[FFmpeg-devel] [PATCH 0/3] AC3 fixed-point encoder sample shifting

Justin Ruggles justin.ruggles
Tue Mar 8 19:21:43 CET 2011

On 03/08/2011 01:18 PM, Justin Ruggles wrote:

> This patch set changes how and when the fixed-point encoder does sample 
> value shifting.  Currently it left-shift normalizes samples to 16-bit in 
> each block for each channel, then adjusts the exponents accordingly so 
> that the final coefficient scale will be correct.  With the addition of 
> channel coupling, it now becomes important to have greater than 16-bit 
> accuracy in the coefficients.  The floating-point encoder converts the 
> coefficients to 25-bit fixed-point since that is the internal scale for 
> AC3.  This patch does the same for the fixed-point encoder by shifting 
> the coefficients back to their original scale, but in the 25-bit range.
> Patch 1 is needed to handle the higher coefficient scale in channel 
> coupling.
> Patch 2 is what I describe above.
> Patch 3 adds x86 SIMD optimization for both pre-MDCT and post-MDCT 
> shifting.
> Total encoding time:
> current: 4.16s
> patch 1: 4.16s
> patch 2: 4.07s
> patch 3: 4.01s
> Justin Ruggles (3):
>   ac3enc: use MUL64() to multiply fixed-point coefficients
>   ac3enc: shift coefficients to 24-bit following MDCT rather than using
>     an     exponent offset.
>   ac3enc: add SIMD-optimized shifting functions for use with the
>     fixed-point AC3     encoder

I forgot something... PEAQ comparison.



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