[flac-dev] "keep qlp coeff precision such that only 32-bit math is required"

Brian Willoughby brianw at audiobanshee.com
Sat Apr 18 12:46:13 PDT 2015

Ok, I just did a comparison of 1.2.1 with 1.3.2, and the change you're suggesting was already there before. So, now the question becomes: why was the code changed in the first place? Was there a bug that was fixed by changing 17 to 16, or did someone just get overzealous in a code review and thought that 17 was a bad choice? Perhaps 32 bits isn't actually large enough to handle the overhead of processing 17 bit values - I haven't done the analysis to say whether it's safe or not. I do recall discussions that might have been referring to this sort of thing, so look at the change log to see why the change came about.

Brian Willoughby
Sound Consulting

On Apr 18, 2015, at 12:20 PM, Brian Willoughby <brianw at audiobanshee.com> wrote:
> Hmm, I don't know the history of the code, but flac 1.2.1 stream_encoder.c has
> 									min_qlp_coeff_precision = FLAC__MIN_QLP_COEFF_PRECISION;
> 									/* try to ensure a 32-bit datapath throughout for 16bps(+1bps for side channel) or less */
> 									if(subframe_bps <= 17) {
> 										max_qlp_coeff_precision = min(32 - subframe_bps - lpc_order, FLAC__MAX_QLP_COEFF_PRECISION);
> 										max_qlp_coeff_precision = max(max_qlp_coeff_precision, min_qlp_coeff_precision);
> 									}
> in process_subframe_(), whereas evaluate_lpc_subframe_() has the comments that you refer to.
> I've not done an analysis to determine whether these constants should match, or if maybe they should remain distinct.
> Brian Willoughby
> On Apr 18, 2015, at 8:05 AM, lvqcl <lvqcl.mail at gmail.com> wrote:
>> stream_encoder.c has the following code:
>> 	/* try to keep qlp coeff precision such that only 32-bit math is required for decode of <=16bps streams */
>> 	if(subframe_bps <= 16) {
>> ...
>> But FLAC can convert 16-bit input to 17-bit if mid-side coding is used.
>> So, does it make sense to compare subframe_bps with 17?

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