THE L3 SYSTEM — EQUALIZATION AND REGULATION 853 



3. The controls are easily adjusted using methods to be described. 



4. If better equalization is required at any point the existing equahzer 

 is retained, additional harmonic terms are added. 



5. The major portions of the equahzers require only one value of 

 inductance and two values of capacitance to form the delay Hues used 

 in the networks. This also assists in the application of distribution re- 

 quirements. 



6. The manual equalization can be designed with a minimum of 

 information about the system characteristics. 



7. The equahzation is on a least square error basis rather than mini- 

 mum peak error. 



The networks used to realize these cosine shapes are constant re- 

 sistance Bode regulating networks^ employing second degree all-pass 

 sections. If the phase of the all-pass sections were made proportional 

 to frequency, the transmission performance within the frequency band 

 of interest would be that provided by a Fourier series composed of 

 cosine terms in the variable oj. By appropriate choice of the all-pass 

 sections the frequency-phase relationship can be warped to give greater 

 weighting to a specified portion of the frequency range. The phase of 

 the all-pass section is given by 



* = -»'-[l(7-s)]- »>> 



Small h and high fc weights the low frequencies, the Unear phase case 

 h = 1.2, fc = 10.2 mc gives uniform weighting and large h weights the 

 high frequencies. A 6 = 2, /c = 13.75 mc, was selected for the L3 equal- 

 izers because it weights somewhat the higher frequencies where the 

 television signal is transmitted and second, each unbalanced bridge T 

 network section can be constructed with only four elements, two like 

 inductors and two capacitors. For 6's smaller than 2, coupling between 

 the two like coils is required, and for 6's larger than 2, an additional 

 element is required. 



The all-pass networks are designed on a 75-ohm impedance level 

 and thus the flat, normal setting of each regulating network is 4.18 db 

 maximum. The 75-ohm level makes the series and shunt networks iden- 

 tical and also facihtates manufacturing testing. A special dual variable 

 resistor is used as the control element. It has a resistance range of 15 

 to 375 ohms and provides a regulation range of ±2.78 db maximum. 



A schematic of this network is shown on Fig. 8. In the case of the 

 harmonic, flat gain, the phase sections are omitted. For the nth harmonic 



