The present invention relates to receiver equalizers used in television broadcasting to provide signal delays compensating for complementary delays in television receivers.
Government established broadcasting standards for color television service set forth a uniform format for the color television signal which are transmitted by licensed television stations. In this format the color television signal is amplitude modulated onto the station carrier, with the resulting double sideband signal being filtered by a vestigial sideband filter so as to eliminate a large portion of the lower sideband thereof. The upper sideband, however, remains intact and extends out to more than 4.2 MHz above the carrier frequency. The sound, or aural signal is transmitted on a subcarrier located 4.5 MHz above the picture carrier.
In order to eliminate interference between the sound carrier and the higher frequency video information, receivers must include filters with high roll-off rates in the vicinity of 4.5 MHz. When the composite video signal is passed through these filters, the upper frequencies of the color signal experience a time delay which must be corrected. This was recognized at the time that the format was adopted by the Federal Communications Commission (FCC) of the United States government, and, to simplified the manufacture of television receivers, it was decided that the correction would be provided at the transmitter, rather than at the receiver. The FCC therefore now mandates that each transmitter include a time delay versus frequency characteristic selected to be the complement of the time delay versus frequency characteristic produced by the sound filter in subsequent receivers.
In TV transmitters currently in use, the receiver equalizer is implemented as a separate functional block including plural discrete components which are designed and adjusted such that the amplitude versus frequency characteristic of the filter is substantially flat over the frequency range of the video signal, but that its time delay versus frequency characteristic is such as to provide the receiver equalization set by FCC standards.
The receiver equalizer adds somewhat to the size of the visual exciter with which it is employed, and includes plural adjustable components which must be calibrated by the manufacturer during the manufacturing process, and which are thereafter susceptible to aging and failure. The receiver equalizer also, obviously, adds to the cost of the visual exciter. For these reasons, then, it would be desirable if the receiver equalizer could be done away with. Receiver equalization is, however, required by law in all transmitters.