1. Field of the Invention
The present invention relates to an apparatus for generating a velocity error signal which is used in a time base corrector (TBC).
2. Description of the Prior Art
The TBC (Time Base Corrector) to remove the time base fluctuation in the color video signal reproduced from a record medium such as a VTR is known. In the TBC, a write clock is formed from a continuous wave synchronized with a burst signal in the color video signal, the digitized color video signal is written into a memory by means of this write clock, and the digital color video signal is read out from the memory by a reference read clock.
Since the burst signal exists for every one horizontal period (1H), it is possible to generate the instantaneous phase ,difference between the current burst signal and the clock synchronized with the phase of the burst signal which is IH precedent to the current burst signal, namely. In other words, the velocity error is generated. The velocity error and its correction will be described with reference to FIGS. 1A to 1F.
FIG. 1A shows a color video signal reproduced from, for example, a VTR. The color video signal has a horizontal sync signal HD and a burst signal S.sub.b for every 1H. The burst signal S.sub.b is separated by a burst gate circuit and supplied to an APC (Automatic Phase Control) circuit, so that a clock signal synchronized with the burst signal is formed as shown in FIG. 1B. By means of this clock signal, the color video signal is analog-to-digital (A/D) converted and stored into a memory. FIG. 1C shows the true phase variation which is included in the reproduced color video signal.
In this way, the phase difference, namely the velocity error between the clock signal which was automatically phase controlled by the burst signal which is 1H precedent to the current burst signal and the burst signal for the current horizontal period changes as shown in FIG. 1D. To remove the the phase variation in the reproduced color video signal a read clock of the memory of the TBC is phase modulated due to the velocity error.
As one of the conventional corrections of the velocity error, the linear correction to linearly interpolate the values of the phase differences which are obtained on a 1H-unit basis has been known. This linear correction relates to the method whereby the phase variation for 1H is linearly approximated as shown in FIGS. 1E and 1F.
The velocity error obtained by the conventional linear correction (i.e., first order correction) is approximated by a straight line, so that there is a drawback such that it doesn't coincide with the true velocity error variation.