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

[Ralph Sperschneider]

D.Domazet wrote:
> ----- Original Message ----- 
> From: "Ralph Sperschneider" <ralph.sperschneider iis.fraunhofer.de>
> To: "D.Domazet" <danijel.domazet zg.htnet.hr>
> Cc: <mp4-tech lists.mpegif.org>
> Sent: Sunday, August 22, 2023 1:48 PM
> Subject: [Mp4-tech] Re: [audio] - CBR and bit reservoir
> 
> 
> 
>>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
>>
> 
> 
> 
> HI RALPH,
> THE AVERAGE BITRATE IS NOT THE PROBLEM, I WILL RESPECT IT. HOWEVER, WHAT
> CONFUSED ME WERE THE LIMITS OF A SINGLE FRAME. IF MY DERIVATION IS TRUE,
> LIKE YOU SAID, THAN THERE IS NO LIMIT ON A SINGLE FRAME SIZE (OTHER THAN
> 12288 BITS).
> 
> REGARDS,
> DANIEL
> 
> 
> 

Hi Daniel,
you are correct. You therefore need a decoder input buffer of that size. This is 
clearly outlined in the standard.
Best regards,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
Fraunhofer IIS                  | Fax:   +49 9131 776 398
Am Wolfsmantel 33               | mailto:ralph.sperschneider iis.fraunhofer.de
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