THE L3 SYSTEM — TELEVISION TERMINALS 



921 



(a) VIDEO MODULATING 

 SIGNAL 



(b) EXCESS CARRIER RATIO 

 EQUAL TO 1.2 



(C) EXCESS CARRIER RATIO {0} EXCESS CARRIER RATIO 



EQUAL TO 1.0 EQUAL TO 0.5 



Fig, 4 — Carrier waves variously modulated by a composite video signal. 



amplitude. Excess carrier ratio, (ECR), for the waveforms of Figs. 4(b), 

 (c) and (d), respectively, are 1.2, 1.0 and 0.5. 



Modulated signals of the forms of Fig. 4(b) or 4(c) may be detected 

 by rectification, i.e., envelope detection. However, rectification of the 

 waveform, Fig. 4(d), produces a spurious envelope wherein video signals 

 which exceed a particular value are inverted. It is necessary to employ 

 homodyne detection, that is, a demodulator driven by a locally generated 

 carrier which is synchronous in phase angle and frequency with the 

 carrier component of the signal wave. As described later, homodyne 

 detection also makes possible the necessary suppression of the quadrature 

 distortion associated with vestigial sideband transmission. Quadrature 

 distortion associated with envelope detection is tolerated in the LI 

 coaxial system but with tandem operation of terminals required in the 

 L3 system, would accumulate to intolerable values. 



Vestigial Sideband Considerations 



A vestigial sideband signal is produced by a band shaping filter follow- 

 ing the modulator. In this filter the lower sideband is suppressed com- 



