 a signal of a given strength is introduced to the input of the set. Typically a broadcast tuner may give a quieting level of 30 dB for an input level of just over a microvolt. 156 Newnes Guide to Radio and Communications Technology To remove the high level of noise produced when no signal is present, a 'squelch' circuit may be included. This mutes the audio when no signal is present and there is a high level of noise at the output. Many scanners and hand-held VHF or UHF transceivers have a control to adjust the signal level below which the audio cuts off. By using this control the set can be adjusted to detect very low level signals if necessary. Hi-fi tuners also incorporate a squelch circuit in most instances. However, they seldom have a control to adjust as it is usually not a requirement to hear signals which are only just above the noise level. Another effect which is often associated with FM is called the capture effect. This can be demonstrated when two signals are present on the same frequency. In cases of this nature, only the stronger signal will be heard at the output, unlike AM where a mixture of the two signals is heard, along with a heterodyne if there is a frequency difference. The capture effect is often defined in receiver specifications as the capture ratio. It is defined as the ratio between the wanted and unwanted signal to give a certain level of the unwanted signal. Normally a reduction of the unwanted signal of 30 dB is used. To give an example of this the capture ratio may be 2 dB for a typical tuner to give a reduction of 30 dB in the unwanted signal. In other words if the wanted signal is only 2 dB stronger than the unwanted one, the audio level of the unwanted one will be suppressed by 30 dB. A number of circuits can be used to demodulate FM. The very simplest form of FM demodulation is known as slope detection or demodulation. It consists of a tuned circuit that is tuned to a frequency slightly offset Lower limit on Carrier Upper limit on deviation of signal frequency deviation of sig