Four digital Modulation Techniques; QPSK, QPR, 4-VSB-AM, and 16-QAM; capable of providing data rates between 10 MBPS and 20 MBPS in a 6 Mhz channel have been studied. A time domain simulation program has been written that allows accurate modeling of the modulation-transmission-demodulation process. Error performance vs. thermal noise has been calculated as a function of amplitude tilt, amplitude ripple, phase noise, quadrature offset, and timing offset. Finally a "typical" cable channel and demodulator have been modeled and error performance calculated.