BRE (Bandwidth Reduction Encoding) operation started with analogue HD video (even when the source was a digital recorder, it was reconverted to analogue to feed the encoder). It was specified to have a 50 Hz field frequency. It could be interlaced, with 25 frames a second (called 1250/50/2 in the recommendation), or it could be progressively scanned with 50 full frames a second (called 1250/50/1). The interlaced version was the one used in practice. In any case, the number of visible lines was 1152, twice the standard 576 lines vertical definition. The full number of lines in a frame period, included those that cannot be displayed, was 1250. This made for a 32 µs line period. According to ITU recommendation for HDTV standards parameters the active part of the line was 26.67 µs long (see also the LDK 9000 camera document ).