This was something that confused me for a very long time, and of course, when you’re being asked to front up a large amount of cash on antiquated technology, you ask yourself over and over again if it’s really worth the money. Which way you should go depends on a lot of factors (please see the page on archiving equipment comparisons for more on that here).
Like many things in life, you only really appreciate the value something brings, once you see it in action and for me that was very true after running a variety of tapes and hardware setups including both the TBC and the frame sync devices.
Time Based Corrector (TBC)
In short, a Time Based Corrector (for the purposes of a digital archivist) is most likely going to be technology within your VCR that squares up the picture properly by making the video frames, fields and lines come out smoothly and uniformly when connected to digital capturing equipment. This is called a Line based TBC.
If you’ve ever tried to capture analog video to a digital device without one, you most likely have seen the poor quality output and thought, ‘wow VHS tapes were worse than I remember’. This is essentially because digital equipment is a lot more picky about these errors than your old analogue TV was. A line based TBC cleans up a very high portion of the errors that exist on an analogue tape (I’ve seen it said 90% of these errors) and for that reason is the number one recommendation for first purchase when going down the hardware based capture and correction route (more on that here).
Please see below for examples of errors fixed by an internal TBC.
Examples Here
External Frame Sync Device
An external frame sync will retrieve the source video frames sent by the VCR and store them in a buffer in such a way that digital capture cards can receive them without errors. This is because analog equipment sends video in frames, fields and lines whereas digital capture equipment captures video in frames. When inconsistencies are present in the source signal, the external frame sync uses its internal memory to buffer the frame and ensure the signal is being sent in the way the capture card expects to receive it.
For example, when a capture card expects an exact number of frames per second and one of the source frames is out of time, instead of skipping it, the buffer allows the external frame sync to rebuild the output to match the original expected number of frames. This makes the video come out as we expect.
An example of this is when capturing video into a PC utilising black magic converters, without an external frame sync many black frames are received and recorded into the capture. With an external frame sync attached, this problem goes away.
Other Types
Most of the more modern frame sync devices also include a line TBC now too. Such as the BrightEye 75 by Ensemble Designs. This certainly helps if you find yourself in a situation where your VCR has no line TBC built in or you have some analog media in a format that cannot have an in-built TBC, however in my experience it does not match the quality of an in-built line TBC in the VCR.
Summary
So in summary a Line TBC (or just TBC) in the VCR will fix timing errors that cause ugly video errors such as skewing caused by the mechanical nature of the tape and an external Frame Sync will fix whole frame errors such as black frames when capturing to digital equipment. There are many types of errors and scenarios in an analogue – digital capture world, the main thing to know is the starting point basis of discussion is you need both a Line TBC and a Frame Sync to make signal capture the best that it can be. I say starting point basis of discussion because you may find certain ways you can bend this to a ‘good enough’ scenario depending on your particular requirement.
