Theoretical and experimental characteristics of random noise in television

Consideration is given to certain aspects of random noise which may be important in television, namely that due to electron current fluctuations in valves or thermal agitation of electrons in resistances. The relationship between frequency spectrum and the time variation of random noise is shown. In particular, a fluctuating low-frequency envelope is associated with ¿quasi-triangular¿ noise, that is, noise peaked at the high-frequency end of the pass-band. A non-linear transfer characteristic is next considered. Low frequency components are then produced from quasi-triangular noise, these terms arising ftom the fluctuating envelope. Phase distortion after the non-linearity has appreciable influence on noise properties. The relevance of these effects to noise visibility on a picture tube is examined, and oscilloscope photographs are given. For random noise plus a sinusoidal signal, non-linearity gives rise to intermodulation products of the noise and signal. This is of special significance if the signal is near the top of the pass- band, and the noise is quasi-triangular, since in this case the presence of the low- frequency intermodulation products drastically increases the noise visibility on a picture tube. It is shown that this intermodulation effect may be produced by the non-linear characteristic of the picture tube.