Return-Path: From: Siarhei Siamashka To: Jaska Uimonen Subject: Re: [RFC/PATCH] sbc: new filtering function for 8 band fixed point encoding Date: Fri, 12 Dec 2008 19:14:48 +0200 Cc: linux-bluetooth@vger.kernel.org References: <1227879337.20555.12.camel@esdhcp03999.research.nokia.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Message-Id: <200812121914.48340.siarhei.siamashka@nokia.com> Sender: linux-bluetooth-owner@vger.kernel.org List-ID: On Fri, 28 Nov 2008, Jaska Uimonen wrote: > I did some testing on the current 8 band fixed point > encoding and it seems to attenuate frequencies below 800Hz > and above 18kHz. There might be some other stuff happening > also, because at least to me the bass seemed to lack some > "definition". > I didn't quite understand how the current tables are calculated > and how the filtering works so I wrote a new filtering function > and calculated new filter tables for it. It is written > using 16 bit fixed point without any platform specific optimizations. > I only unrolled some loops etc. I tried to follow the > flow chart in MPEG-1 annex c. > +/* > + * to produce this Q15 format table: > + * > + * Get the filter coeffs from the spec and multiply them by 2^15. > + */ > +static const signed short _sbc_proto_fixed8[80] = { > + 0, 5, 11, 18, 26, 37, 48, 58, 65, 68, > + 65, 52, 29, -5, -54, -114, 185, 263, 342, 417, > + 480, 521, 531, 501, 423, 290, 95, -161, -479, -855, > + -1280, -1742, 2228, 2719, 3197, 3643, 4039, 4366, 4612, 4764, > + 4815, 4764, 4612, 4366, 4039, 3643, 3197, 2719, -2228, -1742, > + -1280, -855, -479, -161, 95, 290, 423, 501, 531, 521, > + 480, 417, 342, 263, -185, -114, -54, -5, 29, 52, > + 65, 68, 65, 58, 48, 37, 26, 18, 11, 5 > +}; Just remembered to check this. Precision and audio quality should be a bit better if the original floating values are not truncated, but rounded when put into tables. For example, the fifth element is 26 in _sbc_proto_fixed8 table, but it was 26.998194372608 after multiplication and before conversion to integer. Using 27 would have been a bit more correct here. -- Best regards, Siarhei Siamashka