On Fri, 2010-04-09 at 11:00 -0300, Mauro Carvalho Chehab wrote: > Jon Smirl wrote:
> >>> #define NEC_NBITS 32 > >>> -#define NEC_UNIT 559979 /* ns */ > >>> -#define NEC_HEADER_MARK (16 * NEC_UNIT) > >>> -#define NEC_HEADER_SPACE (8 * NEC_UNIT) > >>> -#define NEC_REPEAT_SPACE (4 * NEC_UNIT) > >>> -#define NEC_MARK (NEC_UNIT) > >>> -#define NEC_0_SPACE (NEC_UNIT) > >>> -#define NEC_1_SPACE (3 * NEC_UNIT) > >>> +#define NEC_UNIT 562500 /* ns */ > >> Have you got a spec on the NEC protocol that justifies 562.5 usec? > >> > >> >From the best I can tell from the sources I have read and some deductive > >> reasoning, 560 usec is the actual number. Here's one: > >> > >> http://www.audiodevelopers.com/temp/Remote_Controls.ppt > >> > >> Note: > >> 560 usec * 38 kHz ~= 4192/197 > > > > In the PPT you reference there are three numbers... > > http://www.sbprojects.com/knowledge/ir/nec.htm > > > > 560us > > 1.12ms > > 2.25ms > > > > I think those are rounding errors. > > > > 562.5 * 2 = 1.125ms * 2 = 2.25ms > > > > Most IR protocols are related in a power of two pattern for their > > timings to make them easy to decode. > > > > The protocol doesn't appear to be based on an even number of 38Khz cycles. > > These are easy things to change as we get better data on the protocols. I just found authoritative data. It is in the datasheet for the uPD6122 authored by NEC Corporation: http://www.datasheetcatalog.org/datasheet/nec/UPD6122G-002.pdf Looking at page 11, especially line (5), it appears that all the timings are derived in terms of 1/3 of a carrier period and powers of 2. So.... Resonator frequency: fr = 455 kHz (AM IF parts are cheap apparently) Carrier frequency: fc = fr / 12 = 37.91667 kHz Duty cycle: 1/3 unit pulse: 64/3 / fc = 562.637 us (Jon was closer than me) header pulse: 16 * 64/3 / fc = 9.002 ms header space: 8 * 64/3 / fc = 4.501 ms repeat space: 4 * 64/3 / fc = 2.250 ms '1' symbol: 4 * 64/3 / fc = 2.250 ms '0' symbol: 2 * 64/3 / fc = 1.125 ms repeat time: 192 * 64/3 / fc = 108.026 ms Page 15 also shows that the older chips had a silence gap that could result in signals coming closer than 108 ms. Whew! I'm glad I've worked through my fit of Obsessive Compulsive Disorder for now. :) Regards, Andy -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
