the local stations were gradually either converted to regional relays or closed entirely and replaced by high-power regional programme transmitters. some local studios were retained to provide for programming from specific areas within each region. most transmitters also carried the bbc national programme on a local frequency to supplement the longwave broadcasts from 5xx, initially these were on three separate frequencies in order to minimise interference, but by 1939, as the regional programme network expanded, the national programme's three remaining mediumwave transmitters – at brookmans park (for london and the south east), moorside edge (for the north), and westerglen (for central scotland) – were all using 1149 khz.