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Updated FAQ (markdown)

Denny Sheirer
2024-02-19 05:39:49 -05:00
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FAQ.md

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# sdtrunk - Frequency Asked Questions (FAQ)
## Decoders
### MPT-1327
#### How do I recover the channel map for an MPT1327 system if I only know the control channel frequency?
Setup a channel configuration to monitor the control channel frequency for a period of time. Using the events tab, catalog all of the logical channel numbers that are referenced for call events. sdrtrunk will tell you that it doesn't know the frequency, but that's ok, we just want the channel numbers being used by the system for traffic channels.
Next, you'll need to discover which additional frequencies are being used by the system. There are two potential options:
**Option 1** - Lookup the frequencies from the license
1. Find the government (FCC?) issued license for the system operator and catalog all of the frequencies that they're authorized to use, including the one you already know (ie control channel).
1. Create MPT1327 decoder channels on each of these additional frequencies and run those channels.
1. At the end of each call the system will transmit a series of CLEAR messages that tell the radio(s) to return to the control channel, using the logical channel number for the control channel. From there you have your first confirmed logical channel to frequency mapping, for the control channel.
1. Continue monitoring the control channel activity to correlate the call event channel numbers to the channel frequency that becomes active for each call event to recover the logical channel number to frequency mappings for the traffic channels.
**Option 2** - Watch the spectrum for traffic channels to become active.
1. Monitor the control channel events tab and watch for channel grant events.
2. Watch the spectral display to see when a channel becomes active at the same time as the control channel call grant.
3. Create an MPT1327 channel to decode that traffic channel frequency so that you can capture the CLEAR messages transmitted at the end of the call.
4. When you're successful, you'll have the channel number to frequency map recovered for the traffic channel where the call occurred, and also the channel number and frequency for the control channel.
##### Recover The Channel Maps
UHF - 450 MHz:
A fairly standard/common channel map to use is one that starts at 450.000 MHz with channel zero or channel 1:
* Base: 450.0000 or 450.0125
* First: 1
* Last: 1023
* Step: 12500
VHF - 150-180 MHz:
VHF frequency allocations normally don't follow a clean 12.5 kHz channel raster/spacing and MPT-1327 systems can use several channel ranges that overlap each other so that they can align the channel number ranges that follow a 12.5 kHz channel spacing with the actual licensed frequencies that don't follow a 12.5 kHz channel spacing.
MPT1327 uses channel numbers in the range of 1 to 1023. A channel step size (ie spacing) of 12.500 kHz is common. System operators can segment the allowable channel number range into smaller sub-ranges with each using a different base frequency, possibly in chunks of 100 or 200, to map all of their authorized frequencies to logical channel numbers that the MPT1327 system can use. For example:
* Base: 150.000000
* First: 1
* Last: 199
* Step: 12500
* Base: 150.006250
* First: 200
* Last: 399
* Step: 12500
From these two examples, a system could map a channel number to any frequency that is a multiple/spacing of 6.25 kHz, despite the fact that the system requires that the sequential channels in a range are spaced 12.5 kHz apart. As an example, here are the first few frequencies for these two ranges using a 6.25 kHz channel spacing:
* 150.00000 = Channel 1
* 150.00625 = Channel 200
* 150.01250 = Channel 2
* 150.01875 = Channel 201
Once you have your first control or traffic channel number to frequency recovery, you can create your first range of your channel map. For example, if you've discovered that channel 215 is frequency 152.6875, you can create the following channel map:
* Base: 152.6875
* First: 215
* Last: 1023
* Step: 12500
Monitor the system for a period of time using this channel map. Watch when call events happen and listen to the call audio. If sdrtrunk creates a traffic channel and there's nothing but static, then the mapping is incorrect for that channel number and that channel number should be excluded from your initial (first-last) channel range. You can do this by decreasing the value for the last channel number in your channel map to just below the channel number that was incorrect. Generally, the channel number ranges will follow a pattern like 1-199, 200-399, 400-599, etc.
You should note that the above channel map is functionally equivalent to this channel map:
* Base: 152.6750
* First: 214
* Last: 1023
* Step: 12500
In this second channel map example, we just subtracted 12.5 kHz from the base frequency and decreased the first channel number by one. Using this new range would still map channel 215 to 152.6875 MHz, the same as the first map. So, in this way, you can adjust your channel map to include lower channel numbers in your range to test if those lower channel numbers are also part of this range, or if they belong in a separate channel map range.
It's best to use a spreadsheet when you're trying to recover the channel number ranges since you can create formulas to auto-calculate the frequencies from the channel numbers using different base frequencies and channel number first-last ranges. However, if you are able to recover several channel number to frequency mappings, post a new topic here on the Discussions tab, or post a question on Radio Reference and you'll probably get some help figuring out the channel map.
## Tuners
### SDRPlay