Francis H. Hilbert

Stereo presence signal for an AM stereo system

In an AM broadcast transmitter, an infrasonic tone is added to the difference channel of a compatible AM stereo system to provide a stereo presence signal. In a stereophonic receiver, the corrected output of a synchronous detector in the difference signal channel is coupled to an infrasonic detector for control of a mono/stereo mode switch and a stereo presence indicator. When the received signal is weak, the infrasonic tone is effectively stronger, because the phase angle is increased. Since the s/n ratio is proportional to the phase angle, the stereo presence signal is self-adjusting. The mode switch and indicator are enabled for all values of L and R. The infrasonic tone could also be removed from the transmitted signal whenever the difference signal goes above a predetermined level during stereo broadcasting, in which case a comparator at the receiver would enable the mode switch and indicator upon detection of either a difference signal or the infrasonic tone.

A Compatible Quadrature System for AM Stereo

The proposal contained herein has been devised to protide a stereophonic broadcasting system which provides a compatible monophonic signal whose spectrum most closely resembles that of the existing monophonic transmission, consistent with the transmission of stereo information, in order to insure the highest degree of compatibility with existing monophonic receivers. In addition the system is designed to provide a minimum of transmitter modification and a stereo receiver of relatively simple design.

Stereo and Color Signal Demodulation with Integrated Circuit Techniques

This paper outlines general considerations of silicon monolithic integrated circuit design. It then reviews principles of gate demodulation with special emphasis on stereo and color signal demodulations. The ideas presented are merged to form the basis for discussion of the design and development of two monolithic integrated circuits. An FM broadcast stereo demodulator is described which encompasses the functions of subcarrier regeneration as well as additional functions of inter-station mute, weak signal demodulator defeat, indicator lamp driver, and inter-stereo-station mute. A versatile monolithic color demodulator is described which is capable of either direct or difference signal color demodulation. In addition, this IC has provisions for blanking and the option of brightness/luminance control.

A Method of Directly Demodulating Color Signals

Color TV signal evolution is examined from a sampling encoder (modulator) viewpoint. This logically leads to an analysis of decoding (demodulating) the NTSC color signal with sampling techniques. In the process of enumerating and solving the problems that arise by attempting to demodulate by time - interruption of the signal, basic concepts relating to direct demodulation of the NTSC signal are derived.