VCRs are imperfect machines. The rotating VCR head that scans the analog magnetic signal on the video tape is affected by tape wear, head misalignment and other mechanical problems that results in the scan lines of the video signal not synchronizing with the remainder of the video frame. And almost all VHS tapes have horrible signals. So what steps can be taken to get the best possible transfer of your home videos to a digital file?
Đang xem: Time base corrector là gì
One of the most important is the use of a time base corrector or TBC. If you have ever tried to copy a tape from one consumer VCR to another VCR, you probably noticed a dramatic drop in picture quality. Using a consumer VCR and a digitizing capture card will also result in a digital file that has jitters or looks unstable in parts of the video. When the video tape is being read by the VCR's head, it stretches a bit and springs back which causes some video lines to be shorter and other lines to be longer. This severe time base or sync jitter in the first lines of the video usually reduces as the VCR head moves further down the tape. The worst affected areas are the top 10% to 20% of the picture which is known as flagging or flag-waving. Every time an analog tape is copied to another tape, the time base errors are compounded further, eventually making it impossible for a VCR to synchronize resulting in a signal that becomes unrecordable.
So how do we correct this time base error? A 'time base corrector' or 'timebase corrector', usually referred to as TBC, is a device that corrects the signal and/or image quality of video tapes, especially VHS and S-VHS tapes. By the most basic definition, video is input into a buffer, and then it is corrected before being output again. There are 3 common types of time base correction devices:
External Full-Frame TBC such as models from Leitch, DPS or For.A. This is a separate electronic device that takes the signal from the playback VCR, corrects the signal, and sends it to the receiving machine that, when digitizing, would be the laptop receiving the digitized file. Most TBCs have controls for black level (brightness), video gain (contrast), chroma level (color saturation), and enhancement/filtering (sharpness). A high quality stand-alone external time base corrector will:
reduce visual on-screen image jitter (mild up and down image bounce)
overwrite “dirty” signal areas with new clean signals
provide a steady signal that prevents dropped frames on capture cards or premature recording stop on DVD recorders
clean up image quality by removing chroma noise or suppressing visual distortions
Professional or Prosumer VCRs with built-in TBC. This is a built-in TBC line that provides cleaner visual quality coming from the VCR when used as the playback machine. While these line TBCs do not replace dirty signals with new clean signals and are not as effective at reducing screen jitter, they do provide the following improvements:
removes or reduces chroma noise (the red/blue colored mist found in all VHS tape formats)
removes geometric distortions from the image such as the wiggling appearance of older video, also known as “horizontal” jitter
often have embedded noise reduction circuits to suppress or remove grain and prevent color bleeding
DVD Recorder “TBC”. The circuitry included in VCR/DVD recording combination machines and stand-alone DVD recorders is not true time base correction despite the fact manufacturers use the “TBC” designation on the outside of the box. It is nothing more than a basic frame synchronizer or circuitry that provides a similar function. This is misleading as it's not really a time base corrector and does not perform the same function or benefits of a TBC.
So when digitizing your own video tapes or using an outside service, insist on full-frame time base correction during the tape to digital transfer for the highest quality conversion. The next best solution is to use a high-end VCR that has a time base corrector built-in to the machine. And avoid using DVD recorders with bogus TBC features or consumer video tape players with no time base correction at all.