Detection of Television Signals in Thermal Noise

This paper deals with the general problem of detecting television signals in thermal noise and with the methods of improving the performance of the second detector. A method of computing the thermal-noise spectrum and signal-to-noise ratios following the diode detector for the video and intercarrier-sound signals is described and experimentally verified. It is shown that the diode-detection process for very weak signals suppresses the desired video modulation and, in effect, creates additional noise above that introduced by the input stages. The principal factors for a given frequency component of the detected signal, are found to be the carrier-to-noise ratio, and the frequency response of the rf and IF amplifiers preceding the detector. Although the television viewpoint is emphasized, this analysis is particularly useful in analyzing the detection process of other AM transmission systems which use complex IF responses and several subcarriers. The theoretical and subjective aspects of improving the detection process are discussed, and a method of predicting the performance for some of the improved detection schemes is developed. Two improved detection circuits, an exalted-carrier IF and a product detector, which were constructed and tested, are described. It is found that such detection systems are capable of small improvements for very weak signals. 1, 1954.