THE L3 SYSTEM — TELEVISION TERMINALS 



923 



normal double sideband output of the modulator supplied with video 

 and carrier signals. The other component, Fig. 5(c), represents the out- 

 put of a modulator supplied with carrier and signal voltages each shifted 

 in phase by 90° and with the amplitude attenuated as shown in the 

 vestigal region Fc d= Fv. The modulation transmitted by a circuit with 

 the response function of Fig. 5(c) is called the quadrature component 

 and is related to the normal modulation, depending upon the shape 

 and extent of the vestigial sideband. Fig. 6 illustrates the real and 

 quadrature components of an idealized rectangular wave form demodu- 

 lated after transmission over circuits having the response functions of 

 Fig. 5, respectively. 



The 90° shift of carrier frequency in the quadrature component of 

 the vestigial sideband signal makes possible the suppresssion of this 

 component. The transmitted vestigial sideband signal may be written 



V{t) = P{t) Cos ct + Q{t) Sin ct, (1) 



where c = 27r times carrier frequency and P{t) and Q{t) are "real" and 

 quadrature modulating functions^ typically as represented on Fig. 6. 

 The demodulator may be regarded as an ideal multiplier of signal and 

 carrier supply. Let the carrier supply be denoted: 



C{t) = Cos {ct - 0), (2) 



where is the phase of the receiver carrier supply relative to the carrier 

 factor of the ''real" component of the signal. The demodulator output 

 is the product V{t) X C{t). A low-pass filter rejects the output com- 

 ponents in the band of frequencies about twice carrier frequency so that 

 the demodulated video signal output is the lower frequency component. 

 Thus, neglecting a factor of 3^, 



Vo(0 = P(0 Cos 4> -f Q{t) Sin 0. (3) 



It is seen that real and quadrature video components in the demodulator 



ji 



_^y 



..(p: 



IT— ^- 



\ ^'quadrature, (Q) 



Fig. 6 — Real and quadrature components of a rectangular pulse after vestigial 

 sideband transmission. 



