Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site nicmad.UUCP Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!genrad!panda!talcott!harvard!uwvax!astroatc!nicmad!brown From: brown@nicmad.UUCP Newsgroups: net.video Subject: Re: Help w/VHS slow dubbing Message-ID: <561@nicmad.UUCP> Date: Sat, 1-Mar-86 17:35:58 EST Article-I.D.: nicmad.561 Posted: Sat Mar 1 17:35:58 1986 Date-Received: Sun, 2-Mar-86 19:51:53 EST References: <5031@alice.uUCp> <451@unccvax.unccvax.UUCP> Reply-To: brown@nicmad.UUCP (Mr. Video) Organization: Nicolet Instrument Corp. Madison WI Lines: 89 In article <451@unccvax.unccvax.UUCP> dsi@unccvax.UUCP (Dataspan Inc) writes: > > The problem is that during slow advance on your PV8000, the relationship >of the equalisation pulses to the vertical sync pulse is destroyed. I doubt >that your sync generator will genlock to the vertical sync pulse which is >provided during slow motion and stop frame reproduction. The reason that this >is done is to essentially force the synchronisation at the monitor to >"noninterlaced" (only one field is being displayed at a given time) and because >there is no control track to provide reconstitution of the vertical sync >during normal playback. (Note also that the "dead zone" occurs during the >vertical blanking interval). I beg to differ. You have the right idea, but some bad information. The funny vertical sync pulse, that is provided by the VHS VCR during special effects playback, is there to help mask some of the noise that can occur. The added sync pulse helps the tv from 'rolling' during special effects. But, this added sync makes it different from the normal NTSC sync. VCRs don't really care if the information being recorded is fully interlaced or not. All they are looking for is the vertical sync, in order the phase the heads correctly, during recording. The control track is ALWAYS recorded on the tape and it is ALWAYS referenced during playback. That is how the machine knows where to place the rotating heads. During special effects, the head placement can be even more critical. Though some machines are better than others. The ones that provide the 'clear picture' special effects use the control track to place the heads more accurately. The vertical sync is not reconstituted by the VCR, during playback. It is part of the original signal that was recorded. Because the vertical sync was damaged during special effects, following VCRs may have trouble recording it, which is exactly what happened. If VCRs recreated the vertical sync (or even the complete vertical blanking area), then we wouldn't have tape duplicators recording "copy guard" stuff on the tape, as the VCR would throw it out. I wish that the VCRs did do that, but they don't. > The solution is to get a time base corrector. Most of them which are >designed for use with the heterodyne VTR send an advanced V-sync tach signal >to the VTR. The advanced signal is used to precorrect the headwheel phase >and the control track is simply ignored. Remaining timebase errors are >removed by digital techniques. Now, when you go into slow/stop motion, >the sync which will be on the TBC out will have the correct synchronizing >waveforms impressed on it. However, still frame reproduction will actually >"jitter" now because the TBC out signal is always interlaced. Sending a VCR sync information, makes that VCR give the TV signal in phase with the incoming sync. Again the control track is used. But this time, the VCR speeds up or slows down the tape, so that it matches the incoming sync. > In short, when you go into slow/stop motion, the signal isn't NTSC >anymore, and your 1/2 inch machines (utilizing digital techniques to find >sync pulses, etc) don't know what to do with non NTSC sync pulses. Try >taping a cheap "Pong" game or something to test the effect. Not all VCRs have trouble recording non NTSC standard sync. One of my machines will record the information correctly, as long as the sync information looks even close. Which is why the old style "copy guard" doesn't work on my machine. The new "macrovision" does, but that is another story. > What this world needs is 1) a GOOD but CHEAP time base corrector with, >say, a 32 line window and 2) VHS and Beta machines with provisions to >disconnect the control track servo and provide advanced Vsync tach signal >to the headwheel servo comparator. Everyone seems to be using video these >days, but no one seems to have TBC's. You don't need a $15,000 Tektronix >110 TBC (which is extremely nice, by the way) to do what you want to do, >but the manufacturers haven't gotten the message yet. As I said above, the CONTROL TRACK IS ALWAYS used. The VCR NEEDS to know where the video information was laid down on the tape. The control track provides that information. External sync just makes the VCR change tape speed to match the incoming sync. If you were to tell the VCR, ANY VCR, to ignore the control track, you WILL end up with garbage on your TV. Last point to remember,; if you don't give a VCR external sync, it uses an internal referenced sync. I suggest you read the ads in 'Videography', as there are TBCs for less than $4000.00. True, for me that is still too expensive, for home use. -- ihnp4------\ harvard-\ \ Mr. Video seismo!uwvax!nicmad!brown topaz-/ / decvax------/