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Time-period modulation transmission system
| Details |
Inventors: Taylor, Thomas M.;
Assignee: Neal; John A. (Los Angeles, CA); Taylor; Thomas M. (Los Angeles, CA)
Primary Examiner: Griffin; Robert L.
Assistant Examiner: Watkins; Albert W.
Attorney, Agent or Firm: Freilich, Hornbaker, Rosen & Fernandez
A system for time-period modulating (TPM) a subcarrier in accordance with the amplitude of an input signal includes generating a ramp signal of constant slope, comparing the ramp signal with the input signal, and resetting the ramp signal when the ramp signal reaches the input signal. Each time the ramp signal is reset, a clock pulse is counted. These clock pulses are counted in cyclic groups of N. After each clock pulse is counted, the then attained count is converted to a value of a sinusoid about some reference. The values for each of N counts define one sinusoidal cycle of the modulated subcarrier. Converting these values to voltage levels produces a sinusoidal stepwave which has a period linearly proportional to the amplitude of the input signal at the time. At a receiver, demodulation of the signal is achieved by squaring each cycle of the TPM signal, and during a predetermined half cycle of each squarewave cycle, generating a ramp signal of constant slope which reaches a voltage level in one half cycle linearly proportional to the period of the TPM cycle. The voltage level attained in each demodulating half cycle is sampled and held until the next demodulating half cycle. In that manner, a step waveform is generated which follows the input signal to the modulator at the transmitter. This system may be used for the L-R signal of a monaural compatible stereophonic transmission system which combines an L+R signal with the TPM L-R signal. The composite is then modulated on a carrier. |
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DETAILED DESCRIPTION In accordance with the present invention, time period modulation (TPM) of a subcarrier is employed, for transmission of a signal on a carrier.
This is achieved by comparing the input signal voltage with a linear ramp, and emitting a clock pulse when the ramp exceeds the input signal voltage to reset the ramp generator.
The period between clock pulses is thus a function of the input signal amplitude.
The slope of the ramp is selected to produce a center frequency of clock pulses equal to Nf.
sub.
c, where f.
sub.
c is the desired carrier frequency, e.
g.
, 31.
5 kHz in the case of a subcarrier for television transmission, and N is a number of samples (comparisons) desired per cycle of the subcarrier.
Each clock pulse resets the ramp generator and triggers a modulo-N counter.
The digital output of the counter is converted to a sinusoidal signal each cycle of which is of a constant amplitude and of a period equal to that of the corresponding cycle of the counter.
The result is a TPM sinusoidal signal at a center frequency f.
sub.
c.
This conversion of the counter output to a sinusoidal signal may be achieved by, for example, using the output of the counter to address a read-only memory (ROM) in which predetermined values of the sinusoidal signal have been stored.
These values are then converted from digital to analog form and filtered through a suitable bandpass filter coupling the converted signal to a transmitter.
The time-period modulator thus produces a cycle of the sinusoidal signal at the center frequency f.
sub.
c having a period linearly proportional to the amplitude of the input signal.
If the subcarrier frequency is significantly higher than the input signal, such as greater than about 100 times the input frequency, the amount of error caused by the input signal amplitude varying during the period of one subcarrier cycle is insignificant.
However, when that is not the case, such as when the input signal has a frequency of 15 kHz and the subcarrier has a frequency of 31.
5 kHz for television transmission, a sample-and-hold circuit may be employed at the input of the time period modulator to avoid any such error
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Telecommunications 
