hi all,
i am studying MP3 file struture. But i don't understand about scalefactors. I have a question:
what does the scalefactor use for? and how to use it in decoder?
if you have any document about it, can you share it to me?

thanks alot

Well as the name implies, scalefactors scale up the spectral data. In AAC the gain is derived from scalefactors with formula 2^(0.25(scalefactor - SF_OFFSET)), probably something of similar nature in MP3.

They provide a means to control quantization noise.
low scale factor + "large" quantized spectral coeffs --> low errors, low data reduction
high scale factor + "small" quantized spectral coeffs --> large errors, high data reduction

It's similar to the JPEG quantizaion matrix. A transform coefficient is first divided by the scale factor and then rounded to the next integer value. In the JPEG decoder the reverse is done: the quantized sample is multiplied with the scale factor before the inverse transform is performed. Obviously small values (coarsly quantized) compress better but also lead to larger rounding errors in a relative sense.

itv: yes, the scale factor resolution is exactly the same (1.5 dB or 3 dB depending on the side info field "scalefac_compress" IIRC)

Cheers!
SG

Thanks for all of your information