From: Cinaed Simson Date: Sun, 9 Jul 2017 23:04:26 -0700 ________________________________________________________________________ > On 07/09/2017 12:15 PM, Andy Walls wrote: > > On Sat, 2017-07-08 at 21:38 -0400, Andy Walls wrote: > >>> Date: Fri, 7 Jul 2017 19:50:55 +0300 > >>> From: HLL > >>> Hi all, > >>> > >>> I'm relatively new to DSP and gnuradio but I tried tons of stuff > >>> and > >>> I couldn't decode a fairly simple FSK data. > >>> baudrate seems around 600-700 bps and fsk deviation is less then > >>> 3k. > >> > >> > >> Hmmm. I took a look at your signal and tried building a coherent 2- > >> FSK > >> demodulator. Under the assumption that it was straight 2-FSK, the > >> signaling tones looked to be at +/- 1200 Hz when properly centered. > > Just to be clear - doesn't 2FSK have 4 tones? I'm not sure what you mean > by signaling tones.
"2 Frequency Shift Keying" uses 2 frequencies (aka tones) to send symbols. When I write "signaling tones" for a 2-FSK, I mean the "mark" frequency and the "space" frequency tones. (Assign symbol values of -1 and 1 to mark and space as appropriate for the encoding scheme in use.) > I just looked at it in inspectrum and there are 4 visually identical > structures - and I would guess the fact there are 4 structures is just a > coincidence. > > They're roughly 1.45 seconds long with blank spacers varying from 116 ms > to 200 ms. Those are the actual RF bursts for each packet. There are 4 packets in the file. > I must be doing something wrong. The modulation scheme for this unit appears to be Audio Frequency Shift Keying: 2-FSK performed with 2 (real) square wave audio tones (at 350 Hz and 940 Hz fundamental frequencies) that are then FM modulated. The baud rate appears to be 233 symbols/second. It does not appear to me that the bits are Manchester encoded. Using square waves for the FSK tones is a unusual, unless a cheap digital microcontroller is generating the tones on a digital output pin. Look at the blue signal trace in the first image in this mailing list post, and ignore the level wobble the author is focusing on: http://lists.gnu.org/archive/html/discuss-gnuradio/2017-07/msg00071.html The blue line is not showing bits directly. It is showing intervals of square waves at two different frequencies: 350 Hz and 940 Hz. The frequencies are what are encoding the symbols, not the amplitudes. This is 2-FSK with the tones generated by a really cheap square wave generator. Given the FCC documents cited, I doubt this signal was generated by the devices in the FCC documents. Probably some other device in the same ISM band generated this signal. -Andy > -- Cinaed _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
