AUTOMATIC FINE TUNING (AFT)
The local
oscillator frequency in the RF tuner is set to provide exact IF frequencies.
However, despite many remedial measures to improve the stability of the
oscillator circuit, some drift does occur on account of ambient temperature
changes, component aging, and power supply voltage fluctuations and so on.
For a
monochrome receiver a moderate amount of change in the local oscillator
frequency can be tolerated without much effect on the reproduced picture and
sound output. The fine tuning control is adjusted occasionally to get a sharp
picture. The sound output is automatically corrected because of the use of
inter carrier sound system. However, requirements of frequency stability of the
local oscillator in a colour TV receiver are much more stringent.
This is
so, because proper fine tuning of colour receivers is judged both by the
sharpness of the picture and quality of colour. If the local oscillator
frequency becomes higher than the correct value, the picture IF, subcarrier IF
and sound IF frequencies will also become higher by the same amount and fall on
incorrect positions on the IF response curve.
This will
result in poor picture quality because the amplitudes of low-frequency video
sidebands clustered around the picture IF will decrease. At the same time
chrominance signal sidebands clustered around the location of subcarrier will
receive more gain and hence become stronger than normal.
Similarly,
if the local oscillator frequency changes to become less than the desired value
opposite effects would result and colour reproduction will become weak.
In case
the decrease in frequency is more than 1 MHz, the colour burst may be lost and
only a black and white picture will be seen on the screen.
Similar
troubles can also result in receivers that employ remote control tuning because
of non-availability of fine tuning control and imperfections of the
mechanical/electronic system employed for channel selection.
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