FM stereo decoder and method using digital signal processing
AssigneeHughes Aircraft Company
InventorsKevin T. Collier, Kevin T. Chan
Life of this patent
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Abstract
A digital FM stereo decoder uses the phase characteristics of linear phase FIR filters, together with a trignometric operation, to generate a 38 kHz subcarrier signal from a 19 kHz pilot. The subcarrier signal is mixed with the input composite signal from which the pilot has been removed to shift its L-R component to baseband; the linear phase FIR filters also maintain phase coherence between the subcarrier and the composite signals. A low distortion output is obtained without the use of a phase locked loop for the regeneration of the subcarrier signal.
Claims26
An FM stereo decoder using digital signal processing, comprising:
- a demodulator and analog-to-digital converter (ADC) for demodulating and converting an FM signal having an L+R (left+right) baseband signal, an L-R signal centered on a subcarrier frequency, and a pilot signal at less than the subcarrier frequency, to a digital demodulated composite signal,
- an analog-to-digital converter (ADC) for converting the FM signal to digital format, said demodulator and ADC together yielding a digital demodulated composite signal,
- a digital bandpass filter for extracting the pilot signal from said composite signal,
- a first digital delay filter for maintaining said composite signal in phase with the extracted pilot signal,
- a differencing node for removing the pilot signal from said composite signal by taking the difference between said composite signal and the in-phase extracted pilot signal, thereby yielding a pilot-free composite signal,
- a plurality of digital filters operating upon said extracted pilot signal to obtain a pair of filtered signals at the pilot signal frequency and in mutual phase quadrature relationship,
- a subcarrier frequency generator operating upon said mutual phase quadrature signals to obtain a mixing signal at the subcarrier frequency,
- a second digital delay filter for delaying said pilot-free composite signal to keep it in-phase with said mixing signal,
- a mixer for mixing said delayed pilot-free composite signal with said mixing signal to shift said L-R signal to a baseband frequency range, and
- an output processor for extracting said L and R signals from said delayed pilot-free composite signal and said baseband L-R signal.
A digital signal processor (DSP) for doubling the frequency of the pilot signal component of an input composite digital signal that has pilot and information signal components, the pilot signal having a given frequency .omega., comprising:
- a digital bandpass filter arranged to extract said pilot signal component from said composite input signal,
- a plurality of digital filters operating upon said extracted pilot signal component to obtain a pair of filtered signals Q and I at the pilot signal frequency, where Q and I are in mutual phase quadrature at the frequency .omega., and
- a frequency doubler operating upon said Q and I signals to obtain an input signal at the frequency 2.omega..
A digital signal processor (DSP) for doubling the frequency of an input digital signal having a given frequency .omega., comprising:
- a plurality of digital filters operating upon said input signal to obtain a pair of filtered signals Q and I, where Q and I are in mutual phase quadrature at the frequency .omega., said plurality of digital filters comprising a pair of filters having phase delays that differ from each other by 90.degree., and
- a frequency doubler operating upon said Q and I signals to obtain an output signal at the frequency 2.omega..
A method of obtaining a mixing signal for mixing with a composite FM stereo signal, said composite signal having a pilot signal component at a frequency .omega., an L+R (left+right) baseband signal, and an L-R signal centered on a subcarrier frequency 2.omega. comprising:
- extracting said pilot signal component from said composite signal,
- generating a pair of signals Q and I from said extracted pilot signal component at the pilot signal frequency .omega. in mutual phase quadrature,
- squaring said Q and I signals to obtain signals in the form Q.sup.2 and I.sup.2, and
- subtracting the Q.sup.2 signal from the I.sup.2 signal to obtain a signal in the form I.sup.2 -Q.sup.2 at the frequency 2.omega..
Dependencies are inferred from the claim text.