Why use external TBC with JVC VHS internal TBC?
 

The title says everything. Why use an external TBC (or TBCish) unit when using a recommended JVC VCR that has internal TBC?

Thanks,
Steve

It sure does, LOL! It says you don't know that there are two basic types of tbc.

When tape plays, some of the the scanlines in each frame will exit the player earlier or later than others as far as the cyclical timing goes. This results in geometric distortion such as "wiggling" vertical lines and upright forms that have notches along their length, fuzzy and often ill-defined edges, bent side borders, flagging or tearing across top borders, and other effects. Because scanline timing varies with each frame, these distortions change shape or location rapidly, resulting in very jittery, "nervous" noise patterns. Scanline timing errors are prevented with the line-level tbc in most VCRs or in pass-thru units used as line tbc's.

Note that a line-level tbc controls scanline timing within individual frames. They have no effect on errors caused by frame-to-frame timing. VCRs seldom play a tape at the absolutely correct frame rates, which can be faster or slower over a period of time. Differences in frame timing result in loss of audio sync and/or dropped or inserted frames. A duplicate frame is inserted when an expected frame doesn't arrive in time for the capture circuit to maintain proper frame rate, or a frame is simply deleted (dropped). The type of tbc that fixes this problem is a frame-level tbc. Frame tbc's are almost always external units.

A line-level tbc has no effect on frame rate timing. A frame-level tbc has no effect on scanline timing errors within frames.

- Internal is line, and just cleans the image.
- External is framesync, and cleans the signal.

You need both.

TBC is a loose term, and can mean many things. So while both of these are TBCs, the exact function is very different. In fact, "TBC" is often even used for things that really should not, as is often the case on DV converter boxes or DVD recorders.

Thanks for the input! I'm still reading so much information that it feels like drinking from a fire hose. It's going to take a while to sort out and absorb everything.

I knew there were two kinds of TBC. I just did not know the two types were needed. Based on other topics on the forum, I had assumed from the thread about best VCRs that JVC's use of line TBC was better than Panasonic's frame TBC.

Steve

Have no idea where you read that. Panasonic VCRs don't have frame tbc's. JVC and Panasonic VCRs with line tbc's use different methods for caching the output of scanlines, but the corrections are equally effective.

I got the idea that Panasonic uses frame level TBC from the VCR Buying Guide in the Restore, Filter, Improve Quality Forum. In the section covering Panasonic units it states "NTSC professional editing decks, with full-field (multi-line) TBCs instead of a standard line TBC." I guess I incorrectly translated full-field to mean full frame.

Thanks,
Steve

A full-field line-level tbc caches all scanlines in a field together and transmits them all at once. Other line-level designs will cache fewer lines and send them in batches or one at a time at precisely the correct timing interval. The working terms here are "field" and "multi-line", not frame. So, yes, if you keep thinking "frame" instead of "field " or "line", it can be misconstrued.

Full-field is essentially multiple lines.

Full field is also known as "infinite window", but infinite window can also be multiple fields (though not full frame).

And full field is not better than line, just different. Sometimes one looks worse than the other, and you never know which that is.

Confused yet? :)

Welcome to TBCs. That's why we try to discern what uses what, and not rely on just stating "TBC".

Oh no! My head's spinning again!

Actually, thank you very much.

Steve