Time Base Corrector
The Time Base Corrector ( TBC ) [ ˈtaɪmˌbeɪs kəˈɹɛktə ] is a memory that temporarily stores the output signal of a magnetic recording device and outputs it again with a delay so that temporal fluctuations in the signal, which are caused by inevitable mechanical tolerances in the magnetic recording process, are compensated. On the one hand, this makes it possible to synchronize the signals of several video sources so precisely that they can be mixed together. On the other hand, in the age of analog television technology, it became possible to broadcast recorded material in such a way that the television signal complies with the specified standards in terms of time accuracy.
In the past, time base correctors were implemented using controllable delay lines made up of inductances and capacitances with multiple taps. Modern devices work on a digital level and can, if necessary, even buffer an entire full image ( frame ).
use
A TBC is required to synchronize video signals with other video signals, for example in order to be able to cut and / or record them in real time.
The TBC used to be used externally; there were certain models for different decks , for the Sony 1-inch C machines it was e.g. B. the BVT-1000, for the U-matic devices of the same company the BVT-800 or BVT-810 (whereby the 810 belongs to the variety of the so-called 4-plate TBCs, the 800 has only 3) which has 4 plates TBC next to the power supply is a small mainboard with a special bus system, into which the 4 boards (about 30 × 30 cm in size) are easily accessible from the front, the function controls are located on the front of these boards, but there are also DIP switches on the boards themselves .
4. Circuit board: Noise reduction (enables chroma enhancing and chroma noise reduction via 2 toggle switches)
3. Circuit board: clock generator
2. Board: Processor (enables setting of the video input level, chroma, black and video level (in this case at the output) as well as Y / C delay, as it works internally with Y / C. All controls can be set individually with the "Preset "switch can be set to a standard value.)
1. Board: PAL - Sync Generator: (enables setting of the H, SC and V phase (without preset option) as well as chroma burst (with preset option) DG (differential gain). You can choose between color and black and white as well as automatic (detection of the burst) can be selected if, for example, a color image should be black and white for aesthetic reasons. A bypass circuit is also provided.)
The external TBC normally receives the reference signal "black burst". It sends a leading signal to the MAZ and receives an unmodulated composite video from external devices or a Y / C signal via a special 18-pin connector. Devices that detect "drop-outs" in the signal can also detect a special signal.
The TBC now does the following: It sets the image content of the signal exactly to the sync signals so that absolutely synchronous work is possible. Dropouts are hidden by keying in the content of the previous intact line at this point. If this damage is greater (due to band creases or the like), a colorful stripe pattern appears instead of the usual noise (ant warfare), since the last intact line is repeatedly keyed in one below the other.
With internal TBCs (with "professional" S-VHS devices or with Betacam machines) the setting options are smaller and different: With S-VHS there are video level, chroma level, black level and chroma phase as well as Y / C delay, H and SC phase adjustable, at Betacam Video Level, Black Level, RY, BY.
With internal TBCs, you don't have to worry about the supply line: you save space and also a loud fan. TBCs can usually be controlled remotely. In practice, this is rarely used, unless you regularly have pictures that are too dark or the white balance is incorrect. Then, using the TBC's capabilities, corrections can be made when importing into NLE systems or during analog editing. A color corrector is then indispensable for further corrections.
With DT-capable devices, it is only possible to play back slow and fast motion with TBC. With U-matic this is possible 1 × backwards and up to 3 times forwards.
In practice it is not possible to keep two signals completely synchronous. For example, a video recorder can be roughly synchronized with a source, but there are still small mechanical inaccuracies that can make the signal arrive faster or slower for a short time. This is also known as jitter .
Full screen TBC
Completely unsynchronized video signals always result when the various video sources cannot be synchronized by means of an external reference signal (e.g. "blackburst"), e.g. because the devices are too far apart or do not have this function at all (e.g. non-professional devices for household use such as DVD players or VHS recorders).
Since a TBC must be able to temporarily store at least two fields in this case, various additional functions are available for the devices:
- Change of H-phase, sub-carrier phase, black level, brightness (luminance), color (chrominance)
- Change in the television standard (so-called standard converter )
- Freeze function: the full image saved at the push of a button is constantly output
If image and sound are used, the associated sound signal must also be delayed by the same time (usually 40 ms) using an audio delay in order not to produce any asynchronicity between image and sound.
functionality
Variable delay lines
The first TBCs were used in early color video recorders ( e.g. quadruplex ). The focus there was not on synchronization with external sources, but on compensating for small temporal errors that can lead to strong color errors in color systems.
These delay lines consist of a large number of capacitance diodes and conductor coils , which are interconnected in such a way that they delay the signal somewhat. The delay time can be changed by applying a DC voltage to the diodes . This method achieves a compensation of a few microseconds and is no longer used today.
Runtime memory
Switched runtime memories were used to compensate for larger errors, such as those caused by poorly maintained video recorders . Each of these runtime memories had a certain delay. If you wanted to delay the signal by a certain amount, you simply sent it through more or less of this memory. The portion that cannot be compensated by this method is compensated by a variable delay line. Here you can correct time errors up to one line.
Digital semiconductor memory
The availability of affordable RAM in the 1970s made it possible to store entire lines in digitized form. The image is often divided into its brightness and color components, which are processed separately. The individual pixels of these lines can be output in any order. In addition, you can also access pixels from other lines and thus achieve calculations such as color noise reduction. Typical time errors that can be corrected are in the range of 3 lines up to a full frame.