EDITING BREAKTHROUGH: HOW TO REPLACE DAMAGED
CONTROL TRACK IN THE MIDDLE OF A TAPE
If you have moved to 100% nonlinear editing, and you print to tape in one continuous pass through the VCR, then skip this article. If, however, you are like the other 15% of us who still perform assemble and insert edits on VCRs, particularly SVHS VCRs, then get ready to read about the most important development since flying erase heads.
This is big. Really big. And hardly anybody knows about it. Even JVC, who discovered how to do this, doesn't realize its impact on the editing community. Except to the cognoscente, mid-tape control track replacement may too obscure and technical to sound very important. Well, like a Picasso sold for $5 at a tag sale, when its importance is realized the editing community will rejoice with the news.
Not wishing to overhype this story, I'm only addressing VHS and Super VHS video tape editing. If you are working in Betacam SP, or a digital format, you can circumnavigate this magnetic pothole by copying the good parts of your tape, reediting the bad part, and recopying the remainder. These professional formats have such high quality that the generational loss is insignificant. If you master in Super VHS, however, a one generation drop in quality is usually out of the question. Replacing a control track glitch is far preferable to duping the tape as described above, or possibly reediting your whole video tape.
The glitch that stole Christmas -
Before I describe how to fix the problem, let me describe the problem better. When you originally record a video tape, the VCR records, along the edge of the tape, a series of pulses called the control track at the same time that it records video (or audio and video). If, for any reason there is a deviation or corruption of the control track, the video tape will not play correctly at this point. Your picture may roll, tear, break into snow or other garbage depending on the circumstances. A TBC generally cannot fix the problem (it just gives you stable garbage), and up till now, no VCR could correct the problem easily (other than reediting the rest of the show from this point on, or copying the good parts of the tape and reediting the bad part, going down one generation). Causes for this glitch:
1. Accidentally hitting the assemble edit button when intending to perform an insert edit in the middle of the tape. Once you hit that assemble edit button, there is no saving the recording; the beginning of the edit will look fine but the end of the edit will be ragged with a glitch and possibly a patch of snow.
2. Assemble editing a tape from camera masters, one of which has a --- possibly unseen --- glitch. Things that cause camera master glitches: a. The original tape was made on a VCR that was malfunctioning or deviated from proper specifications. b. The original tape had a physical defect (a fold, nick, scratch, fleck of dirt, or dropout along the edge of the tape that holds the control track. c. The camcorder or original VCR may have been damp during the original recording causing the tape to be sticky and unable to slide through the mechanism smoothly. d. The camcorder or VCR's battery may have become low and reducing the tape speed before shutting down the mechanism entirely. e. The camcorder or VCR may have spun around quickly, causing gyroscopic errors (the spinning video heads and capstan flywheel change speed if the deck is rotated quickly in any direction). f. The camcorder or VCR is bumped or jostled during recording. g. 101 other possibilities available in appendix C of Murphy's Law book.
3. You assemble edited your video tape instead of insert edited a "black" video tape, and one of the edits was bad.
4. You got a momentary head clog during the assemble editing process.
5. You performed assemble edits using two different editing recorders, and their tracking was mechanically different.
6. You failed to properly adjust tracking on the editing player for each camera master you played.
7. You readjusted the tracking on your editing VCR somewhere along the editing process.
8. You insert edited over a blacked tape but your black source was unreliable:
a. You were using black burst from a sick sync generator. b. The sync generator was genlocked to a deviant source. c. You made black from another video tape, perhaps through a time base corrector, and the tape hiccupped. d. You recorded black from a capped camera, switcher, or character generator that followed a deviant sync source. e. Instead of laying black, you recorded video off the air and sync was interrupted during a commercial break or an airplane flew between the transmitter and your antenna or a bird layed an egg in the feedhorn of your cable company's satellite antenna.
9. A brownout slowed the motors on your original tape's VCR, or on your edit player or recorder during the recording process.
10. Thirty-six other possibilities found in Appendix G of Murphy's Law Book.
All of the above can wreck your control track, but the most likely possibility, the one we all remember doing at least once, is errantly hitting the assemble edit button when we intended to do an insert on a completed --- no doubt otherwise perfect ---master video tape. This one's the killer; it's your own darn fault and you knew better, it's just that you were in too much of a hurry. Typos will occur, even irreparably destructive ones. If the subject of assemble versus insert editing is new to you, the accompanying box will explain it further.
