Differential Phase and Gain Measurements in Digitized Video Signals

The peculiar nature of the standard test signal currently used in analog television system evaluation treats a digitized video signal in such a way that the true performance of the system under test (VTRs in particular) is not revealed. The common test signal for measuring DP and DG in a VTR is a 20-IRE-unit subcarrier superimposed either on a 100-IRE-unit ramp signal or on a stair-step signal. Experience has shown that even though the VTR circuits are only fractionally loaded, the results of DP and DG measurements may be easily interpreted for fully loaded (saturated color) operation. This is not true for combined analog and digital systems because quantizing errors, once introduced, have a fixed peak value so that they actually represent a smaller and smaller proportion of the signal as the signal is made greater. Standard measurements of DP and DG in high-quality digital systems can be so misleading as to show errors of more than 8% DG and 5° DP. Two improvements in measuring techniques are proposed: first, the subcarrier amplitude of the test signal should be doubled to 40 IRE units peak-to-peak, and second, the subcarrier phase should be made random during the ramp or stair-step. These changes will not eliminate the problem but will make meaningful measurements easier to obtain.

Print ISSN

0036-1682

Published

1976-02

Content type

Original Research

DOI

10.5594/J13299