[Mp4-tech] Re: [audio] - CBR and bit reservoir

[Ralph Sperschneider]

D.Domazet wrote:
> Hi all,
> Suppose a 44.1 kHz stereo audio signal is encoded at constant bitrate 128000
> bits.
> 
> Bit-reservoir can go up to max frame size minus average bitrate, that is:
> 6144*channels = 12288 -> max frame size
> 2972 -> average bits per frame
> 12288 - 2972 = 9316 [bits] ->max bit reservoir size
> 
> Does this mean that some of the frames can have
> 12288 bits (average bitrate + bit reservoar), which is 529000 bitrate,
> and encoder still stays in CBR (constant bitrate) mode?
> 
> Thanks,
> Daniel

Daniel,
your derivation is true, it is just not that meaningfull to derive a bitrate 
based on the length of one single frame. As to allow for a frame with 12288 bits 
you have to fill up the bit reservoir before with 9316 bits by sending frames 
being shorter as 2972 bits. After the transmission of such a long frame the bit 
reservoir is empty again, thus you can immediately afterwards only transmit a 
frame that is not longer than the average frame length. This assures that the 
overall bitrate remains constant.
Hope this helps,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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