Generation of electric signals having approximately sinusoidal waveforms
First Claim
1. In a method of generating an electric signal having an approximately sinusoidal waveform, said method being of the kind comprising the steps of(A) frequency dividing a first train of pulses by a factor to derive a second train of pulses having a lower pulse repetition frequency,(B) utilizing the second train of pulses to address in turn locations of a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, and(C) deriving from the signals supplied by the digital store an output signal having an approximately sinusoidal waveform,the improvement comprising:
- (D) making a plurality of progressive alterations to the factor by which the frequency of the first train of pulses is divided to form the second train of pulses so as to change the fundamental frequency of the output signal.
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Abstract
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.
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Citations
12 Claims
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1. In a method of generating an electric signal having an approximately sinusoidal waveform, said method being of the kind comprising the steps of
(A) frequency dividing a first train of pulses by a factor to derive a second train of pulses having a lower pulse repetition frequency, (B) utilizing the second train of pulses to address in turn locations of a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, and (C) deriving from the signals supplied by the digital store an output signal having an approximately sinusoidal waveform, the improvement comprising: (D) making a plurality of progressive alterations to the factor by which the frequency of the first train of pulses is divided to form the second train of pulses so as to change the fundamental frequency of the output signal.
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2. In a generator for generating an electric signal having an approximately sinusoidal waveform and selectively one or other of two fundamental frequencies, said generator being of the kind comprising
(A) a source for supplying a first train of pulses having a repetition frequency F, (B) a divider circuit which is arranged to derive from the first train of pulses a second train of pulses having a repetition frequency F/N, where N is an integer, (C) a digital store which stores in digital form a plurality of quantized amplitudes of a sine wave, (D) a counter which is arranged to respond to said second train of pulses and which supplies signals to address in turn the locations of said digital store at which are stored the quantized sine wave amplitudes, and (E) a digital to analogue converter which is arranged to respond to the digitized sine wave amplitudes supplied by the digital store so that when, during use, the digital store is addressed as aforesaid the converter supplies an output signal having an approximately sinusoidal waveform, the improvement comprising: (F) means operable when the fundamental frequency of the output signal is to be changed to control said divider so that the divisor N is progressively altered in a plurality of steps. - View Dependent Claims (3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
Specification