Noise level circuitry for facsimile transmission

Claim 1

IN A FACSIMILE COMMUNICATION SYSTEM FOR TRANSMITTING VIDEO SIGNALS REPRESENTATIVE OF THE LIGHT VALUES OF INTELLIGENCE ON A DOCUMENT, A TRANSMITTER COMPRISING: CATHODE RAY TUBE SCANNING MEANS FOR REPEATEDLY SCANNING SUCCESSIVE PARALLEL LINE PATHS ACROSS THE DOCUMENT WITH A LIGHT SPOT, PHOTOMULTIPLIER MEANS RESPONSIVE TO THE REFLECTED LIGHT FROM THE DOCUMENT FOR PRODUCING VIDEO SIGNALS IN ACCORDANCE WITH THE REFLECTED LIGHT VALUES INDICATIVE OF THE INTELLIGENCE AND THE BACKGROUND ON THE DOCUMENT DURING EACH SCAN OF SAID SCANNING MEANS, A READER CIRCUIT COUPLED TO SAID PHOTOMULTIPLIER MEANS RESPONSIVE TO CHANGES IN THE BACKGROUND ON THE DOCUMENT, INCLUDING MEANS TO PRODUCE AN OUTPUT VOLTAGE LEVEL INDICATIVE OF THE BACKGROUND LEVEL IN THE VIDEO SIGNAL RESULTING FROM THE CHANGES, A SECOND CIRCUIT COUPLED TO SAID READER CIRCUIT FOR RECEIVING THE OUTPUT VOLTAGE THEREOF, SAID SECOND CIRCUIT INCLUDING MEANS TO PRODUCE A NOISE CLIPPING OUTPUT VOLTAGE LEVEL AS A FUNCTION OF SAID BACKGROUND OUTPUT LEVEL WHEREBY THE WAVEFORM OF THE NOISE CLIPPING LEVELS IS SIMILAR TO THE WAVEFORM OF THE BACKGROUND LEVEL,