In analogue TV technology, residual carrier is the ratio of carrier level which is modulated by the maximum video signal to the unmodulated carrier level.
Video signal (VF) or more formally composite video signal (CVS) is the signal which carries the video information as well as some auxiliary signals for synchronizing. In all systems 0-300 mV level is reserved for auxiliary signals and 300-1000 mV is reserved for video information. In monochrome TV, 300 mV (or sometimes 350 mV) corresponds to black and 1000 mV corresponds to white.( In color TV depending on the system used, superimposed color subcarrier may have slightly higher values. )
The signal modulates a carrier (usually an IF carrier) by AM (C3F) technique. This modulation is known as negative modulation. Because unlike more familiar positive modulation, increase in modulating signal (in this case VF) level decreases IF level and vice versa. So, when there is no modulating signal IF has the full level and when there is a modulating VF signal the level of IF is lower. Since VF black level (level when transmitting a black scene) is lower than the white level, the level of IF when transmitting black is higher than the level of IF when transmitting white.
The modulators are set to yield % 10 or % 12.5 of carrier level when modulated by 1 V signal (which corresponds to white scene) This percentage is known as residual carrier.
An example of the modulated carrier is shown in the accompanying figure. In this figure, the level of the modulating VF signal is 1 volt (white level). 1 V. yields residual carrier (% 10) . Short duration full carrier ( % 100) is the sync pulse and the % 73 is so called back porch (black level) . For clarity, the frequency of the carrier in the figure is chosen to be much less than IF carrier. The duration of the signal is approximately 100 µs.
References and notes
- Reference Data for Radio Engineers ITT-Howard Sams Inc.,New York, 1977 Chapter 30.
- Actually the modulation is known as Vestigial side band modulation which is a version of amplitude modulation.
- Broadcast Engineers'Handbook