output). As a result, impulse interference would cause visual dark spots before it was large enough to affect the synchronisation of the picture. If the interference was large enough, the picture was probably unwatchable anyway. The later introduction of flywheel sync circuits rendered the picture much more stable, but these could not have alleviated some of the problems with positive modulation. Almost all television systems that succeeded the 405-line system adopted negative modulation for this reason alone. Automatic gain control The AGC circuit was problematic. First-generation AGC merely detected the average value of the transmitted signal; however, due to the positively modulated carrier, peak power represented peak white – not guaranteed to be present. Thus for a completely black picture, the AGC circuit would increase the RF gain to restore the average carrier amplitude. The result was a screen that was not black but mid-grey. In fact, the total light output of early TV sets was practically constant regardless of the picture content. By the mid-1950s, several manufacturers started to introduce gated-AGC systems to avoid this issue. A delayed pulse was derived from the recovered line-sync signal. This pulse would trigger a gate which would sample the received video signal during the "back porch" which was a guaranteed black-level transmitted between the end of the line-sync pulse and the start of the picture information. The introduction of negative modulation in later systems simplified the problem because peak carrier power represented sync pulses (which were always guaranteed to be present). A simple peak-detector AGC circuit would detect the amplitude of only the sync pulses, thus measuring the strength of the received signal. Whistle due to line output transformer magnetostriction The 405-line system produced a noticeable 10,125 Hz whistle in many sets, equal to the number of lines per second. This high-pitched whistle was caused by magnetostriction in the line output transformer. This is a common artifact in sets that use a cathode ray tube. While all CRT-based television systems produce such a noise, the higher number of lines per second in later standards produces frequencies (PAL's 15,625 Hz and NTSC's 15,734 Hz) that are at the upper end of the audible spectrum, which not all people are able to hear. Modern sets using plasma, LCD or OLED display technology are completely free of this effect as they are composed of a million or more individually controllable elements, rather than using a