# Demodulating a MPT1327 signal **URL:** \ **Category:** GNU Radio\ **Created:** [August 6, 2014, 9:54pm UTC](https://www.ruby-forum.com/t/demodulating-a-mpt1327-signal/231419 "2014-08-06T21:54:59Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [August 6, 2014, 9:54pm UTC](https://www.ruby-forum.com/t/demodulating-a-mpt1327-signal/231419/1 "2014-08-06T21:54:59Z")
MPT1327 uses MSK on its control channel. I am attempting to use NBFM Receive to demodulate the signal into a WAV file that I can play back and use sdrtrunk to decode it. So far my best attempt is set up as follows: USRP Sample Rate: 250k Center Freq: 652.625M Gain: 100 Frequency Xlating FIR Filter Decimation: 1 Taps: firdes.low\_pass(100,samp\_rate,10e3,1e3) Center Frequency: 0 Sample Rate: 250k Rational Resampler Decimation: 250k Interpolation: 192k NBFM Receive Audio Rate: 192k Quadrature Rate 192k Wav File Sink Sample Rate: 192k Bits per Sample: 16 The output of this seems to be just noise and is not working. Thank you. –tyler ---
**Author:** ![WeaverS\_Tyler\_SGCS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/df788c/32.png) [@WeaverS\_Tyler\_SGCS](https://www.ruby-forum.com/u/WeaverS_Tyler_SGCS)\ **Post date:** [August 7, 2014, 3:57am UTC](https://www.ruby-forum.com/t/demodulating-a-mpt1327-signal/231419/2 "2014-08-07T03:57:43Z")
Post your GRC file. I tried it just as you implemented and it is working here. I would have choose different sample rates and topology, but nonetheless it works. Gain of 100 (dB) should not be a valid UHD value as it tops out at 81 for the B200, and 31 for the WBX. It still works though. Can you trace the signal down the flow with the GUI instrumentation? Weaver, Tyler [GCS] wrote > Decimation: 1 > Quadrature Rate 192k > > * * * > > Discuss-gnuradio mailing list > Discuss-gnuradio@ > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – View this message in context: [http://gnuradio.4.n7.nabble.com/Demodulating-a-MPT1327-signal-tp49824p49828.html](http://gnuradio.4.n7.nabble.com/Demodulating-a-MPT1327-signal-tp49824p49828.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![WeaverS\_Tyler\_SGCS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/df788c/32.png) [@WeaverS\_Tyler\_SGCS](https://www.ruby-forum.com/u/WeaverS_Tyler_SGCS)\ **Post date:** [August 9, 2014, 11:48pm UTC](https://www.ruby-forum.com/t/demodulating-a-mpt1327-signal/231419/3 "2014-08-09T23:48:41Z")
OK got your file, and it is working fine here on a N210+WBX, and a B200, when fed with a signal generator. 1. You must be using GR 3.6 or earlier as some blocks don’t import into 3.7.5. I replaced the rational resampler and nbfm\_rx with your same settings. 2. I assume you are using an USRP2 or N2x0 since you are using an IP address. The gain should be 31 maximum for the WBX. I measure the minimum detectable signal at this gain is about -90 dBm, for 1 kHz tone on NBFM with 5 KHz deviation. 3. I also added a resampler to 48k, 10x gain, and an audio sink. I did verify the wav file is being written. Are you sure you are on frequency and signal is strong enough? Any tune warning from UHD? Can you tune a strong signal like 162.55 MHz NOAA, or equivalent in your area? At least you are getting noise. [http://gnuradio.4.n7.nabble.com/file/n49889/uhd\_rx\_sample.png](http://gnuradio.4.n7.nabble.com/file/n49889/uhd_rx_sample.png) Thanks, Lou KD4HSO madengr wrote > > use sdrtrunk to decode it. So far my best attempt is set up as follows: > > Sample Rate: 250k > > Sample Rate: 192k > > Bits per Sample: 16 > > > > The output of this seems to be just noise and is not working. > > > > Thank you. > > > > –tyler > > > > * * * > > > > Discuss-gnuradio mailing list > > Discuss-gnuradio@ > > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – View this message in context: [http://gnuradio.4.n7.nabble.com/Demodulating-a-MPT1327-signal-tp49824p49889.html](http://gnuradio.4.n7.nabble.com/Demodulating-a-MPT1327-signal-tp49824p49889.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com).