Optimized demodulation of RDS signals in digital radio
View Patent ↗A method for correcting RDS demodulation in a vehicle radio system including a digital core with an RDS demodulation block, a numerically controlled oscillator, a digital mixer that mixes the input signals with the output of the numerically controlled oscillator, a low-pass filter for recovering baseband RDS signals including a sequence of symbols, and a phase-estimating block configured to estimate a phase deviation of the baseband signal. The method including a first correction acting, depending on the estimation of the phase deviation of the baseband signal, on a phase equalizer, downstream of the low-pass filter, in order to cancel out the phase deviation, and optionally a second correction forming a feedback loop and acting on the numerically controlled oscillator depending on the drift in the phase deviations of the baseband signal.
1. A method for correcting RDS demodulation in a vehicle radio system, the method comprising:
receiving, by an input for digital signals of a radio data system (RDS) demodulation block, RDS signals;
mixing, by a complex digital mixer, the RDS signals with a complex value output by a numerically controlled oscillator to produce mixed RDS signals;
filtering, by a low-pass filter, the mixed RDS signals to recover baseband demodulated RDS signals;
estimating, by a phase-estimating block, a phase deviation of the baseband demodulated RDS signals based on a plurality of time delayed symbols in the baseband demodulated RDS signals;
correcting, by an error loop filter, the complex value output by the numerically controlled oscillator depending on the estimation of the phase deviation of the baseband demodulated RDS signals; and
decoding, by a decoding block, the baseband demodulated RDS signals.
2. The method as claimed in claim 1 , wherein the phase-estimating block performs a calculation of the phase deviation on N last received symbols, N being an integer comprised between 4 and 16.
3. The method as claimed in claim 2 , wherein the calculation of the phase deviation between a frequency of the numerically controlled oscillator and a frequency of an emitter is estimated using the following formula:
θ
^
=
1
2
arg
(
∑
k
=
0
N
-
1
(
S
(
k
)
)
2
)
=
1
2
arctan
(
∑
k
=
0
N
-
1
(
S
(
k
)
)
2
)
.
4. The method as claimed in claim 2 , wherein a conjugate e −j{circumflex over (θ)} of the estimated phase deviation is calculated, which is injected into a phase equalizer, where it cancels out the phase deviation of the symbol transmitted to the decoding block, the conjugate e −j{circumflex over (θ)} of the phase deviation estimated being calculated using the following formula:
e
-
j
θ
^
=
conj
(
∑
k
=
0
N
-
1
(
S
(
k
)
)
2
∑
k
=
0
N
-
1
(
(
Sk
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)
2
)
.
5. The method as claimed in claim 1 , wherein the method comprises a second correction forming a feedback loop and acting on the numerically controlled oscillator depending on a drift in the phase deviations of the baseband signal, in order to reset a frequency of the numerically controlled oscillator to as close as possible to a received RDS subcarrier.
6. The method as claimed in claim 5 , wherein the frequency of the numerically controlled oscillator is corrected only if the drift in the phase deviations is higher than a preset threshold.
7. The method as claimed in claim 5 , wherein the drift in the phase deviations is calculated from an average drift over M consecutive phase deviations, M being an integer.
8. The method as claimed in claim 5 , wherein the second correction forming the feedback loop is undersampled with respect to a first correction.
9. The method as claimed in claim 1 , wherein the phase-estimating block performs a calculation of the phase deviation on N last received symbols, N being an integer equal to 8.
10. A vehicle radio system comprising:
a digital core with an radio data system (RDS) demodulation block;
an input for receiving digital signals, the input receiving RDS signals from the RDS demodulation block;
a numerically controlled oscillator;
a digital mixer that mixes the RDS signals with a complex value output by the numerically controlled oscillator to produce mixed RDS signals;
a low-pass filter for filtering the mixed RDS signals to recover baseband demodulated RDS signals;
a phase-estimating block for estimating a phase deviation of the baseband demodulated RDS signals based on a plurality of time delayed symbols in the baseband demodulated RDS signals;
an error loop filter for correcting the complex value output by the numerically controlled oscillator depending on the estimation of the phase deviation of the baseband demodulated RDS signals; and
a decoding block for decoding the baseband demodulated RDS signals.
11. The vehicle radio system as claimed in claim 10 , furthermore comprising a second correction forming a feedback loop and acting on the numerically controlled oscillator depending on a drift in the phase deviations of the baseband signal, in order to reset a frequency of the numerically controlled oscillator to as close as possible to a received RDS subcarrier.