Base frequency (how to find)
To work out a base frequency for a system using a logical channel planScan, and find voice channels that are using MPT. Run TrunkView, feed it with audio from a voice channel and look at the 'Call clear-down' messages.
Hopefully you will see something like:
09:18 - CLEAR| Call clear-down | Ch: 787 Freq: 000.0000 MHz | Return to Control Ch: 895 |or
09:18 - MAINT| Call maintenance message | (021) Org: 1425 Oper: 6 | Ch: 787 Freq: 000.0000 MHz |
From that and your scanner readout you will know:
1. the frequency (voice frequency 180.6375 MHz assumed here)
2. the channel number (787)
3. the control channel number (895)
The 'MAINT' message is not always transmitted.
Calculate base frequencyBase frequency = Voice frequency - (channel x step)
Almost all European systems use 12.5 kHz channel steps, so the above example works out:
|Base frequency (channel 0)||= 180.6375 - (787 x 0.0125) MHz
= 180.6375 - (9.8375)
So now you know the base frequency: 170.8000 MHz.
From the more useful 'Call clear-down' message you would also know that:
|Control Channel frequency||= Base frequency + (channel x step)
= 170.8000 + (895 x 0.0125)
= 170.8000 + (11.1875)
As you can see, the above (real) example is a Band 3 system with standard numbering (as used by Fleetcomm in the UK).