Some methods proposed for evaluating the spectra of modulated pulse trains are discussed. The basic pulse-frequency, -phase, -length and -amplitude modulation systems are defined and the spectrum of a train of rectangular pulses sinusoidally modulated in any one of these ways is derived. Modulation by more than one tone is also considered. It is shown that in none of these methods is there any harmonic distortion or audio crosstalk. Anharmonic distortion arises from sidebands of harmonics of the pulse repetition frequency, and graphs are given to illustrate its magnitude. The formulae are extended to cover non-rectangular pulses, and it is shown that the distortions to be expected are practically the same as for rectangular pulses. The reproduction of transients is briefly examined. It seems that they suffer little distortion in form but that their timing is modified by the pulse modulation, the maximum time-shift being half the pulse repetition period.

The spectrum of modulated pulses

Formulae for random noise in the audio-frequency output in pulse-communication receivers are given for pulse-length-, pulse-phase- and pulse-frequency-modulation systems in which the received pulses are sliced; they are also given for pulse-amplitude modulation. Noise resulting from random distribution of the i.f. phase at the onset of pulses is also considered. Optimum slicing level giving minimum output noise is evaluated.Signal/noise ratio and threshold formulae are obtained from these data for the pulse-modulation systems and are compared with those for conventional amplitude- and frequency-modulation systems having the same mean or peak power. Final conclusions as to the merits of all the above systems are drawn from graphs of output signal/noise ratio, in which the performance below threshold is also taken into account.

Noise problems in pulse communication

During 1945 and 1946 a complete sequential television system was constructed and tested. This was followed by the development of a simultaneous system, compatible with the present commercial monochrome television. This paper is the introduction to a group of two papers which describe the transmitting and receiving apparatus used in the simultaneous system.

An Experimental Simultaneous Color-Television System: Part I-Introduction

A microwave communication system giving two-channel voice transmission over a single carrier is described. Diplexing is accomplished by the use of two types of modulation, rather than by time or frequency division. One channel is transmitted by a form of pulse-time modulation using 1-microsecond pulses at a mean repetition rate of 10 kc, and the other channel by frequency modulation of the pulsed microwave carrier (FM burst). The addition of the pulsed-FM channel requires no additional carrier power or change of duty cycle. Cross talk is negligible. The double system is applicable to most of the common forms of pulse-time modulation. Time-division techniques are as applicable as in systems employing pulse-time modulation alone. If a time-division index of n is employed, i.e., that necessary to give n channels in a straight pulse-time time-division multiplex system, the employment of the diplex system described gives 2n channels. Compared to a straight pulse-time time-division system giving 2n channels, the double-modulation system results in economies in synchronizing equipment, and provides a better signal-to-noise ratio on the pulse-time portion of the system, but requires some additional bandwidth.

http://ieeexplore.ieee.org/iel5/10933/35788/01697832.pdf

Multiplex Employing Pulse-Time and Pulsed-Frequency Modulation

Thesis (M.S.) Massachusetts Institute of Technology. Dept. of Electrical Engineering, 1947.

Pulse-width and pulse-time modulators

Several pulse methods for the transmission of television sound on the picture carrier during the line-blanking intervals are analyzed from the points of view of signal-to-noise ratio, audio fidelity, and transmitter and receiver design. The advantages of duplex transmission are: (1) elimination of a separate sound transmitter; (2) elimination of the ambiguity and difficulty which may occur when a standard frequency-modulated sound signal is tuned in; (3) freedom of the audio output from the type of distortion which occurs in frequency-modulated receivers as a consequence of excessive drift of the frequency of the local oscillator; and (4) improvement of the phase characteristic of the picture intermediate-frequency amplifier resulting from elimination of trap circuits. The highest audio-modulation frequency in duplex systems must not exceed one half of the line-scanning frequency. This is a disadvantage under the present television standards which specify a line frequency of 15,750 cycles per second. With the exception of pulsed frequency modulation, the signal-to-noise ratios of sound in duplex systems are not so great as the ratio offered by the transmission of a standard frequency-modulated carrier. The comparison is subject to the condition that the amplitude of the frequency-modulated carrier is 0.7 of the peak amplitude of the duplex carrier. The signal-to-noise ratio of a pulsed frequency-modulated signal may equal the ratio of a standard frequency-modulated signal up to a critical distance from the transmitter, but is less at greater distance.

G.L. Fredendall

Papers

Transmission of Television Sound on the Picture Carrier