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AMR-NB codec :

Started by ashwin_c29 August 4, 2008
Hi All,

Had a small doubt regarding the AMR codec. Why is the file size (file
obtained at the output of an AMR-NB codec) constant for all AMR rates
even though there is a different bit allocation for each 20 ms speech
frame of the input file corresponding to each of these rates.(Besides
the file size is also larger than that of the input file). Am still a
novice as regards this codec and haven't been able to figure this out.
It would be nice if someone could throw some light on this.

Thanks,
Ashwin.
Hi Ashwin,

Please find answers below.

Regards,
Rama

On 03/08/2008, ashwin_c29 wrote:
>
> Hi All,
>
> Had a small doubt regarding the AMR codec.
> Why is the file size (file
> obtained at the output of an AMR-NB codec) constant for all AMR rates
> even though there is a different bit allocation for each 20 ms speech
> frame of the input file corresponding to each of these rates.
[Rama]: AMR-NB codec contains both Encoder and Decoder.
Encoder encodes each 20ms speech frame to corrresponding bit rate
specified at the encoder.This is the place where compreesion happens.
for example if you are encoding using 12.2kbps,same 20ms will be
represented by 244 bits.
At the decoder,using these 244 bits we get 160 samples (20ms * 8Khz).

>(Besides the file size is also larger than that of the input file).
[Rama]: So once it got decoded the file size should be same as input file.
But perceptually both will be same.

> Am still a novice as regards this codec and haven't been able to figure this out.
> It would be nice if someone could throw some light on this.
>
> Thanks,
> Ashwin.
hehe,Rama,u r so funny.

The constant said by Ashwin must refered to output from encoder,not from decoder instead,
altough Ashwin did not say it clearly and i did not meet the situation.

cheers

HyeeWang

>Hi All,
>
>Had a small doubt regarding the AMR codec. Why is the file size (file
>obtained at the output of an AMR-NB codec) constant for all AMR rates
>even though there is a different bit allocation for each 20 ms speech
>frame of the input file corresponding to each of these rates.(Besides
>the file size is also larger than that of the input file). Am still a
>novice as regards this codec and haven't been able to figure this out.
>It would be nice if someone could throw some light on this.
>
>Thanks,
>Ashwin.
>