THE L3 SYSTEM — TELEVISION TERMINALS 925 



determined from a predominantly black picture. A carrier oscillator, 

 phase synchronized to the average carrier phase of the signal would 

 execute 180° phase reversals as picture content changed from average 

 white to average black, producing sudden video polarity reversals at 

 the demodulator output. 



This signal carrier polarity ambiguity which is momentary in charac- 

 ter can be exchanged for one which is not time variable by a multiplica- 

 tion operation. The modulated signal is squared, i.e., multipHed by itself 

 on an instantaneous basis, in a square law circuit. Such an operation 

 squares carrier amplitude and doubles carrier frequency and phase angle, 

 the latter effect converting 180 degree phase reversals into 360 degree 

 changes which are indeterminable in the average phase detector. Under 

 these conditions the phase synchronized demodulator carrier supply, 

 stably locked to the average phase of the squared signal, experiences no 

 phase reversals with change in picture content. The ambiguity now is in 

 the determination of incoming signal polarity. The squaring operation 

 eliminates any basis for determining polarity so that the demodulator 

 carrier may with equal likelihood lock to either polarity relative to the 

 signal and thereby at the demodulator output produce video signal wave- 

 forms of either polarity. 



The method used to secure phase synchronization of the local receiver 

 oscillator to the received signal is described next with reference to Fig. 7. 

 Signal from the line together with the output from the carrier oscillator 

 are brought to the demodulator where the desired video output signal 

 is obtained as the lower sideband of the modulation product. This process 

 has already been described, equations 1 to 3. In the carrier regeneration 

 process signal and carrier phase shifted by 45 degrees (equations 4 & 5) 

 are each squared in square law circuits. 



V(t) = P Cos ct + Q Sin ct, (4) 



C(t) Z45° = ± Cos (ct - <l> - t/4:). (5) 



Band pass filters select from the squaring circuit output signal frequencies 

 in the neighborhood of twice carrier frequency. From the signal squarer, 



i(P' - Q') Cos 2ct + PQ Sin 2ct (6) 



and from the carrier squarer, 



i Sin {2ct - 2<^). (7) 



The two squared signals at twice carrier frequency are multiplied to- 

 gether in a product modulator. This product contains signals in two 



