Hi Rob

I am new in using the MAD Library and upon working with the code, I have got doubt in the Scaling (1st operand shifted right by 12 and 2nd operand shifted right by 16) being done during the mad_f_mul operation.

I like to know about the sbsamples from the IMDCT routine. Is it in Q4.28 format. If so why we need to scale it down during SYNTHESIS Filter operation. I have read the comments in fixed.h telling that

" * The scale factors are constant as this is not used with SSO. * * Pre-rounding is required to stay within the limits of compliance. " I like to know how this scaling has been taken care in the further calculations.

Please find below my doubt.

I have defined as FPM_DEFAULT and doesnt define OPT_SPEED.[OPT_SSO will be defined as FPM_DEFAULT is defined]. With that the mad_f_mul (A,B) is defined as [((A+(1L<<11))>>12) * ((B+(1L<<15))>>16)]

This has been used during the synth_full operation. When the synth_full function calls the dct32() function,

sbsamples<2><2><576> (in Q28.4 format) from IMDCT is passed on to the function where it will be multiplied with costab<1..31>.

During this multiplication the mad_f_mul MACRO is used. After that the result from dct32 is stored as the filter output. While storing the filter output, the multiplied value is scaled down by a factor of 12.

Then windowing[D array values being scaled down by 12] was applied over the filter ouput and the PCM output is stored with a scale down of 2.

The operation of SSO is understandable but the intial scaling that happens with the mad_f_mul MACRO needs explanation.

Thanks in advance

Balaji Shanmugam

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