How to perform the magic -
The only machines I'm sure can do this trick are the JVC 22 Series SVHS professional VCRs. This includes the $6405 BR-S822U editing recorder/player, the $4725 BR-S622U recorder/player, and the $4245 BR-S522U feeder/player, with or without the help of the editor controller RM-G870U. Control track insertion is not a standard feature of the 22 Series VCRs, but rather a service feature normally available only to technicians. Read pages 1-10 and 1-11 of the service manual (part # 9246C) for these machines and the instructions tell you how to go into an "adjustment" mode that directly programs the VCR's circuits to create control track signals. And yes, even the BR-S522U player can record control track signals; all three machines share the same circuits. In general, you feed your VCR a stable replacement signal (it could be black). Put the machine in the "adjustment" mode and start it playing before the glitch. Dial up menu 7 while the machine is playing, and it will begin recording new CTL (control track) pulses. Continue the process until after the glitch, and then dial out of menu 7. Then stop the machine. Exit the "adjustment" mode. The new control pulses are now on the tape and you can go back to insert video if you wish. The accompanying table lists a step-by-step procedure. If you don't have one of the above VCRs and are handy with electronics and soldering, you can turn a normal editing VCR into a "magic" one that allows assemble edits amidst a recorded program. The trick is to find the erase head for the control track. You would snip one of the wires and run it to an on/off switch which you mount on the deck. This will allow you to manually shut off the mechanism that erases the control track and leaves a gap when you assemble edit mid-tape. Now to use it: You've found the "bad" place on your tape where the accidental assemble edit had occurred. With your modified VCR, you feed the VCR a stable signal, like from a camera (it doesn't matter what the picture is because you will insert edit a replacement picture in a later step). Switch the control track erase switch to "off", so it doesn't function. Next play your tape, and just before the "bad" part, press the assemble edit button. The machine is now making new control track, and the control track recording head is ALSO miraculously erasing the old control track (not very well, but good enough). As the machine continues, you estimate how long to let it run to "get past" any problems; 30 seconds will probably do it. Then press STOP or END ASSEMBLE or whatever normally ends an assemble edit on your machine. With luck, you will have now patched up your control track. Switch your erase override switch ON before you forget; now your machine is back to normal. Your video is still screwed up, but now you can fix it using 2 or 3 insert edits. Note: Before experimenting with an important tape, try the process on a scrap recording to make sure everything works.
REVIEW: Insert versus assemble editing -
There are three ways to record a video tape:
1. Press record/play. This records new picture, sound, and control track over a blank or prerecorded (erasing the old recording entirely) video tape.
2. Assemble editing. This also records new video and sound and control track on a blank or prerecorded tape, erasing the old signal. The VCR makes a "clean" edit-in. It does this by listening to the incoming video signal and matching it to the signal playing from the tape. After the two are lined up and the edit-in point is reached, the VCR instantly changes from the play to the record mode during the vertical interval, the brief period between video pictures. A flying erase head clears away old video for a moment so that the video record head can add a new picture to the tail of the old.
The VCR simultaneously listens to the existing control track while playing back the tape. This assures that the video playback heads follow exactly the same path that the record heads took when the tape was originally made, and assures a clear playback without tracking problems. When the VCR switches to the record mode during an assemble edit, it creates new control track pulses based on the incoming video signal. Because the two signals were matched before the machine went into the record mode, the control track pulses maintain the same smooth cadence without a glitch. At the end of an assemble edit the VCR simply stops recording; it doesn't line up anything with anything. The end of the edit is ragged. It may contain snow because the VCR's erase heads were clearing the tape upstream of the record heads (leaving a gap between the two). No effort was made by the machine to line up incoming video (which created control track pulses) with prerecorded control track pulses. In fact this would be impossible to do because the machine is erasing the control track pulses --- the information is disappearing as the editor records. When the end of an assemble edit is played back, it may include snow, rolling picture, diagonal lines, a loss of sync, mistracking, flashes of color, and tape speed aberrations.
3. Insert edits. Insert edits record only video and/or sound, but not control track. They can only be performed over prerecorded video tape. Old video (or black) is erased while new video is recorded in its place. The VCR coordinates the incoming video and existing control track so that the two are locked together resulting in a video picture for every control track pulse. Insert edits can involve audio, video, or both. The end of an insert edit is clean. The VCR simply stops recording video and switches to PLAY during the vertical interval between pictures. Because the control track was never touched, the VCR was always able to synchronize the incoming video with the control track pulses on the tape (the control track pulses each represented a spot for a new video picture to be recorded).
A bad edit or deviant sync in the video signal or any other corruption of the video signal can cause an insert edit to appear glitchy. The edit can easily be saved, however, by recording over it --- in the insert mode. The tape doesn't care how many times you strip off the old video and put in new video, as long as the control track remains untouched; thus mistakes are always fixable.
More about control track: Control track, like sprocket holes in a movie, guide a machine when it plays back a tape. During the recording process, as the spinning video head lays down a picture, the control head spits out a pulse along the edge of the tape. When the tape is played back, those pulses control a mechanism called a drum servo, which makes the spinning video head ride along the same magnetic path taken by the record head when the tape was made. This helps the playback head read good information rather than the chaff between the tracks. You see this intertrack chaff when you misadjust the tracking control on a video tape player. The heads are playing part of one track, part of another, and the jibberish or snow between them. Since no two video players are exactly the same mechanically, a tracking adjustment allows you to make up for slight physical imperfections in their mechanisms.
