Generation of electric signals having approximately sinusoidal waveforms

摘要

For the purpose of generating an output signal having a sinusoidal waveform, a digital store stores at different locations binary-coded amplitudes at equally spaced instants of time throughout a complete cycle of a sine wave. Those locations are addressed in turn by means of a pulse counter and the signals supplied by the memory are passed to a digital/analogue converter from which the required output signal is derived. The pulse signal supplied to the counter is obtained from a train of clock pulses by a programmable divider. To effect a frequency change in the output signal (for the purpose of signalling binary data), the divisor of the divider is changed in a sequence of steps so that the overall change in output frequency is less abrupt than would otherwise be the case. The arrangement described is suitable for use in a modem working with the Viewdata system currently adopted by the British Post Office.