A NEW AFTCONCEPTFOR USEWITHA MONOLITHICl.C.
Peter J. Janssen · 1970
1.Summary Unlike blackandwhitetelevisionreceivers, acolortelevisionreceiver onlypermits moderate detuning before colorpicture quality deteriorates. Thisisduetothecloseproximity oftheuppercolorsidebandandaccompaning soundcarrier, together withahigh degreeofsoundcarrier suppression required onthe luminance-chrominance detector. Conventional automatic finetuning circuits withadiscriminator onpicture I.F.carrier frequency notnecessarily adjust thesoundI.F.carrier tooptimumsound suppression: duetomisalignment, temperature drift or ageing, thefrequency ofoptimumsoundcarrier suppressionmaynotdiffer exactly 4.5MHz fromtheAFT discriminator zerofrequency. Uptill recently itwasthought impossible tousetheintercarrier frequency fortuning purposes, becausethisfrequencyisonlydetermined inthetransmitter andequalto thepicture tosoundcarrier frequency separation. A novelsystemofgenerating AFT control voltage onintercarrier frequency willbedescribed. Useismadeoftheabrupt phasechangeofthefirst I.F. soundcarrier onbothside's' ofoptimumsoundcarrier suppression.systemhastheadvantage, thatthesoundtrapitself determines thecorrect tuning, thusnoseparate adjustmentoftheAFT discriminator zeroonsoundtrapfrequency+4.5MHz isrequired. TheAFT function nowbeingperformed on4.5MHz,the amplifier-limiter andphasediscriminator canbebuilt in onemonolithic integrated circuit.