Post by thread:PIC to video synchronization

I wonder if anybody tried to synchronize PIC to video signal. The problem is (as far as I understand) interrupt latency. I am using 14.31818 MHz crystal (k x horizontal sync frequency) and enable external interrupt by horizontal sync pulse. Then I generate levels I need in a video line. I assume that due to chosen crystal frequency interrupt latency should be equal for each line in a field. Nevertheless vertical line on a screen have jitters of about one cycle size (~280 nS). According to datasheet (I am using 16C71) interrupt latency = 3-4 cycles depending on where interrupt occurred during an instruction cycle time. It looks like this +/- 1 cycle gives jitters observed. If anybody faced this problem and found the way around it, please let me know. Thank you in advance. Gennady Palitsky Jefferson Audio Video Systems spam_OUTgennadypTakeThisOuTmainlink.net

On Fri, 13 Mar 1998 01:05:43 -0500 Gennady Palitsky <.....gennadypKILLspam@spam@MAINLINK.NET> writes: >I wonder if anybody tried to synchronize PIC to video signal. [...] > Nevertheless vertical line on a screen have jitters of >about one >cycle size (~280 nS). According to datasheet (I am using 16C71) >interrupt >latency = 3-4 cycles depending on where interrupt occurred during an >instruction cycle time. It looks like this +/- 1 cycle gives jitters >observed. Yes indeed it does. The problem is that the PIC can't resolve events of smaller than one instruction clock (xtal/4), and that your crystal (even if it's a very good one) is never exactly the same frequency as the incoming video. 14.318 MHz is a bad choice anyway, as it makes the PIC instruction rate 3.579545 MHz, or 227.5 PIC instructions per NTSC line. So the PIC will always be 1/2 cycle off of the video. Several people have used RC or LC oscillators in circuits that stop them before each line. The horizontal sync pulse from the video signal is used to restart the oscillator, always in consistent phase with the video. I've had good results with a continuously running LC VCO as the PIC oscillator, and phase locked it to the video. You should be able to find a detailed post about that in the archive. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]

Mike Keitz wrote: {Quote hidden}> On Fri, 13 Mar 1998 01:05:43 -0500 Gennady Palitsky > <gennadypKILLspamMAINLINK.NET> writes: > >I wonder if anybody tried to synchronize PIC to video signal. > [...] > > Nevertheless vertical line on a screen have jitters of > >about one > >cycle size (~280 nS). According to datasheet (I am using 16C71) > >interrupt > >latency = 3-4 cycles depending on where interrupt occurred during an > >instruction cycle time. It looks like this +/- 1 cycle gives jitters > >observed. > > Yes indeed it does. The problem is that the PIC can't resolve events of > smaller than one instruction clock (xtal/4), and that your crystal (even > if it's a very good one) is never exactly the same frequency as the > incoming video. 14.318 MHz is a bad choice anyway, as it makes the PIC > instruction rate 3.579545 MHz, or 227.5 PIC instructions per NTSC line. > So the PIC will always be 1/2 cycle off of the video. > > Several people have used RC or LC oscillators in circuits that stop them > before each line. The horizontal sync pulse from the video signal is > used to restart the oscillator, always in consistent phase with the > video. > > I've had good results with a continuously running LC VCO as the PIC > oscillator, and phase locked it to the video. You should be able to find > a detailed post about that in the archive. > > _____________________________________________________________________ > You don't need to buy Internet access to use free Internet e-mail. > Get completely free e-mail from Juno at http://www.juno.com > Or call Juno at (800) 654-JUNO [654-5866] *NEWBIE DISCLAIMER* I am very new to PIC programming, and more curious than productive at this point... What sorts of applications would video synchronization enable? What have you done with it? I've studied lots of projects people have done using PICs, but have never seen any that deal with video. I thought the PIC was too slow for that. Thanks! Carey -- ## An Apparition of her lover # # She recognizes with dismay; # # And later on she will discover ###### That he himself had died today. # # # # -Edward Gorey-

---Original Message----- From: Carey Pembroke <.....careyKILLspam.....TIPPETT.COM> To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU <PICLISTspam_OUTMITVMA.MIT.EDU> Date: Thursday, March 12, 1998 9:18 PM Subject: Re: PIC to video synchronization {Quote hidden}> > *NEWBIE DISCLAIMER* I am very new to PIC programming, and more curious >than productive at this point... > >What sorts of applications would video synchronization enable? What have >you done with it? > >I've studied lots of projects people have done using PICs, but have never >seen any that deal with video. I thought the PIC was too slow for that. > >Thanks! >Carey > Well, with most instructions executed in a single cycle and up to 20 MHz crystal you can get resolution as little as 200 nS, what is good even for character generation (though I never did that, there are enough standard characters generator ICs on a market). But video effects like video switching, frames, splitters etc. - your imagination is a limit. Gennady Palitsky Jefferson Audio Video System @spam@gennadypKILLspammainlink.net

On Fri, 13 Mar 1998 01:05:43 -0500 Gennady Palitsky ><KILLspamgennadypKILLspamMAINLINK.NET> writes: >>I wonder if anybody tried to synchronize PIC to video signal. >[...] >> Nevertheless vertical line on a screen have jitters of >>about one >>cycle size (~280 nS). According to datasheet (I am using 16C71) >>interrupt >>latency = 3-4 cycles depending on where interrupt occurred during an >>instruction cycle time. It looks like this +/- 1 cycle gives jitters >>observed. >Yes indeed it does. The problem is that the PIC can't resolve events of >smaller than one instruction clock (xtal/4), and that your crystal (even >if it's a very good one) is never exactly the same frequency as the >incoming video. 14.318 MHz is a bad choice anyway, as it makes the PIC >instruction rate 3.579545 MHz, or 227.5 PIC instructions per NTSC line. >So the PIC will always be 1/2 cycle off of the video. > >Several people have used RC or LC oscillators in circuits that stop them >before each line. The horizontal sync pulse from the video signal is >used to restart the oscillator, always in consistent phase with the >video. > >I've had good results with a continuously running LC VCO as the PIC >oscillator, and phase locked it to the video. You should be able to find > detailed post about that in the archive. I once had to solve the same problem. I ended up creating a "software" PLL with the pic. I had a TTL oscilator generate the clock for the pic and then run the clock into a NAND gate then to the pic. Using a JK flip flop to control the gate I was able to have this circuit "rob" a single clock cycle from the pic. The principle was to sample the sync line just before the transition and then send result to one pic pin connected to the flip flop. When I sampled just before the transition, nothing happened but if I sampled after the transition the sample level would clock the flip-flop and make it "rob" one cycle. I was able to get 1/4 instruction cycle resolution that way and that was enough to eliminate the apparent jitter. This needed two extra chips though. I have since got a look on a similar principle design that did not used any extra chip. The PLL clock speed change was done, if I remember well, by putting a Zener diode in parallel with one of the 15pf crystal load capacitor. One pic pin was used to change the voltage seen by the diode and it looked to me that the generated change to the internal capacitance of the diode was enough to slightly change the crystal oscillation rate. Just enough to get the PLL action. I want to play with that idea myself !!! looked neat :-) one of the pic pin used to Michel Tremblay Network specialist (CBS) Sidus Systems Inc. RemoveMEmtremblayTakeThisOuTmontreal.sidus.ca