What does a time based corrector actually correct?

[Rudyard]

This may sound dumb but ive been reading a lot of posts about capturing VHS and a TBC always comes up.
Problem is, and ive tried a google but dont get anything I can understand, I dont actually know what is being corrected.
What is the difference between a non-TBC capture and a TBC capture? What about the time is incorrect the needs to be corrected?

Im sorry if that is all mumbled but in laymens terms what is actually happening that needs a TBC?

[redwudz]

Mostly a TBC stabilizes the video signal timing pulses so they will match up with the video standards and be compatible with other video hardware.

From Wikipedia:

Time base correction is a technique to reduce or eliminate errors caused by mechanical instability present in analog recordings on mechanical media. Without time base correction, a signal from a videotape recorder or videocassette recorder cannot be mixed with other, more time stable devices found in video studios. Most broadcast-grade VCRs have simple time base correctors built in though external time base correctors ("TBC"s) are often used.

More here: http://en.wikipedia.org/wiki/Time_Base_Corrector

You won't likely find two VCRs that are exactly the same for playback as the recording VCR, hence the need for TBCs.

[Rudyard]

Thanks for the answer, but if you could dumb it down a shade for me that would be great!

When looking at a video what is evident that it needs a TBC?

Video/audio synch? Video noise? etc

[redwudz]

Wavering video, lines drifting up and down the screen, rolling picture, picture offset, maybe color shifts, brightness problems. One recent example: https://forum.videohelp.com/topic361964.html

[Rudyard]

BINGO! Thankyou, I now see what it is doing and have seen that type of thing on many VHS tapes.

Many thanks again.

[jagabo]

From a little further down in redwudz link:

https://forum.videohelp.com/topic361964.html#1929675

Notice how the line TBC straightened up the picture (the waves are caused by variations in the rotational speed of the VCR heads while recording and during playback). This is critical for MPEG encoding because MPEG is largely based on not including parts of the picture that don't change from frame to frame. Time base errors cause the picture to wave randomly horizontally -- forcing the MPEG encoder to waste lots of bitrate.

Here's an animated gif that shows the problem (less severe than the above video):

https://forum.videohelp.com/topic369003.html#1975151

The job of a full frame TBC is to provide a stable signal when the time base errors get so severe a capture card or TV can't even sinc to the signal.

[Rudyard]

Thanks, very informative.

[jimdagys]

...dumb it down a shade...really does work.

[Rudyard]

I stole that from Homer......

[edDV]

Think of a 640x480 or 720x480 raster. A VCR has mechanical playback jitter (think wow and flutter) so horizontal pixels are moving left and right line to line in disorder. A perfect TBC will act like a drill sergeant and get those civilian pixels lined up into rows and columns in each frame.

Did that help?

[Rudyard]

Yeah, its all helping, I have been struggling with this for a while but now its.

Great analogy by the way, im imagining that drill seargant hammering those waywood pixels!

[Rudyard]

Could someone please actually tell me of some brands/models of TBC that are reasonable (or on the flip side, models/brands to stay away from).

Im thinking since I have a heck of a lot of VHS to convert it may be a wise purchase.

please note these are videos for non professional use, so something in the home user price range not pro range.

Thankyou.

[videobruce]

TBC's correct what you don't see; the horitzontal and vertical blanking intervals. Sync and burst specifically that analog devices rely on to work correctly.

The two major problems are incorrect sync and burst levels that causes increases or decreases in luminance and chroma levels that get worse down the line. Specifically, video recorders. Images get brighter or darker, color levels get higher or lower. Usually the chroma winds up going through the roof because of the poor frequency response of consumer recorders, causing increases in color levels.

A TBC maintains these important signals to prevent this. The other benifets are what was posted above. To stabilize timing issues due to poor tape transports. Flagging; bending at the top of the picture.

[wanolan]

Ok, might seem like a dumb question but I've been told by people who know more than me that I need a timebase corrector for dvd capture as well as VHS capture. Is that so? I don't see really any discussion of that on these forums. Is this just because people generally go a software route for capturing DVDs?

[redwudz]

I don't think they know more than you. Most people don't 'capture' a DVD, they just digitally copy or transfer it. The disc just contains a lot of numbers. Much different than a analog VHS tape. I'm not sure what they would use a TBC for when copying DVDs, but there may be some reason.

But others can probably add to that.

And welcome to our forums.

[videobruce]

"Capture' really is not the correct term here. To make a long story short, TBS's fixed what you don't see.

TBS's were originally developed for 3/4" ENG footage for broadcast to compensate for time based errors from helical recording. This included VHS, or any 'tape' format for that matter (other than 'Quad'). There is no need for a TBC for coping DVD's, just tape. Especially 2nd or 3rd generation material. The other function of a TBC is the 'Proc Amp' portion to adjust and control levels; Y, C, phase (tint) and saturation. that can get messed up especially with 1/2" material. Chroma levels usually suffer with each generation due to the roll off of the burst causing the AGC circuits to increase those levels resulting in a over saturated C level.