[Mp4-tech] Re: Regaring PNS in MPEG4 AAC

[Ralph Sperschneider]

Mody, Mihir wrote:
> Hi,
> 
> I have couple of queries on PNS module in AAC.
> 
> 1)
> According to AAC standard, noise energy (nrg) is calculated as follows.
> nrg = global_gain - NOISE_OFFSET - 256.
> Does anybody know significance of 256? Does encoder have to to add 256 
> in nrg? In that case, can we define NOISE_OFFSET = 90 + 256 = 346?

dpcm_noise_nrg[g][sfb], which is the first differentially coded noise_nrg value, 
is transmitted as 9 bit uimsbf, i. e. it has the domain [0,511]. By reducing the 
start value by 256 you move that domain to [-256; 255].
> 2)
> According to AAC standard, the quantization of noise_energy during PNS 
> is done with resolution of  1.5 db. Does it mean following line is valid?
> 
> quantized_noise_energy = (INT) (log2 (noise_energy))

A factor of two seems to be missing, since for the energy the following is
true:
   log2(nrg)  -> 3dB
2*log2(nrg)  -> 1.5dB
> Regards,
> Mihir

Best regards,
Ralph
-- 
Dipl.-Ing. Ralph Sperschneider  | Phone: +49 9131 776 344
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