Receiver Pilot Tone Extraction via 19 kHz Frequency Shifting
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Solution Overview
Problem
Existing stereo decoders face challenges in efficiently extracting and processing stereo signals due to noise power spectral density issues in double sideband amplitude-modulated signals, particularly with suppressed carriers, where noise is higher in the upper sideband, affecting audio quality.
Innovation Solution
A method involving frequency shifting of the analog stereo multiplex signal by 19 kHz to center the pilot tone at DC, followed by low-pass filtering and decimation, with an optional complex halfband filter to separate and extract the stereo difference signal, allowing for improved noise reduction and audio quality by utilizing a simple receiver architecture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If both side bands are used in DSB-SC demodulation, then the receiver can decode stereo signals, but noise in the upper side band degrades audio quality
Solution Approach 1:
The patent segments the DSB-SC signal into lower and upper side bands using a complex halfband filter. The lower side band is processed for stereo decoding while the upper side band is suppressed, eliminating noise contribution from the upper side band while preserving stereo signal integrity.
Solution Approach 2:
The patent introduces asymmetric processing by applying a complex halfband filter that treats the lower and upper side bands differently. The filter suppresses the upper side band while passing the lower side band, creating an asymmetric noise rejection profile that improves audio quality without sacrificing stereo decoding capability.
2Object-affected harmful factors
If a complex halfband filter is added to suppress upper side band, then audio quality improves, but computational requirements increase
Solution Approach 1:
The patent changes the processing parameters by introducing complex arithmetic operations instead of simple real-valued filtering. The complex halfband filter uses complex coefficients and complex arithmetic to achieve superior noise rejection, accepting increased computational complexity as the trade-off for significantly improved audio quality.
3Ease of operation
If frequency shifting by 19 kHz is applied, then pilot tone is centered at DC for easier extraction, but additional processing steps are required
Solution Approach 1:
The patent performs preliminary frequency shifting by 19 kHz before the main filtering and decoding operations. This preliminary action centers the pilot tone at DC, making subsequent extraction and phase-locking operations simpler and more robust, even though it adds an initial processing step.
Data Source
AI summary
The invention refers to a method for a receiver (1) having a signal path (3) incorporating a tuner (4), a frequency demodulator circuit (5) for supplying an analog stereo multiplex signal comprising a baseband stereo sum signal, a 19 kHz stereo pilot and a stereo difference signal, which is double sideband amplitude-modulated on a suppressed 38 kHz subcarrier, a sampler (6) for converting the analog stereo multiplex signal into a time discrete digital stereo multiplex signal and a stereo decoder (7) for decoding the time discrete digital stereo multiplex signal into a time-discrete digital stereo sum and a time discrete digital stereo difference signal. According to the invention the analog stereo multiplex signal is converted into a time discrete digital stereo multiplex signal and then the time discrete digital stereo multiplex signal is shifted over a frequency of 19 kHz to extract the pilot tone.