Editing Analog television (section)

===Timebase circuits===
{{Further|Oscilloscope}}
In an analog receiver with a [[cathode-ray tube|CRT]] display sync pulses are fed to horizontal and vertical ''timebase'' circuits (commonly called ''sweep circuits'' in the United States), each consisting of an oscillator and an amplifier.  These generate modified [[sawtooth wave|sawtooth]] and [[parabola]] current waveforms to scan the electron beam.  Engineered waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface.  The oscillators are designed to free-run at frequencies very close to the field and line rates, but the sync pulses cause them to reset at the beginning of each scan line or field, resulting in the necessary synchronization of the beam sweep with the originating signal. The output waveforms from the timebase amplifiers are fed to the horizontal and vertical ''deflection coils'' wrapped around the CRT tube. These coils produce [[magnetic field]]s proportional to the changing current, and these deflect the electron beam across the screen.

In the 1950s, the power for these circuits was derived directly from the mains supply. A simple circuit consisted of a [[Series circuit|series]] voltage dropper [[Electrical resistance|resistance]] and a [[rectifier]]. This avoided the cost of a large high-voltage mains supply (50 or 60&nbsp;Hz) [[transformer]]. It was inefficient and produced a lot of heat.

In the 1960s, [[semiconductor]] technology was introduced into timebase circuits.  During the late 1960s in the UK, [[Synchronization (alternating current)|synchronous]] (with the scan line rate) power generation was introduced into [[Solid state (electronics)|solid state]] receiver designs.<ref>{{cite web|url=http://www.oldtellys.co.uk/otpsupps.html|title=TACKLING THE POWER SUPPLY|work=Publication date&nbsp;– unknown|publisher=Old Tellys.co.uk|access-date=24 November 2010|url-status=live|archive-url=https://web.archive.org/web/20120303043022/http://www.oldtellys.co.uk/otpsupps.html|archive-date=3 March 2012}}</ref>

In the UK use of the simple (50&nbsp;Hz) types of power, circuits were discontinued as [[thyristor]] based switching circuits were introduced. The reason for design changes arose from the electricity supply contamination problems arising from [[Electromagnetic interference|EMI]], and supply loading issues due to energy being taken from only the positive half cycle of the mains supply waveform.<ref>{{cite web|url=http://www.yorkemc.co.uk/conferences/emcYork2003/potm/2004-01_SMPS-and-SELC.pdf|title=An Investigation into the EMC Emissions From Switched Mode Power Supplies and Similar Switched Electronic Load Controllers Operating at Various Loading Conditions|publisher=York EMC.co.uk|access-date=24 November 2010|archive-url=https://web.archive.org/web/20120315084947/http://www.yorkemc.co.uk/conferences/emcYork2003/potm/2004-01_SMPS-and-SELC.pdf|archive-date=15 March 2012}}</ref>