Tutorial Multitone Paging

Tutorial: Multitone Paging

What it is. Multitone Electronics is a UK maker of pagers used in hospitals and on industrial sites. The air interface is narrowband FM with FFSK or tones. The message layout is not public, so this mode is a detector: it tells you that a transmission is there and shows what is measurable, but it does not show message text.

Read this first. Pager traffic is private. This mode does not decode addresses or messages, but a hospital or site paging system is still someone else's communication. Check the law where you live. See Legal and Privacy.

What you need

• A radio that tunes 25 to 480 MHz (the mode table range). The mode asks for 2 Msps and works down to 250 ksps, so an RTL-SDR is fine.
• A VHF/UHF antenna.
• A frequency. The mode starts on 446.475 MHz (the frequency database lists it as an app default example: on-site multitone systems are licence specific). Real systems sit on frequencies assigned to the site, so you usually have to look for them with the spectrum or the Scan tab.

Before you start

Keep the gain moderate. The channel width is 25 kHz and signals are short.

Step by step

1. In the mode list open Pagers and choose Pagers, tones and FFSK.
2. Choose the radio under Source and start the receiver.
3. Tune to the suspected channel.
4. Open Transmissions and wait for a burst.
5. Look at FSK data and Tones for the details of what was heard.

What you should see

A row per transmission in Transmissions: the time, the length, the carrier, the FM deviation and the state. For FSK data, the measured baud rate, the bit stream in hex and the 32-bit words that pass the BCH(31,21) and parity check (marked with an asterisk). Tones lists steady audio tones and spectral lines.

Reading the screen

• Transmissions: Time UTC, Heard, Last, Frequency, Deviation, SNR, Length, Kind (unverified). The line below counts transmissions and those with FSK data.
• FSK data: the measured baud rate, the bits as hex and the words that pass the POCSAG BCH check.
• Tones: steady tones with their frequency and level.

Everything shown is unverified. The audio FFSK was found in real recordings (tones f and 2f, 585 baud, a 0x605A frame word); the address and message fields are not worked out, so no message text is shown.

Options and settings

Clear forgets every transmission, burst and tone heard so far. Copy copies the lists.

Troubleshooting

• Nothing is heard. The site system may be on another frequency, or quiet. Use the spectrum to look for short FM bursts.
• Transmissions but no FSK data. The pager system may be tone-only (selective call tones). Look at Tones.
• A POCSAG sync is flagged. Then the channel is probably a POCSAG one: use Tutorial-POCSAG instead.

Tips

Record IQ of an interesting burst (see Settings-and-files) so that you can look at it again later.

Legal and privacy notes

Receive only. No message text is shown.

Under the hood

Narrowband FM with audio FFSK or selective tones. The detector measures carrier, deviation, baud rate and tones, and applies the POCSAG BCH(31,21) check to words in the bit stream as a plausibility test. The format itself is not public.

Related

Mode-Multitone-Paging, Tutorial-POCSAG, Tutorial-FLEX, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.