Quantcast VHS tearing even WITH TBC and JVC SVHS player. - digitalFAQ Forum
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07-13-2010, 11:08 AM
ramrod ramrod is offline
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Hey guys!
So its been a while since i was last here, mainly due to the fact that thanks to this site, my transferring of tapes has gone effortlessly! However, a VHS tearing issues has arised in a few tapes that i need help with!

This is my setup.
SVHS player (JVC SR-V10U) + Grex TBC (http://www.xdimax.com/grex/grex.html) + DVD recorder (pioneer, really good quality one) + S-VIDEO cables.

I can transfer tapes pretty well with this setup. I click the button on the front of the SVHS player which enables TBC/DNR, set the mode to AUTO in settings for picture, video stabiliser to off, D3 to off etc. Most of the time this works perfectly, no issues.

But on a few copyright protected tapes there is tearing. It happens like this. I start transferring....its fine, but the length in which there are no issues with tearing can vary. Sometimes its 10 minutes until the tearing occurs, sometimes its 5 minutes.
Ive noticed that if i have turned all the equipment off. and leave it off for quite a few hours (at least 14 hours to a day), then the time it takes before tearing occurs is longer (it kind of resets or fixes itself...), and when it starts flickering, i must then pause the recording, turn off the SVHS player and grex, wait a few hours and try again. I dont know why i have to do this, maybe it gets overheated? or overloaded? im not sure, its frustrating though! If i turn it off for only a couple of hours, the flickering almost starts instantly again... If i leave everything off for 12 hours, the flickering comes back after like 5 minutes.... then i have to restart the cycle again.

Can anyone explain why this happens? or how i may be able to fix it. I only have this one 20 minute tape left to transfer..but i cant just pause and record from the tape because the timing will be all wrong, and its music videos!

Thankyou for any help
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07-13-2010, 02:50 PM
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Why does this happen? Well, it all goes back to VHS/analog recording theory, and the importance of time-base and sync.

Short answer: The timing is uneven. The signal is be requested or provided at uneven intervals, and the byproduct is the signal is stretched linearly along the upper portion of the signal. Visibly, it's appears to be tearing, or bending (or ripping or flagging, depending on the layman word of choice).

Looooooooooong answer: Read the following two-page excerpt from the book VCR Troubleshooting & Repair by Greg Capelo and Robert Brenner...

Quote:
Figure 4.11 described severe time-base error. Figure 6.1 shows a slight shifting (horizontal time-base error) of the horizontal sync pulse at the video head switching point as seen on the monitor. This is caused by a small (few grams) difference in forward holdback tension between record and playback. The tape tension servo maintains the tape at a specified amount of forward holdback tension at the point where the video tape enters the head drum assembly (25 grams/cm for VHS). Meanwhile, the capstan and head drum servo circuits ensure that the video head contacts the recorded tape at a prescribed location.

As the tape is drawn around the drum, however, the tape tension gradually escalates due to the increased friction coefficient between the moving tape and the stationary portion of the lower drum assembly. This increase in tape tension causes the tape to stretch slightly as it travels around the drum. Since tape tension rises as it winds around the drum, the tape stretching also elevates proportionally. During record, the horizontal sync pulses are written onto the tape as it moves around the video head drum. The synchronization pulses are recorded on the tape as it is being stretched. Since the tape is stretched less at the initial head-to-tape contact point than it is at the drum exit point, the distance between the various horizontal sync pulses on the tape can be altered if the tape is not played back under the same conditions as it was recorded. This is because the elastic nature of the tape causes it to return to its normal (unstretched) condition after it has passed through the machine. In order for the recorded horizontal sync pulses to be played back at the same interval as recorded, the tape must be returned to the same length that it was during record. Tape tension, and the condition of upper and lower video head drum assembly (which also affects the tension coefficient) must be consistent between record and playback modes to prevent serious horizontal time-base errors.

