As shown in Fig.8, the correlation muting circuit consists of all-pass filter AP2 connected in series with fm. demodulator all-pass filter API and adjusted by an external capacitor to provide a total phase shift of 180*. The output from AP2 is applied to mixer M3 which determines the correlation between the undelayed limited i.f. signal at one of its inputs and the delayed and inverted version of it at its other input.The output from mixer M3 controls a muting circuit which feeds the demodulated audio signal to the output when the correlation is high, or feeds the output from a noise source to the output to give an audible indication of incorrect tuning when the correlation is low. The switching of the muting circuit is progressive (soft muting) to prevent the generation of annoying audio transients. The output ftom mixer M3 is available externally at pin l and can also be used to drive a detuning indicator.
Figure 9 shows that there are two regions where the di. modulated audio signal is fed to the output because the muting is inactive. 'One region is centred on the correct tuning point fL. The other is centred on the image frequency -fL. The image response is therefore not suppressed by the muting system when the frequency-locked loop is open. When the loop is closed, the time-constant of the muting system, which is determined by external capacitor Cl, prevents the image response being passed to the audio output. This is described under the next heading.