[FFmpeg-devel] [PATCH] add fixed coeff LPC mode to alac encoder

[Justin Ruggles]

Jai Menon wrote:

> On Sun, Oct 04, 2009 at 08:53:03PM -0400, Justin Ruggles wrote:
>> Michael Niedermayer wrote:
>>
>>> On Thu, Oct 01, 2009 at 08:03:09PM -0400, Justin Ruggles wrote:
>>>> Hi,
>>>>
>>>> We currently do not have a regression test for the ALAC encoder like we
>>>> do for FLAC.  The ALAC encoder has 2 compression levels, one
>>>> uncompressed, which does not cover much of the code, and one which uses
>>>> floating point when calculating LPC coefficients, which does not lend
>>>> itself to reliable regression tests across all test platforms.
>>>>
>>>> The way we do regression testing with FLAC is to use the fixed LPC
>>>> coefficient mode, which skips the floating-point parts but still covers
>>>> prediction, entropy coding, and other parts in the regression test.
>>>>
>>>> Here is a patch to the ALAC encoder to add a fixed-coeff LPC mode using
>>>> compression_level == 1.  This will allow us to add a round-trip
>>>> encode/decode regression test.  The default compression level is changed
>>>> to 2 and keeps the same behavior.  The fixed-coeff mode is about 20%
>>>> faster but has about 6% worse compression (about the same as FLAC with
>>>> compression level 2).
>>>>
>>>> Thanks,
>>>> Justin
>>>>
>>>>  alacenc.c |   13 +++++++++++--
>>>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>>> f1f4235083438379b9b828846668c463de9addf2  alac_compr1.patch
>>>> Index: libavcodec/alacenc.c
>>>> ===================================================================
>>>> --- libavcodec/alacenc.c	(revision 20134)
>>>> +++ libavcodec/alacenc.c	(working copy)
>>>> @@ -132,12 +132,21 @@
>>>>      int shift[MAX_LPC_ORDER];
>>>>      int opt_order;
>>>>  
>>>> +    if (s->avctx->compression_level > 1) {
>>>>      opt_order = ff_lpc_calc_coefs(&s->dspctx, s->sample_buf[ch], s->avctx->frame_size, s->min_prediction_order, s->max_prediction_order,
>>>>                                     ALAC_MAX_LPC_PRECISION, coefs, shift, 1, ORDER_METHOD_EST, ALAC_MAX_LPC_SHIFT, 1);
>>>>  
>>>>      s->lpc[ch].lpc_order = opt_order;
>>>>      s->lpc[ch].lpc_quant = shift[opt_order-1];
>>>>      memcpy(s->lpc[ch].lpc_coeff, coefs[opt_order-1], opt_order*sizeof(int));
>>>> +    } else {
>>>> +        s->lpc[ch].lpc_order = 4;
>>>> +        s->lpc[ch].lpc_quant = 3;
>>>> +        s->lpc[ch].lpc_coeff[0] = -4;
>>>> +        s->lpc[ch].lpc_coeff[0] =  6;
>>>> +        s->lpc[ch].lpc_coeff[0] = -4;
>>>> +        s->lpc[ch].lpc_coeff[0] =  1;
>>> these coefficients do not look optimal, did you generate them using the LPC
>>> code over a large number of audio samples? You should IMHO!
>> you're right.  i did some more testing and found coeffs that come within
>> about 1% of compression level 2 with most samples tested.
>>
>>> besides, are you sure you want to assign them all to lpc_coeff[0] ?
>> oops... yeah, that would help. :)
>>
>> new patch attached.
> 
> Feel free to apply anytime :) Sorry for the late reply, I was away for
> quite sometime..

applied.

Thanks,
Justin