Assuming that the upper and lower sections of the video head drum assembly are working correctly, and the tape tensions between the record and playback modes are die same, the distance between the horizontal sync pulses on the tape will appear consistent to the monitor as they are read by the playback head. If not, the timing between horizontal sync pulses will vary as the video head scans up the RF track.

To illustrate, let's assume that a tape is recorded with lower than specified forward holdback tension and then played back on a second machine with correct tension settings. (For this example, assume that while the tape tension is lower than it is supposed to be, it is high enough to hold the tape against the spinning head cylinder for recording.) During playback, the detected horizontal synchronization pulses appear to stretch farther apart as the head scans the RF track. This occurs because the tape is stretched more as it passes around the drum on the second VCR than it was during the recording process. The monitor attempts to compensate for the playback horizontal timing error using its horizontal automatic frequency control (HAFC) circuit. As the video head exits the tape, the horizontal synchronization pulses reach a point where they are most separated. At this time, however, the next head begins to contact the video tape at the video tape entrance point and detects horizontal synchronization pulses that are closer together. It is at the point where the video heads switch to the next field (the next RF track) where the HAFC circuit in the TV monitor receiver must rapidly compensate for the new horizontal pulse read off the tape. The HAFC circuit makes a dramatic jump between horizontal time-base errors occurring at the end of one RF track (the drum exit point) and the horizontal time-base errors occurring at the beginning of the next RF track (the tape entrance point). The result is a bending or tearing of vertical objects in the upper portion of the TV picture (see Figures 4.11 and 6.1).

If the tape recorded with the incorrect forward holdback tension is played back on the same VCR with the same incorrect tension, the differences between the record and playback horizontal sync pulse timing will be minimal and the HAFC circuit will not have to make the dramatic jumps between fields previously described. The bending at the top of the picture will thus be gone. As long as the tape recorded with the incorrect tension is played back in the same machine, no tearing will be seen in the displayed picture.
Images of the pages attached.

I highly suggest buying this book, if you have any interest in better understanding how VCRs and analog signals work. It will take a lot of mystery out of video, even if it's not necessarily the easiest of reads. Note that it's been out of print for years now, so don't expect to take a stroll to the nearest Barnes & Noble and find it on the shelf.


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07-13-2010, 02:55 PM
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Now then, how to fix it? This is easy for me to answer, too!

Short answer: Use a Panasonic ES10 DVD recorder as a pass-through device.

Longer answer: Read the sticky topic (FAQ Answer) concerning timebase correction. What is a TBC? Time Base Correction for Videotapes.
Quote:
DVD Recorder "TBC":

This is where we start to enter the land of "it's a TBC because we wrote it on the box". In many cases, the "TBC" is nothing more than a basic frame sychronizer, or circuitry that provides a similar function.

These tend to only be good at one thing: removing "tearing" that can sometimes be seen on the top of a VHS signal, a visual distortion on screen. You'll have to disable the VCR TBC and often remove the standalone TBC, to get benefit of this feature, as needed.

NOTE: These DVD recorders often have bad capture/recording quality, unfortunately, so you'll want to use it in "passthrough" mode. This means you feed a signal into the DVD recorder, and then output it to a better capture device further on. It is not used for recording. You can often re-add the standalone TBC after the DVD recorder, because it's still not necessarily the best analog signal yet.

The Panasonic "ES" series from 2005-2006 is known for this (ES10, ES15, etc).

DVD recorder "TBCs" will do next to nothing (or outright nothing) in terms of visual OR signal cleaning. Some of the DVD recorders do have digital NR, but it can be overly strong. The ES10, for example, over-processed the video with NR engaged, causing temporal blurring and posterizing/banding the video (compressed color palette).
That will fix you right up.

The only other device commonly known to correct this is the JVC D-VHS VCR. Certain older broadcast TBCs can do it too, but it varies widely between brands, models and sub-models of TBC. The DVD recorder is easiest, and often cheapest, and therefore most suggested.

Good luck on the project.

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