920 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



the carrier frequency its multiplication by the input signal generates 

 a series of double sideband output spectra, each centered about one of the 

 harmonics of the carrier. It happens in this case that the lower sideband 

 of the third harmonic spectrum of the carrier modulator contains fre- 

 quencies low enough to overlap the high frequencies of the carrier spec- 

 trum upper sideband and this overlap results in quite visible picture 

 distortion. 



It is possible, by employing a second modulator paralleling the first 

 but driven with a frequency three times the carrier frequency, to generate 

 a signal spectrum centered at carrier third harmonic which will cancel 

 the corresponding output of the carrier modulator. Successful translation 

 of the video spectrum to the L3 carrier band in a single modulation step 

 depends upon the maintenance of this and other modulator balances to 

 unusually stringent requirements. 



The carrier supply oscillators for the modulators and demodulators 

 are of the Meacham bridge type^ with quartz crystal frequency control 

 and thermistor amplitude control. Frequency stability of two parts per 

 million is required for successful carrier regeneration at the receiver. 

 A constant temperature oven for the quartz plus the inherent stabihty 

 of the bridge type circuit is expected to provide the required frequency 

 stability between monthly maintenance periods. 



A feature of the signal transmitted over the L3 system is a degree 

 of modulation which exceeds the value commonly referred to as 100 per 

 cent modulation. The resulting waveform contains a maximum ratio of 

 information to peak carrier, important from the standpoint of optimum 

 signal to noise performance. Fig. 4 shows progressively the reduction 

 in peak carrier amplitude which may be effected by subtraction of carrier 

 component from a modulated signal. Figs. 4(b), (c) and (d) each contain 

 the same amplitude of video modulation. Fig. 4(b) represents a video 

 modulated carrier signal with maximum carrier occurring at tips of 

 synchronizing pulses and a minimum carrier, equal to 20 per cent of 

 maximum carrier, corresponding to picture white. Fig. 4(c) represents 

 the same signal as Fig. 4(b) except that the 20 per cent excess carrier 

 has been subtracted. This is the 100 per cent modulation case. Fig. 4(d) 

 shows the effect of further carrier subtraction, (addition of negative 

 carrier), to reduce to a minimum the peak amplitude of the modulated 

 signal. The waveform of Fig. 4(d) employed for L3 transmission requires 

 lyi db less maximum carrier power than that of Fig. 4(b) for the same 

 transmitted information. The term "excess carrier ratio" has been de- 

 vised to describe degrees of modulation which exceed 100 per cent. It 

 is the ratio of peak carrier amplitude to the peak-to-peak modulation 



