Video Processors FAQ - under development - please help - Page 2

Originally Posted by Glimmie

I cut this from one of my prevoius posts. It gives a very technical description of time base correctors and how they relate to consumer devices and processors. Thought this might be good in the FAQ
Glimmie 01-16-2003

There is no such thing as a perfect bearing. There is no such thing as a perfectly balanced wheel. Therefore VTR heads and mechanical components wader and jitter. Same goes for laser disks, DVD's and even hard disk drives.

This mechanical jitter causes the timing of the recorded signal to jitter as well. In even early B&W video recorders this jitter was not an issue. When color came about, it was intolerable. The sensitive phase modulation of the chroma signal would be upset to the point where at a minimum you get the wrong color to total loss of color. This happens several times a second. So at least the chroma portion of the signal must be corrected and that's just what the consumer machines did. The correction was analog done by frequency mixing - basic AM radio technlogy. An out of phase jittery refernece signal was made from the lumanince and mixed back with the chroma. Result was the error was subtracted from the chroma and it was now stable.

But the B&W or lumanance signal still has the jitter. In the early days of consumer VTR's this was fine. As higher quality displays came into being with 3 line comb filters, this "half way" time base correction showed it's faults. Furthermore any digitizing of the video signal requires it to be totally time base corrected or broken down to RGB before being digitized. And decoding this non-coherent video has it's own set of problems. decoding is best done with coherent signals.

A TBC corrects the whole signal (both B&W Luminance and the Chroma). It works by keeping a memory buffer half full of video. The video fills the buffer in an unstable manner and the output of the buffer is controlled by a stable reference. Suppose you have a bucket with a 1 inch pipe in the bottom through a pressure regulator. The water comes into the bucket in splashes. As long as the bucket doesn't run out or overflow, you will have constant pressure water out the pipe and not lose any of the "input" water. This is exactly how a TBC works. Now to keep the memory buffer half full which is the safest operating point, the controller actually speeds up and slows down the head or disk motors to feed video into the system as needed. Even consumer VTR's with TBC's built in use this feedback principle. This is an important point as I will expalin next.

In order to keep the video centered in memory, the TBC must be able to control the VTR or disk players motor. Only professional VTR's allow this feature. So how does a consumer machine work with an external TBC? Another form of TBC is a "full frame or field" TBC. This device has the same memory but much more, a full televuision field or even two making a full frame of storage. There is so much memory versus the amount of error it can be thought of as a bottomless pit. But it's not. Sooner or later if the video is coming in slower than the stable output reference, the buffer will empty and the controller will repeat the last field or frame while it waits for the buffer to partially re-fill. Likewise if the video is entering faster than the output reference the controller will drop a frame to catch up. For general viewing and even most editing applications, this process is invisible. Most computer input cards or video processors that can accept raw VHS use this system.

The full frame system suffers another problem in that a VHS or Laser Disk has stable chroma but unstable luminance. The chroma must be un-stabalized to match the lumanince jitter. This can be done by demodulaing the choma but there is a quality hit for this step. Many processors now do this processing digitally but that too has it's artifacts.

TBC are also used in digital VTR's and even CD players. Your lowly computer hard disk has a TBC of sorts to buffer the data. But these are simpler due to the all digital signal path.

Bottom line is that an internal TBC with the source device works best. Some consumer and most prosumer VHS machines have TBC's bult in and these are the best. Same for laser diskplayers. I have an old $500 Sony LD player which has a TBC built in. DVD, OTA, and DBS are stable signals to begin with and don't require TBC's. In fact DVD and DBS produce textbook perfect NSTC signals at least in timing requirements. Video quality is another issue with DBS. The only problem remaining is Macrovision. Most Capture cards and processors, even external TBC's choke on Macrovision. Often a cheap Macrovision buster is all that is need to fix it and for the most part they don't degrade the signal. Processor manufactures can't include them outright for obvious reasons but they can offer more sophisicated back porch clamping. This is just a Macrovosion buster wearing "legal" clothes.


Last edited by Glimmie on 05-07-02 at 06:45 PM