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  1. Preservationist davideck's Avatar
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    The following is a brief description of what a typical TBC does; the design of any specific TBC may differ in detail, but the fundamental operation remains the same. Please feel free to embellish, correct, etc.

    A TBC has three major sections; input processing, memory, and output processing.

    Consider the output processing section first. A stable reference clock is used to synthesize a "Black Burst" Sync reference made up of the Vertical Sync, the Horizontal Sync, and the Chroma Burst elements. This clock is either a free running crystal oscillator, or a Phase Locked Loop (PLL) that is locked to an external "Gen Lock" input.

    The actual Video samples stored in memory are read using the reference clock, converted to analog, and added to the Black Burst Sync reference, creating the Video Output signal.

    If the Black Burst Sync Reference remains valid at all times, then the TBC also provides "Frame Synchronization". A TBC/Frame Synchronizer will continue to provide a valid Black Burst Sync Reference output even when the Video Input drops out, becomes discontinuous (like when playing through a gap between recordings) or is removed completely. This behavior is desireable and eliminates many audio/video skew problems when capturing.

    Another function provided by the output processing section of some TBCs is Vertical Interval Blanking. This function substitutes blank (black) lines for the horizontal lines in the non-picture area at the top of each field that may contain information such as closed caption and copy protection. Any information that existed on these lines at the Video Input is eliminated at the Video Output.


    Now consider the memory section. It might be a full frame, or just several lines, but in both cases the memory serves as a FIFO to allow the timing characteristics of the Video output to be distinct from that of the Video input. Think of the memory as a funnel. Video input samples (water) can be added in variable amounts at various times while the output stream remains constant. As long as the memory is not overfilled or emptied, its output rate remains stable.

    If only several lines of memory exist, then the Video input timing must be slaved to the Video output timing. Line TBCs often have an "Advance Reference" output to be fed to a VCR's Gen Lock or Reference input for this purpose. This adjusts the VCR Video output timing (TBC Video input timing) to keep the TBC's memory approximately half full. Consumer VCRs do not provide this connection.

    If a full frame of memory exists, then the Video input timing can be completely independent and freely drift with respect to the Video output timing. An occasional image disturbance will be created whenever the input Vertical Sync frame timing drifts through the output Vertical Sync frame timing (as a Frame is lost or gained), but the need to slave the Video input timing to the Video output is eliminated.


    Finally, consider the input processing section. In order to provide Video samples in memory that can be added to a stable Black Burst Sync reference, the input processing must remove the timing instability and store the Video input as stable Video samples. This requires a sophisticated PLL to synthesize a write clock with the same timing instabilities as the Video input itself. By digitizing the unstable Video input with a clock that is unstable in "identical" fashion, the instabilites are removed during the Analog to Digital conversion process. These stable Video samples are then written into memory.

    The characteristics of the write clock sythesis determines the actual timebase correction capabilities. Different TBCs perform differently. Generating a write clock that is unstable in "identical" fashion to the Video input is a theoretical goal but a practical limitation. Some TBCs generate write clocks that are more closely "identical" than other TBCs. Some TBCs eliminate timebase instability to within nanoseconds such that even chroma phase (purity) errors are eliminated. Other TBCs may only eliminate more significant errors like tearing at the top of the picture and luminance jitter. Some TBC/Frame Synchronizers provide Frame Synchronization but little if any actual timebase correction.


    As examples;

    The DataVideo External TBCs provide Frame Synchronization and Vertical Interval Blanking.

    A typical DVD Recorder has an internal TBC/Frame Synchronizer that provides Frame Synchronization but does not provide Vertical Interval Blanking.

    The TBC/DNR Digipure system internal to some JVC VCRs does an excellent job of timebase correction, but it does not provide Frame Synchronization or Vertical Interval Blanking. If the video off tape is lost or discontinuous, then the Black Burst Sync reference output will also be missing or discontinuous. This is why a DataVideo TBC or a DVD Recorder is often advantageous in addition to using the JVC TBC/DNR when capturing.
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  2. Member lumis's Avatar
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    are you lordsmurfs brother?

    good info.
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  3. Member The_Doman's Avatar
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    Also good reading is this I found:

    What is a TBC* and why do I need one?
    http://www.questronix.com.au/info/info_tbc.htm
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  4. Member
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    ...well they do fight like brothers...

    Good info, davideck, I have no idea on the accuracy, but it sounds right.
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