This invention relates in general to television signal processing equipment and in particular to means for automatically adjusting the chroma/video response characteristics of such equipment.
The characteristics of broadcast NTSC television signals are closely regulated by the FCC to conform to defined standards. While standards for over-the-air and cable transmissions are not the same, minimum standards are imposed, for example, on the signals delivered by cable converters to television receivers, for retaining signal quality which can be adversely affected by a nonuniform frequency response in the tuner (tuner tilt), intermediate frequency amplifier (IF) response characteristics and non-linearity in the converter circuitry. Generally the major source of difficulty is tuner tilt. In a cable environment, where signals are converted from one frequency to another, the response characteristic at the picture carrier (video) frequency and at the color subcarrier (chroma) frequency may not be within prescribed limits. Quite apart from any resultant degradation in the quality of the processed signal from such a response characteristic, FCC regulations dictate that there be no more than a .+-.2 dB variation in response between the picture carrier and color subcarrier frequencies. This requirement has militated against the use of so-called single conversion tuners in cable converters.
Single conversion tuners include a local oscillator that is adjustable in frequency for generating, when mixed with an incoming television carrier, a fixed intermediate frequency (IF) output of 45 MHz. The tuner may include three or four varactor diode tuned circuits that must track each other in frequency change over a significant range of D.C. tuning voltage change to achieve a flat frequency response characteristic while one of the tuned circuits (the oscillator) is tuned to a frequency 45 MHz higher. The difficulty of achieving a satisfactory response over all of the television frequency bands and the above-noted FCC requirement of no more than .+-.2 dB variation between the response at picture carrier and color subcarrier frequencies on cable converters, has dictated the use of double conversion tuning systems. A double converter, or Up Converter as it is often referred to, uses a local oscillator to convert all incoming television signals to a very high frequency (such as 600 MHz), filters the output to select desired channel frequencies and mixes the resultant with a fixed local oscillator at 555 MHz to generate a 45 MHz IF. This arrangement, while cumbersome and expensive, enables the FCC requirement to be met. With the present invention, the need for a double conversion tuning system in a cable environment is obviated and indeed, as will be seen, the bandpass limit specifications on conventional tuners may be relaxed significantly to render usable many tuners having heretofore unacceptable forward and reverse tilts.