Comparison of Analogue (NFM) and Digital (FDMA) Signals in PMR446 band
In the UK and other European Union countries, a set of radio channels have been allocated for use by the general public licence-free. Known as PMR446 (Public Mobile Radio on 446 MHz), a set of 8 analogue narrowband FM channels and 16 digital narrowband FDMA (Frequency Division Multiple Access) channels are available.
Popular amongst hikers, mountain bikers and skiing enthusiasts in locations where cellular signals are weak and unreliable, PMR446 FM walkies-talkies are cheap and easy to use. Digital PMR signals are less common because of the greater expense of equipment for not much extra gain (you’re just talking to friends after all!).
Analogue PMR uses 8 channels separated by 12.5 kHz from each other and uses Frequency Modulation (FM / F3E) with a maximum bandwidth of 11 KHz and maximum signal deviation of 2.5Hz. The frequencies are:PMR ChannelFrequency (MHz)1446.006252446.018753446.031254446.043755446.056256446.068757446.081258446.09375
Digital PMR (dPMR) uses 16 digital voice channels separated by 6.25 kHz from each other (so half as much channel separation as FM) with 4-Level Frequency Shift Keying (FSK) modulation at 3.6 kilobits per second. A voice vocoder is used to convert the microphone audio to digital bits. At 3.6 kbps the sound quality is equivalent to its FM counterpoint albeit with a slight ‘robot’ timbre when listening to the speech being transmitted.dPMR ChannelFrequency (MHz)1446.1031252446.1093753446.1156254446.1218755446.1281256446.1343757446.1406258446.1468759446.15312510446.15937511446.16562512446.17187513446.17812514446.18437515446.19062516446.196875
Comparing the two signals using SDRSharp and an RTL-SDR compatible USB ‘dongle’ I get this image (click for larger version).
On the left in channel PMR 8 is an FM signal and on the right in channel dPMR 1 is the dPMR PSK signal On both transmissions am saying saying ‘hello’ and ‘aaaaaaaah!’ slowly.
The top half of this image is the signal strength (vertical) and bandwidth (horizontal), and in the bottom half is the same but in a ‘time waterfall’ scrolling downwards with the ‘just received’ signal t the top and the bottom for signal received about 5 seconds ago.
Note that the FM signal has responded directly to my vocals, but the digital signal bears no relation! Where I stop speaking for a brief period (clearly shown half way down the waterfall on the FM side), the digital signal does not vary.
Also note that the digital signal is taking up less bandwidth (horizontal width in the image) than the analogue signal.