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.