Assemble edits are dangerous because they create control track as they record. If anything goes wrong during the recording process, the control track is corrupted. You saw a list earlier of the many things that can effect the video signal and thus foul up the control track. Once the control track was damaged for any reason, it could not --- up to recently --- be fixed. If you try to do an assemble edit, you can lay down good clean control track over bad control track but you cannot stop the edit; if you do, it leaves a glitch at the end of the edit. Thus you cannot replace an edit in the middle of a tape, or the beginning, or anywhere except the very end, if you want. You are forced to reedit the entire tape from where the glitch occurred, on. This could be a real pain if you just finished an otherwise perfect job with three hundred excellent choices and no precise edit decision list to work from. It is also a sad waste of electrons if the glitch occurs in the middle of an animation where your machine has been laying down pictures for days and there is just one frame missing from the middle.
Assemble edits are Russian roulette played with editors. The only safe editing technique is a two step process: First record black (called blacking or black bursting a tape) from a stable source (you could use any video signal instead of black, but black is neater and doesn't leave Shopping Channel between edits or at the end of the tape). Second, switch to the insert edit mode and do your edits. You can perform these edits in any order or in chronological order as if you were assemble editing your tape.
Step-by-Step Procedure for Replacing Control Track Using a JVC 22 Series VCR.
1. Feed a stable video or black burst signal into the video input of the recorder. The content of the signal is unimportant as it is used only for its synchronizing value. On the BRS422U player, feed video or sync to the machine's external reference input. This provides a guide to help the machine record perfect control track pulses, and to line up the old pulses with the new pulses before the edit begins.
Note: Once the edit begins in step 4 below, only luck keeps the two in line. If the edit is short, and the original control track pulses were stable (except for the glitch area), the two sets of pulses should line up when the edit finishes, even though the machine does nothing to line them up. It's like getting two drummers to beat a rhythm together, and then covering the ears of one of the drummers for a few moments. As long as you don't do this for too long, the drummers will stay in time. Most glitches last less than 15 seconds, and are well within the accuracy of the machinery.
People may tell you that the VCRs don't need external sync or video, and can manufacture it themselves if fed nothing. This is true, but chancy. "Auto detect" circuits in the machines are designed to lock to themselves or lock onto external signals if they're there. Problem: "Auto detect" circuits make mistakes. Forcing the deck to "listen to" a stable external source is a sure thing.
2. If at all possible, try to make you repairs using the same VCR that originally edited the tape. This will assure that the tracking will remain at the center position. If the glitchy tape has been recorded on a different machine, check to see that the editor that you are using for this procedure tracks the tape in the center position. If it does, you'll be okay. If it doesn't, there is an increasing risk that the procedure may not work, especially if the tape tracks near the edge of the editor's range. Reason: If the tape plays without tracking adjustments, that means the tape was recorded to exactly the same specifications as the present editor. If, however, the tape tracks at the edge of the editor's range, the editor has to have its tracking knob readjusted to play the tape properly. But when the editor goes into the RECORD mode, it starts recording control pulses based on its own internal workings, paying no attention to the tracking control. The resulting tape will be much like a tape recorded on several different editors; the tracking will jump all over the place if the editors weren't perfectly aligned to one another.
3. a. Switch your VCR's "local-remote" switch to "local" so you can operate it directly. b. Locate the beginning of the glitch. c. Back up the tape to 5 seconds before the glitch. d. Reset your counter to zeros or memorize the scene so you can locate it again "by eye". e. Locate the end of the glitch and go 5 seconds past it. Note the counter numbers or memorize the shot. It may simplify matters if you switch the deck's menu set ON (a switch under the control panel), then connect a video monitor to the deck's monitor output. This way you can see time code readings (control track or otherwise) on the TV screen and know precisely where you are on the tape. Watching the minute and seconds counter there, you can manually play the tape, start recording control track before the glitch, and stop recording control track after the glitch. On the other hand, you could just memorize the scenes and just eyeball them. f. Reverse the tape back to 5 seconds before the glitch (where you were in 3c above) and STOP there.
4. Switch the deck to the "adjustment" mode. To do this:. a. Turn the deck power off. b. Depress the COUNTER RESET button, the FF button, and the REW button together while pressing the POWER switch to ON. The counter will display "....88...." and then "...00...", the monitor enters "adjustment mode 0". If you don't see the "...88...", repeat the procedure (sometimes pushing 4 buttons at once doesn't "take"). c. Turn the JOG dial to change the mode. You want adjustment mode 4, which is 04 on the counter display. d. Press PLAY. The deck will run up to speed. e. Before the glitch, rotate the jog dial to adjustment mode 7 (counter display 07). The machine will begin recording CTL pulses. Watch the numbers on your monitor screen or use a stopwatch or view the picture to determine the length of the glitch. When you reach the end, turn the JOG dial to adjustment mode 4 (counter display 04). f. STOP the machine after about 5 seconds.
5. Take the machine out of the adjustment mode by turning off the power. Then turn the power back on normally.
6. Go back and check your tape by playing it and seeing if the control track pulses keep good time. (Your counter and monitor should display continuous numbers as the tape plays over the bad spot.)
7. Now perform a normal INSERT video or audio-and-video edit to cover any video glitch.
8. Play back the result and shake your head in amazement.
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