The present invention relates to a video signal processing apparatus for outputting an inputted video signal after subjecting the video signal to time axis correction, particularly to a video signal processing apparatus preferable when the video signal is subjected to image compression.
In a reproduced signal processing of conventional VTR for household use, there is used a time axis correcting circuit or a time base corrector (hereinafter, abbreviated as TBC) for correcting a variation in an input signal with respect to a time axis. TBC corrects a time axis error by executing writing to a memory based on a write control signal having a time axis variation substantially the same as the time axis variation in the input signal and executing reading from the memory based on a read control signal substantially in synchronism with a period of a synchronizing signal of a standard signal. The technology is described, for example, in “Home video technology” p.115 edited by Nihon Hoso Kyokai.
FIG. 2 shows an example of a block diagram of a related TBC. A reproduced video signal inputted from a terminal 1 is subjected to digital conversion by an A/D converter 2 and is inputted to a synchronizing signal separator 4 and a line memory 6. The synchronizing signal separator 4 detects a synchronizing signal of an input signal and generates a composite synchronizing signal pulse csync in synchronism therewith. The pulse csync is inputted to a pulse generator 5 and a horizontal synchronizing signal pulse hsync is generated. The pulse hsync is inputted to a write control circuit 7 and an output from the A/D converter 2 is subjected to a clamp processing by a clamp circuit 3 and is thereafter stored to the memory 6. In the meantime, a synchronizing signal generator 9 generates a standard composite synchronizing signal tsync in synchronism with a standard signal. The signal tsync is inputted to a read control circuit 8 and data stored in the memory is read at a period substantially in synchronism with the standard signal. Thereby, from the memory 6, there is outputted a video signal a time axis variation of which has been corrected.
According to the above-described TBC operation, an amount of a deviation between periods of the input signal including the time axis error and an output signal corrected with the time axis error, is absorbed by the capacity of the memory 6. When an average period of the input signal does not coincide with a period of the standard signal, the deviation amount is gradually accumulated and when the accumulated deviation amount cannot be absorbed, the TBC operation is failed.
There is shown one method for avoiding the problem as follows. The synchronizing signal generator 9 generates a V correction pulse indicating a vertical synchronizing signal portion of the signal tsync and outputs the V correction pulse from a terminal 22. Thereby, a rotating cylinder is controlled. That is, although a servo unit executes a control such that 1 field of a reproduced signal constitutes a half period of the rotating cylinder, the servo unit further executes a control such that a period of rotation of the rotating cylinder is conformed to a period of the V correction pulse, described above. By the control, a field period of the input signal inputted from the terminal 1 substantially coincides with the signal tsync and accordingly, the average period of the input signal coincides with the period of the standard signal and the deviation amount does not accumulate. Such a control of the rotating cylinder by the V correction pulse is referred to as servo feedback.
Further, there is presented a video signal processing apparatus in which a time axis correction circuit is connected to an image compression circuit for generating image compression data in DVD recorder block of FIG. 2 in “Development of a DVD recorder using DVD-RW; Image information media society technical report, Nov. 24, 1999”. In the drawing, TBC is provided at a prestage of MPEG1/2 Video Encoder, however, no description has been given of operation of TBC in this case.