The MER and EVM help to estimate the proximity of the RF Video digital cliff. But once the MER begins to reach its minimum value (and EVM begins to reach its maximum value), the BER of the received signal is so great, that the quality of the picture and sound becomes unusable. MER measurements are able to measure small changes in RF Video transmitter and system performance and are one of the best single figures-of-merit for any cable and satellite transmission system. EVM and more traditional BER are useful for standard RF Video cross-equipment checks and as an aid to identify short-term signal degradation.
The constellation display can be considered as an array of 2-D eye diagrams of the RF Video signal. The closer the points are together in the cloud of received symbols, the better the RF Video signal quality. Since the diagram maps amplitude and phase on the screen, the shape of the array can be used to diagnose and determine many system or channel faults and distortions, and help pin down the cause. Following installation and adjustment of modulators, amplifiers, and splitters, modulation impairments can appear due to various I and Q vectors being distorted in amplitude or phase. Constellation displays help provide a reliable health check for RF Video transmission systems by indicating artifacts, distortion, or equipment drift.
By combining these critical RF measurements with comprehensive MPEG transport stream monitoring and alarming, RF Video system problems can be detected at an early stage, before viewers are affected.