videotapes still survive. However, the majority of surviving 405-line programmes are in the form of black and white film telerecordings, usually with optical soundtracks. Occasionally video re-recording would be employed instead, with a 625-line camera pointing at a 405-line monitor. This preserves the original 50-field interlaced format, but with some geometrical distortions owing to the curvature of the CRT monitors used at the time. Modern 405-line programming may be recorded and played on an unmodified VHS or Betamax video recorder, as long as the input to the recorder is baseband rather than RF. Thus, various modern video recordings of 405-line programming also exist. Betamax was sometimes preferred for this, as the dropout compensator could be switched off on certain models for use with PCM digital audio decoders. System A 405-line is System A in the CCIR assignment of broadcast systems. The audio uses amplitude modulation rather than the frequency modulation in use on modern analogue systems. In addition, the system was broadcast in an aspect ratio of 5:4 until 3 April 1950, when it changed to the more common 4:3 format. All System A transmitters used vestigial sideband transmission, with the single exception of Alexandra Palace in London, which closed down in 1957 when it was replaced by Crystal Palace. Why 50 fields per second? Since the mid-1930s it has been standard practice to use a field frequency equal to the AC mains electric supply frequency (or a submultiple thereof), 50 Hz in most countries, (60 Hz in the Americas) because studio lighting generally uses an alternating current supply to the lamps and if these were not synchronized with the field frequency, an unwelcome strobe effect could appear on TV pictures. Secondly, the smoothing (filtering) of power supply circuits in early TV receivers was rather poor, and ripple superimposed on the DC could cause visual interference. However, the main problem was the susceptibility of the electron beam in the CRT being deflected by stray magnetic fields from nearby transformers or motors. If the picture was locked to the mains frequency, this interference would at least be static on the screen and thus relatively unnoticeable. The very earliest TV sets used a mains transformer; care had to be taken in design to prevent the transformer's stray magnetic field from disturbing the electron beam in the CRT. Why 405 lines? Because an interlaced system requires accurate positioning of scanning lines, the horizontal