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In-band high frequency noise in FM demodulation

Started by williamch January 24, 2007
Hi all,
    I have implemented FM demodulation(including Mono and Stereo) on FPGA.
When it work on Mono mode, the audio quality sounds good. But if I changed
the mode to the stereo, the audio quality sounds not very well. I have
checked the spectrum of m(t), and there are high noise floor over high
frequencies(looks like the response of a high pass filter). Even I changed
the differentiator from [1 -1] to atan2(z(n)*conj(z(n-1)), the result is
same. The phase of pilot tone looks like ok, because the separateness of
L-R channel is fine. What could be the major cause to make this kind
noise? What/How can I do?
    My demodulator work on Fs = 256k. The NCO used for IF Mixer and
downconvertor(8.192M(Real)->2.048M(Complex)). 
    If you can give me some suggestions, I will be very appreciate your
kindly help.





williamch wrote:
> Hi all, > I have implemented FM demodulation(including Mono and Stereo) on FPGA. > When it work on Mono mode, the audio quality sounds good. But if I changed > the mode to the stereo, the audio quality sounds not very well. I have > checked the spectrum of m(t), and there are high noise floor over high > frequencies(looks like the response of a high pass filter). Even I changed > the differentiator from [1 -1] to atan2(z(n)*conj(z(n-1)), the result is > same. The phase of pilot tone looks like ok, because the separateness of > L-R channel is fine. What could be the major cause to make this kind > noise? What/How can I do? > My demodulator work on Fs = 256k. The NCO used for IF Mixer and > downconvertor(8.192M(Real)->2.048M(Complex)). > If you can give me some suggestions, I will be very appreciate your > kindly help.
Are you on the edge of adequate sifnal strength? Stereo costs 3 dB. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

williamch wrote:

> Hi all, > I have implemented FM demodulation(including Mono and Stereo) on FPGA. > When it work on Mono mode, the audio quality sounds good. But if I changed > the mode to the stereo, the audio quality sounds not very well. I have > checked the spectrum of m(t), and there are high noise floor over high > frequencies(looks like the response of a high pass filter).
But of course. If you use the discrimimator with the differentiation of the phase, the resultant noise will look like that. Even I changed
> the differentiator from [1 -1] to atan2(z(n)*conj(z(n-1)), the result is > same.
No wonder. The phase of pilot tone looks like ok, because the separateness of
> L-R channel is fine. What could be the major cause to make this kind > noise? What/How can I do?
Most of the noise is probably related to the quantization artifacts of your processing. Take a close look at the precision loss in the computations.
> My demodulator work on Fs = 256k. The NCO used for IF Mixer and > downconvertor(8.192M(Real)->2.048M(Complex)). > If you can give me some suggestions, I will be very appreciate your > kindly help.
Your appreciation means too much for me. $100 will be just all right. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com