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

Justin Ruggles justin.ruggles
Fri Mar 11 23:08:39 CET 2011

On 03/08/2011 01:49 PM, Michael Niedermayer wrote:

> On Tue, Mar 08, 2011 at 01:21:43PM -0500, Justin Ruggles wrote:
>> 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.
>> http://www.flickr.com/photos/justinruggles/5431711148/
> great work, all commited will push later

Michael, I also recommend reverting the other 2 patches.  I'm sending a
new patch set which does something similar but better.


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