Many individual impairments could be immediately identified by certain distortions of the picture. This is no longer possible with digital. Digital signals degrade differently than analog. D igital channels will provide full-quality pictures up to a substantial impairment level, thanks to forward error correction, data interleaving and other techniques. But this also means that technicians are no longer able to judge signal degradation by simply looking at the television picture.
Measurements are the primary means to understanding digital channel transmission performance.
MER is one of a class of measurements that are made in-service and use data derived from the signal's constellation. MER is defined as the ratio of average power in an ideal QAM constellation to average power in the error vectors. It is expressed in dB, a familiar measurement unit. Very short-term BER providing early indication of error rate activity. Further, MER provides the earliest view of transmission quality degradation resulting from noise, ingress and composite distortions.