Video noise detector

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS.
1A and 2A, there are shown opposite polarities of the standard NTSC composite sync portion of a video signal.
The composite sync signal includes horizontal synchronizing pulses, identified by the number 2, each transmitted with a respective video line for defining horizontal line synchronization of the scanned image.
The composite sync signal further includes a vertical blanking interval during which are provided a set of pre-equalizing pulses 3 and a set of broad pulses 4 followed by a set of post-equalizing pulses 5.
The pre- and post-equalizing pulses, only two of the normal complement of six each being shown for convenience, occur at twice the line frequency and serve to reduce the effect of line-frequency pulses on the interlaced horizontal scan lines of the scanned image and to coordinate vertical timing for proper interlace.
The broad pulses, which also occur at twice the line frequency and which comprise the vertical sync pulse interval, are six in number according to NTSC standards (only two again being shown in the drawing for convenience) and are transmitted at the end of each field for establishing the vertical timing of the scanned image.
The broad pulses are also known as the vertical sync pulse.
Under excessively noisy conditions, not enough of the horizontal sync pulses of the correct width occur and too many pulses resembling the broad pulses appear outside of the vertical sync pulse interval degrading the integrity of the composite sync signal.
Referring to FIG.
3, there is shown in simplified block and schematic diagram form a video noise detecting system 10 in accordance with the present invention.
A baseband video signal is provided to a sync separator 18 for processing in a device such as a CATV encoder.
In accordance with the invention, noise detecting system 10 detects the noise level in a received composite video signal.
Upon detection of excessive noise, control of the CATV encoder can, for example, be switched to a local stand-by crystal oscillator (not shown)