Time base correction
The Time Base Corrector (TBC) [ taɪm ˌ beɪs kəɹɛktə ] is a digital image store ( Digital Frame Store), which temporarily stores one frame ( frame) of a video signal and at a different time (between 0 ms and the duration of one frame ) outputs again.
- 2.1 variable delay lines
- 2.2 Period of storage
- 2.3 digital semiconductor memory
Use
A TBC is required to synchronize video signals with other video signals, for example to cut it in real time and / or be able to record.
The TBC was formerly externally usual, there were certain models for different decks, for the Sony 1 inch C- machine example, it was BAT -1000, for the U -matic devices of the same company of BAT -800 or BAT -810 (where the 810 for the variety of the so-called 4-ring TBCs heard of only 3 800 ) of 4 plates TBC has next to the power supply a small motherboard with a particular bus system, are inserted into the front of the readily available 4 boards ( about 30x30cm in size ), the front of these boards are the function controls, but there are also DIP switches on the board itself
4 board: Noise reduction (allows chroma enhancing and Chroma Noise reduction over 2 toggle switch)
3 board: Clock Generator
2 board: Processor (allows setting the video input levels, chroma, black- and video level ( in the case at the output ) and Y / C Delay, as it works internally with Y / C All controllers can individually with the " preset. be " set switch to a standard value. )
1 board: PAL - Sync Generator: (allows adjustment of the H -SC and V - phase (without Presetmöglichkeit ) and chroma burst ( with Presetmöglichkeit ) DG ( differential gain) You can choose between color and black and white and automatic ( detection of the burst ). be selected when eg a color photograph for aesthetic reasons to be black and white. Similarly, a bypass circuit is provided. )
Normally, the external TBC receives the reference signal " black burst ". There is a voreileindes signal to the deck on and receives from external devices an unmodulated composite or a special 18 -pin connector Y / C signal in devices can recognize the signal interference "drop- outs ", a special signal.
The TBC now does the following: it sets the image content, the signal very precisely to the sync signals, so that an absolutely synchronous operation is possible. Dropouts are hidden by the contents of the preceding intact line is keyed in at this point. If this damage is greater ( by tape crease or similar) is displayed instead of the usual noise ( ant war ) a colorful stripe pattern, because the last line is intact, with each other keyed.
For internal TBCs (for " professional" S -VHS equipment or Betacam machines ) the setting options are less and differently: at S - VHS video level, chroma level, black level, and chroma phase and Y / C Delay, H and SC phase adjustable in Betacam video level, black level, RY, BY.
Via the supply line you need not to worry with such internal TBCs: only saves space but also a noisy fan. TBCs are usually controlled remotely. In practice, this is rarely used, unless you regularly too dark, or white balance is wrong. Then corrections are possible about the possibilities of TBCs already warming up at NLE systems or the analog interface. Then a Color Corrector is indispensable for further corrections.
In DT -enabled devices, it is possible only with TBC, Slow and fast -motion playback. In U-matic this is 1x backwards and up to 3 -fold forward possible.
In practice it is not possible to keep the two signals synchronized completely. For example, can be roughly synchronized with a source of a video recorder though, but there are still small mechanical inaccuracies that may be the signal arrive faster or slower for a short time. This is referred to as jitter.
Fullscreen TBC
Completely unsynchronized video signals arise whenever the different video sources are not using an external reference signal ( for example, " black burst " ) can be synchronized, for example, because the devices are separated too far or this function does not own (eg. non-professional appliances for household use such as DVD player or VHS recorder ).
As a TBC in this case, to cache at least two fields must, various additional functions for the devices to offer:
- Change of H- phase subcarrier phase, black level, brightness (luminance), color ( chrominance)
- Change the TV standard (so-called standards converter )
- Freeze function: the full image stored on a button is constantly being re-issued
If you work with image and sound, it is necessary that the associated audio signal by means of an audio delay also around the same time (usually 40 ms ) is delayed to produce no asynchrony between image and sound.
Operation
Variable delay lines
The first TBCs were used in early color video recorder ( for example, quadruplex ). There was not synchronizing with external sources in the foreground, but the compensation of small temporal errors that can result in color systems to strong color errors.
These delay lines are composed of a larger number of variable capacitance diodes and conductor coils which are interconnected so that they delay the signal bit. By applying a DC voltage across the diodes, the delay time can be changed. This method achieves a balance of a few microseconds, and is no longer used.
Run-time memory
To compensate for large errors, such as, for example, stemmed from poorly maintained VCRs, switched run-time memory were used. Each of these run-time memory had a certain delay. If you wanted to delay the signal by a certain amount, so you simply sent it by more or less of these memories. The portion that can not be compensated by this method is compensated by a variable delay line. Here you can already correct time error to a line.
Digital semiconductor memory
By the availability of affordable memory in the 1970s it became possible to store digitized entire lines. The image is thereby often divided into its luminance and color components, which are processed separately. The individual pixels of these lines can be output in any order. Additionally, one can but also access pixels of other lines and thus achieve calculations, such as for example a color noise reduction. Typical time correctable errors are in the range of 3 lines up to an entire frame.