Quadrature amplitude modulation preserving one channel zero crossing for

Abstract

The present invention provides a QAM modulation scheme compatible with a single channel demodulator. The inphase (I) channel is provided with luminance information encoded on a series of sine wave symbols. The quadrature (Q) channel is used for color difference values with a sine or cosine symbol being used for each color difference value. Alternate ones of these color difference symbols are inverted so that, on the average, their effect on the I channel zero-crossings simply cancels out. Thus, when sent to a single channel demodulator which expects a black and white picture, the quadrature channel will be filtered out; leaving luminance information which will be decoded as a black and white picture. The color information does not disturb the zero crossings of the luminance signal on an averaged basis, allowing the carrier recovery phase lock loop to maintain synchronization for demodulation purposes. When received by a QAM demodulator, the color difference values are recovered on the Q channel, to give a color picture. In one embodiment, the cosine symbols on the channel are offset with a positive or negative DC value, depending on whether the symbol is inverted, to shift the Q channel zero-crossings so they are aligned with the I channel zero-crossings.