Several proposals have been made to augment the channel capacity of existing or proposed CATV systems by adding compressed digital video signals to conventional analog VSB signals. Such a proposal brings with it the difficulties of quantifying the degradation of broadband distortion performance (CTB, XMOD, DSO and CSO) associated with the additional loading caused by the new digital signals. Conventional techniques that measure device and system distortion performance by modeling the digital signals with CW carriers at the picture carrier frequencies of conventional analog VSB channels will give extremely disappointing results.
A new technique is described that models the digital signals in a manner that accurately represents the digital channel energy spectral density. Measurements using this technique are given that accurately predict distortion performance that is far better than that predicted by conventional CW carrier techniques.
A simple means of generating this signal is described. Test results are given that compare the performance of test devices when loaded with only conventional analog VSB signals to the performance of the test devices when loaded with mixed analog and digital signals.