FM Receiver Gain Control Using Audio Noise Measurement
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Solution Overview
Problem
Existing gain control circuits in FM receivers either fail to effectively reduce noise when the received radio wave is weak or excessively reduce the audio output, leading to discomfort due to noise or incorrect assessment of signal strength.
Innovation Solution
A gain control circuit comprising a noise level measurement circuit and a preamplifier, where the noise level measurement circuit extracts the noise component from the demodulated audio signal and generates a control signal to adjust the preamplifier output based on the noise level, rather than the electric field strength, thereby reducing the reproduced audio output accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the output level is kept constant irrespective of received electric field strength, then high quality audio output with high S/N ratio can be obtained when electric field strength is high, but significant noise occurs in the audio output when electric field strength is low
Solution Approach 1:
The preamplifier's output level is made dynamic rather than constant. The control signal adjusts the preamplifier gain based on measured noise level, allowing the system to adapt to varying received electric field strengths and minimize noise while maintaining acceptable audio quality
Solution Approach 2:
A noise level measurement circuit measures the noise component in the demodulated signal and feeds back a control signal to adjust the preamplifier gain. This closed-loop feedback mechanism enables automatic adaptation to changing signal conditions, reducing noise when received signal is weak
2Object-affected harmful factors
If the preamplifier output is reduced according to electric field strength, then noise level decreases when electric field strength is low, but the reproduced audio output is reduced too much irrespective of noise component magnitude
Solution Approach 1:
The noise level measurement circuit extracts and measures only the noise component from the demodulated signal, separating it from the useful audio signal. This allows independent control of noise reduction without affecting the overall audio output level based on incorrect electric field strength assumptions
Solution Approach 2:
The control parameter changed from electric field strength to noise level magnitude. By measuring actual noise component magnitude rather than inferring from electric field strength, the system accurately adjusts preamplifier output to reduce noise while maintaining appropriate audio levels
3Object-affected harmful factors
If squelch circuit cuts off audio output when noise component is large, then discomfort due to noise is reduced, but user has to listen to intended signal amid significant noise until squelch is operated
Solution Approach 1:
The noise level measurement and preamplifier gain adjustment occur continuously and preliminarily, preparing the system to handle noise conditions before they become problematic. The gradual gain reduction prevents sudden squelch activation, allowing users to hear signals gradually as they improve rather than waiting for squelch release
Data Source
AI summary
The present invention provides a gain control circuit for an FM receiver that can reduce a reproduced audio output according to the magnitude of the noise component of an intended signal included in a received signal.The gain control circuit of the present invention is constituted by a noise level measurement circuit 26 and a preamplifier 25. The noise level measurement circuit 26 measures a noise level in an audio signal Sa1 output from an FM detector 4 and outputs a control signal Sc corresponding to the noise level. The preamplifier 25 reduces an output according a value of the control signal Sc. With the gain control circuit of the present invention, the volume of the speaker is reduced as the noise component in the audio signal Sa1 increases due to a decrease in received electric field strength, and therefore discomfort caused by unpleasant noise can be reduced.