THE L3 SYSTEM — TELEVISION TERMINALS 



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in the distortion of brightness values of the transmitted picture and 

 presumably will distort chromaticity values of color television signals. 

 With carrier transmission apparatus, in addition to these effects non- 

 linearity produces extraneous interference patterns at harmonics of the 

 carrier frequency. In L3 with a carrier modulated spectrum concen- 

 trated near 4.139 mc, the second harmonic distortion of line repeating 

 amplifiers produces a new spectrum concentrated near 8.278 mc, which 

 is demodulated by the receiving terminal to the region near 4.139 mc' 

 This form of distortion is considerably more disturbing in the final 

 picture than a comparable distortion of brightness values and constitutes 

 a limit to transmission signal-to-noise performance. 



Advantage is taken of the spectral distribution of energy of television 

 signals to ameliorate somewhat the effects of second harmonic distortion. 

 A pre-emphasis network is employed in the transmitting terminal to 

 accentuate the amplitude of high frequency components of the signal 

 before transmission. At the receiver a restorer network introduces a 

 complementary frequency characteristic to make the overall transmission 

 characteristic constant Avith frequency, (see Fig. 8). The restorer net- 

 work, de-emphasizing the high frequency components, likewise sup- 

 presses the second harmonic distortion signals. A limit to the amount of 

 predistortion permitted is set by the maximum amphtudes expected 

 of the high frequency picture components particularly in anticipation of a 

 high frequency color sub-carrier in color television systems. Tentatively, 

 the characteristic of Fig. 8 is chosen as a compromise of these factors. 



12 3 4 5 6 7a 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 8 — High frequency pre-emphasis characteristic. 



