A quadrature amplitude modulation (QAM) transmitter separates an input digital level into I and Q components. In a variation, a QAM transmitter uses every other input digital level as an I or Q component. A QAM receiver receives a QAM modulated signal and outputs digital levels. A QAM transmitter for transmitting analog levels uses a pair of input analog levels as the I and Q components. A QAM receiver receives a QAM modulated signal and outputs analog levels. The digital and analog input levels are produced by encoding N samples using L orthogonal codes.