By Marc Ryba and Joseph B. Waltrich, Motorola Broadband Communications Sector
In a cable system carrying both analog and digital signals, the digital signals are typically run at ñ10 dB relative to the analog channels. This is done in order to minimize effects of digital third order distortion on the analog signals. Since distortions produced by digital signals are noise-like, their effect appears as a decrease in Carrier/Noise ratios in adjacent analog channels.
Selection of digital power levels is a compromise between minimizing effects of analog distortions (i.e.- burst errors) on the digital channels and maintaining an acceptable noise floor in the analog channels. This is of particular concern as the industry moves toward higher density modulation formats such as 256QAM. This paper will discuss the results of testing to determine optimum levels of digital signals added to a 77 channel analog multiplex.