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Topic: AAC BitRate Issue. (Read 6336 times) previous topic - next topic
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AAC BitRate Issue.

hai all,

i have some questions in aac related to bitrate.

a. in ADIF stream, the bitrate can be extracted from header. is this bitrate of individual channel? i use a stereo ADIF stream for testing. the bitrate it shows is 128 kbps. is this 2 channel's bitrate (64 + 64)?

b. in ADTS, there is no separate bitrate field in both fixed and variable headers. ISO-IEC-13818-7 doc tells the max bit rate calculation can be (6144 / 1024) * Fs * channel_count. can i use this value for bitrate? or, by formula, Fs * bitdepth * channel_count ?

c. can anyone provide worst case AAC Stream (.aac/.adif/.adts)? i want the stream with the following parameters : 96K Fs, Stereo (currently v r supporting 2 ch decoding), 320kbps, TNS present. please correct me if anything wrong.

Advance Thanx to Audio Buddies.
Prove the worth of being You.

AAC BitRate Issue.

Reply #1 –
Quote from: vasanthfriend on
b. in ADTS, there is no separate bitrate field in both fixed and variable headers. ISO-IEC-13818-7 doc tells the max bit rate calculation can be (6144 / 1024) * Fs * channel_count. can i use this value for bitrate? or, by formula, Fs * bitdepth * channel_count ?


I don't understand what you're asking. What would be bitdepth in your last equation? What do you actually want to know?

Quote
c. can anyone provide worst case AAC Stream (.aac/.adif/.adts)? i want the stream with the following parameters : 96K Fs, Stereo (currently v r supporting 2 ch decoding), 320kbps, TNS present. please correct me if anything wrong.


Why would this be worst case?
You can produce such a stream with Nero AAC.

AAC BitRate Issue.

Reply #2 –
Thanks Garf.

b. we plan to deliver the bitrate information to the user, such as in winamp. the bitdepth is 16 bits per sample. generally the formula for bitrate (bits/sec) calculation is sampling rate (samples/sec) * bit_depth (bits/sample) * number_of_channels. my question is, whether we have to use this simple formula or the formula mentioned in the doc.

c. worst case stream in the developer's sense is, during decoding of the 'worst' stream, all of the code blocks have to be executed. and 'worst stream' in one more sense is, the stream must have the maximum supportable features of the codec. simply, to calculate the highest MCPS taken by the decoder.

i will try with Nero AAC Encoder. Special Thanks for this.
Prove the worth of being You.

AAC BitRate Issue.

Reply #3 –
Don't you think that [6144*Fs/1024*nChannels] looks very much like constant? And it definitely exceeds 128kbps.

a) total bitrate
b) adts->frame_length
c) conformance bitstreams

note that all bitrate related fields are optional