A level compensator for telephotograph systems

SINCE 1936, a number of different telephotograph systems1 have been operated over the lines of the Bell System on a leased-wire basis. The requirements which a line facility must meet for such service are in several respects more severe than the requirements for voice-message service. Telephotograph ¿ transmission requirements have usually specified that abrupt variations in line net loss should be limited to ±0.2 decibel, single frequency noise power should be at least 50 decibels below maximum signal power, and throughout the useful frequency band (1,200 to 2,600 cycles per second), the envelope delay should be equalized within ±300 microseconds. Such requirements were originally satisfied by using 4-wire H-44¿25 side circuits in cable, where available, and elsewhere by using 2-wire open-wire side circuits. These facilities were delay-equalized over the necessary frequency range, and precautions were taken to minimize various transmission disturbances.