>
> Any guesses as to why MP3Enc 3.1 upsamples to 48kHz by default at 320kbps?
> To increase time resolution?
>
> -- Mat.
>
I think this is related to the 7680 mp3 buffer limitation.
An mp3 frame can be no larger than 7680 bits. However,
look at the framesizes at 320kbs:
320kbs/32khz 11520 bits
320kbs/44.1khz 8369 bits
320kbs/48khz 7680 bits
Thus a 320kbs/44.1khz frame is *required* to waste 689 bits
(8369-7680) per frame by padding the ancillary data.
And a 320kbs frame cannot make use of the reservoir,
since this only increases the framesize further.
This also comes up with, for example, 256kbs frames. A 256kbs/44.1khz
frame is 6687 bits. So the bitreservoir can be at most 993 bits. If
the reservoir happened to be larger (for example, with VBR and the
previous frame was 128kbs which allows for a larger bitreservoir) then
again bits must be wasted only because of the 7680 bit mp3 buffer
limitation.
I think the only valid reason for this limitation is so hardware
decoders know in advance that they need at most a 7680bit
buffer to hold the decoded data. That is not even 1K of memory!
I imagine most decoders could afford to allocate a 2K buffer
instead, and then they could play back mp3 files which ignore
this limitation. This is why I suggested we add some kind
of option to lame to allow this.
Maybe Alex Broadhead could comment on if the decoder he just
wrote could handle this?
Mark
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