In a message dated 7/5/2006 12:00:27 Pacific Daylight Time, [EMAIL PROTECTED] writes:
Recall that AT-cut crystals have a third-degree temperature-to-frequency curve. > The basic design was for locking an MPEG video stream to a broadcaster's > 27MHz master clock using the MPEG time stamps in a digital PLL loop. Actually, when running in such an application you can autotrim very cheaply. The naive approach would be just to lock it (through a software loop) and then use holdover (i.e. just keep the last frequency correction prior to breaking the loo). A little more advanced approach would be to correlate the needed correction with that of the temperature. You can either do a lock-up table or better yeat, figure out the 4 unknowns in the third-degree equation f = a*T^3 + b*T^2 + c*T + d and then use that as a separate correction method. Whenever you are tracking, you also record the required total correction for a few different temperatures. When you have recent measures for four different temperatures, you can solve the equation. T may be any unit really, so it hasn't have to be scaled to match "real" scales. It should just be whatever the temperature measurement cranks out. > When free-running (eg playing back video from a hard disk etc), the > temperature compensation should work great to keep the 27MHz within specs. Indeed. By providing the temperature compensation in combination with tracking, the necessary tracking-dance will be lowered. Cheers, Magnus Hi Magnus, the trick is to get away without any compensation if possible, since the Varicap diodes, temp sensor etc all cost money that consumer type products can't afford. Then, the crystal total deviation (temperature, aging, thermal effects, transport-related changes etc) has to be within NTSC or PAL spec (I think its <= 50ppm). This is the tough part to do for cheap. Even if you get the crystal adjusted at the factory to say 5ppm, can it be guaranteed that all the other effects combined stay at <45ppm over the lifetime of the product (these days it's about 6-12 months depending on the warranty period)? One would be surprised how many products ship that are way out of spec (DVD players, Set Top Boxes, Game consoles etc). Only the TV's very wide range of color-carrier-lock capability prevents more isses (e.g. a black an white picture). Typical TV's can lock up to 100ppm or so. BTW: doing Audio/Video Synchronization using the MPEG time stamps is something that not all DVD players and Digital TV Set Top Boxes do. The ones that don't have to skip/repeat a frame every so often and loose lip-sync after some time. It's surprising how many name-brand products don't do the AV-sync at all to save BOM cost. I personally know of a large Asian manufacturer that shipped hundreds of thousands of units that were totally out of spec (due to a component value problem). During production they tested with TV's that would be able to lock to the bad color carrier without any issues, but in the field the frequency was so far off that almost all units failed. This cose them millions $$ to fix. A simple frequency counter test at the assembly line would have prevented this. bye, Said _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
