
> bit reservoir, can actually have frames which use as many bits as a 320
> kbps frame. ABR/VBR modes minimize the use of the bit reservoir, and
> thus need to allow 320 kbps frames to get the same flexibility as CBR
> streams. This is useful only if you need to circumvent a buggy hardware
> device with strange bitrate constrains.
> Default value is the maximum allowed bitrate for the MPEG versio (i.e.
> no limit)
>
Using ABR still makes the most sense. It uses a variable bitrate, so
individual frame rates vary according to encoding need, but the overall
streamed bitrate is shaped to fit a particular bandwidth level, as set
in the LMS player settings. Isn't that the whole idea behind bitrate
limiting, rather than just putting a cap on the maximum bitrate of a
tiny part of the encoded stream?
You can fix it simply by placing the following rule in
custom-convert.conf:
Code:
--------------------
flc mp3 * *
# FB:{BITRATE=--abr
%B}T:{START=--skip=%t}U:{END=--until=%v}D:{RESAMPLE=--resample %D}
[flac] -dcs $START$ $END$ -- $FILE$ | [lame] --silent -q $QUALITY$
$BITRATE$ $RESAMPLE$ - -
--------------------
------------------------------------------------------------------------
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