924 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



output exist ill the same proportion as the components of the demodula- 

 tor carrier supply in phase and in quadrature respectively with the real 

 carrier component of the vestigial signal. By providing carrier exactly in 

 phase with the real component of the signal the quadrature component 

 in the output may be suppressed completely. It has been determined 

 that to suppress the quadrature component resulting from the L3 ves- 

 tigial band shape to barely perceptible (threshold) values the phase angle 

 of the carrier regenerated at the receiver must be maintained to an ac- 

 curacy of plus or minus 2.5 degrees. A requirement for one demodulator, 

 when six pairs of terminals contribute to produce quadrature distortion 

 at threshold value, becomes 2.5 degrees divided by the square root of 

 six, or about one degree. 



The regeneration of carrier at the receiver is one of the principal L3 

 terminal features. Here a 4.139-mc carrier must be provided to de- 

 modulate the "over-modulated" L3 signal. The required carrier must be 

 reconstituted from information carried in the signal itself. It would be 

 possible, of course, to transmit separately a signal from which carrier 

 frequency could be derived but carrier frequency is really the smallest 

 part of the required information. It is the phase angle of the carrier of 

 the received signal which must be duplicated closely at the demodulator 

 and separate transmission of carrier phase angle does not seem feasible. 

 A phase controlled oscillator is employed for the carrier supply at the 

 receiver, with phase control obtained from information residing in the 

 signal itself and frequency synchronization an additional burden upon 

 the phase control system. 



The basis for synchronizing the receiver oscillator to the carrier of 

 che received signal lies in the phase angle of the carrier frequency com- 

 ponent of the vestigial sideband signal averaged over a period of time of 

 the order of one frame scanning period. Referring to Fig. 5(b) and 5(c) 

 again, it may be noticed that the quadrature response function is zero 

 at carrier frequency. This means that the quadrature component of 

 the transmitted signal contains no carrier frequency component and will 

 not affect the determination of the real carrier component phase angle 

 based upon averaging over a sufficient period of time. Another signal 

 characteristic presents more serious problems. The degree of modulation 

 employed in L3, shown in Fig. 4(d), makes the, average carrier polarity 

 indeterminate. That is, the carrier polarity for a video amplitude cor- 

 responding to picture white is opposite to that corresponding to picture 

 black or sync pulses. The polarity reverses as the composite signal 

 changes through its half peak-to-peak value. The average polarity 

 determined from a predominantly white picture is thus opposite to that 



