Signal injection technique

Abstract

In a digital information transmission system having an analog-to-digital converter (ADC), an information transmission medium, and a digital-to-analog converter (DAC) all located between a transmitting station and a receiving station, signal transmission is improved by injection of a controlled signal in the form of a symmetric triangle wave sweep having a maximum peak-to-peak amplitude exceeding the magnitude of a few quantization intervals of the ADC and the DAC, introducing frequency components concentrated outside the system frequency spectrum. A class of controlled signals is disclosed. In the preferred embodiment, the controlled signal is injected into the analog information input signal prior to conversion to digital form. The injected signal predominates whenever the amplitude of spurious noise signals is less than the amplitude of the injected signal and the level of the input of the ADC is near a quantizing interval transition point so that subsequent analog output signals from the DAC have frequencies concentrated outside the frequency spectrum of the system. The DAC output signals are subsequently filtered by a post-sampling filter having a pass-band characteristic coextensive with the system frequency spectrum so that the injected signal components are removed before coupling to the signal output utilization device. Alternatively, a digital representation of the controlled signal may be added to the information signal after conversion to digital form and subsequently filtered from the information signal.