Video-optimized modulator-demodulator with adjacent modulating

Abstract

A video optimized, low cost modulator-demodulator is provided. Each modulating symbol has a one-to-one correspondence with a particular pixel value of brightness. A multiple amplitude two-phase modulation method is used. The black and white values are assigned to the highest amplitude level at opposite phases. As the pixel value becomes less black or white and more gray, it is assigned lower and lower amplitude levels with its phase being determined by whether it is closer to black or to white. Errors in amplitude thus result in a pixel having an adjacent shade of gray, thus minimizing the effect of the error. Errors in phase are less likely at the higher amplitude levels, which would result in an error from black to white or vice versa. The errors in phase are more likely at lower amplitude levels, which would result in changing a mid-gray pixel to another mid-gray value. In one embodiment, each pixel value is digitized and supplied to a ROM along with the digital representation of the carrier signal. These signals combine to form an address for the ROM look-up table to select a digital representation of the modulated carrier. This is then fed to a D/A converter to produce an analog waveform for filtering and transmission. The received signal is demodulated with a phase-lock loop to recover the carrier which is then combined with the received data in a chopper demodulator. An A/D converter reconstructs the original digital data which is supplied to an image display subsystem.