BERT mode doesn't count lost frames

[MustBeArt]

opv-demod processes everything a frame at a time. Bit error rate test (BERT) mode counts error bits and total bits in the frames it processes and reports the bit error rate on screen. That means it doesn't count lost frames as bit errors! That's not what we expect for the bit error rate.

We want the receiver to handle BERT mode automatically, and to be able to join a BERT transmission in progress, so it still can't do anything about frames lost before reception starts. But it can count lost frames until the BERT measurement ends, if we add a highly reliable end-of-BERT-mode postfix to the BERT transmission.