As far as P25 systems go, whenever a voice grant occurs, the control channel communicates frequency information to your scanner using a channel number instead of the frequency itself.
The scanner calculates the receive frequency using a table which is usually downloaded from the control channel.
Tables vary, but here is what one might look like:
Code:
LOW HIGH BASE
CHAN CHAN OFFSET FREQUENCY STEP*
--------------------------------------------
0 2877 0 851.00625 6.25
4415 8191 4415 764.00000 6.25
8192 12287 8192 136.00000 2.50
12288 16383 12288 146.00000 2.50
16384 20479 16384 156.00000 2.50
20480 21760 20480 166.00000 6.25
24576 28671 24576 136.00000 6.25
28672 30657 28669 161.57500 6.25
*Step value is in KHz
The table works by taking the received channel number and finding the row in which the channel value falls within the Low/High range.
The offset value from that row is then subtracted from the channel number, then multiplied by the step, then added to the base frequency.
((CHANNEL - OFFSET) * STEP) + BASE FREQUENCY = CHANNEL FREQUENCY
As an example, lets say the channel is 17767.
Because 17767 falls between 16384 and 20479, the offset value 16384 is subtracted from 17767, resulting in a value of 1383.
1383 is then multiplied by step value of .0025 (2.5 KHz) = 3.4575
The value 3.4575 is added to the Base Frequency 156.0, resulting in a channel frequency of 159.4575.
Displaying the channel number on your scanner is probably of little use, unless you're attempting to trouble shoot a manually entered table. In this case, you would want to note the channel number and frequency from the display.