Simulcast TX and RX Adjustment

Simulcast TX Audio Adjustment

In order for your simulcast and voted system to function correctly, we need to be diligent and consistent with the audio level adjustments. Luckily, the method I employ using a decent service monitor is pretty easy.  There are a few different ways to do this with less equipment, but if you are putting up a simulcast system you should have a decent service monitor with a Demodulated Audio output port which nearly all service monitors meant for LMR (Land Mobile Radio) use have. We will cover the 3 options I have used and know of, the first being my preferred option. All these instructions assume you have your RTCM and/or Voter board up and working with your repeater(s) and they are working with the Allstar software on the host computer. 

NOTE: This particular simulcast system does not transmit DPL (Digital PL). You can set your system up with PL/CTCSS on the transmit side and set your receivers to receive and decode DPL for a split DPL/PL system if you wish. 

Option #1 Service Monitor and Volt Meter
What you will need:
1. Service Monitor with demodulated audio output port
2. A decent voltmeter (preferably Fluke) that can measure AC RMS voltage and can measure out to preferably 3 decimal places Ex. 1.234 Vac

So the first thing here that needs to be done is the setup of the service monitor. This will be highly dependent on the service monitor you have but they should be similar. You need to set the service monitor audio filters, one filter should be a High Pass filter that prevents the PL/CTCSS tone from coming out the demod port. A filter that passes 300Hz and above will do. Next is a Low Pass filter which also needs to be in place, this should ideally be a 5 KHz and below low pass but 10 or 15 KHz will also work. The reason for this is we want to keep noise on the demod output audio to your voltmeter to a minimum which will make taking a precise AC RMS voltage measurement easier. You will be measuring the AC voltage from a 1 KHz test tone generated from the Allstar software or a signal generator on the receivers input.  

Now with your service monitor connected to the transmitters out and your volt meter connected to your demod port we can now perform our baseline adjustment and measurement. Now login to your host computer and get yourself to Asterisk command line by issuing this command in the Linux command line:

asterisk -r

Your command prompt should have changed to your_host_name*CLI>
Now that you are here you can issue the command which will produce a continuous 1 KHz test tone out of your transmitter.
Now issue this command in the Asterisk command line:

rpt cmd your_node_number cop 4 xxx

If you have a radio listening to the output of the transmitter (or speaker on the service monitor) you should be hearing a 1 KHz constant test tone. The level of this tone from the system assumes it to be coming out of your transmitter at 3 KHz FM deviation. If you are hearing the tone, look at the deviation level on your service monitor, adjust the TX pot on the RTCM/Voter Board so you achieve 3.0 KHz of deviation. Now that is set, look at the reading on your voltmeter and write down that number (it is usually in the ballpark of 1 Vrms but yours may be different) because that is the number you will use to adjust all the other transmitters in your system, +/- .01 Vrms. To turn off the test tone, just issue the above command again in the Asterisk CLI and it will stop. 

DO NOT use a different service monitor or different volt meter when adjusting the other transmitters in the system. You must use the same equipment to adjust every transmitter. I would also suggest that before adjustment are made to any transmitter level that your service monitor has had at least 15 to 30 mins of warm up time before you start. The goal here is the make everything the same , or as near as we can get it. 

Now you can perform this on all your transmitters so they are all in alignment with each other. 

Option #2 Bessel Null Method
What you will need:
1. A RF Spectrum Analyzer - One of the cheap USB SDR spectrum analyzer will work for this believe it or not. 
2. Sine Wave Audio frequency generator - This can be a separate piece of equipment or Allstar doing it.
3. The PL/CTCSS Tone must be turned off in the Voter.conf file  Example: txctcsslevel = 0

Without going into detail on why this works here, let us concentrate on the more practical application of it. 
The short explanation of this method is that at certain levels of deviation of an FM carrier a particular audio frequency modulating the carrier will create a series of nulls in the center carrier of the signal at particular deviation levels. 
If you want more detail on this go HERE.  
For your purposes I will skip to the end and give you the answer. If you modulate your FM carrier with a 1247.4 Hz sine wave tone you will null the center carrier of your transmitter at 3.0 KHz of deviation. 
All that needs to be done is to observe the output of your transmitter on a spectrum analyzer and start with you TX pot all the way down. Increase the TX pot very slowly because the null can be easy to miss. As you increase the audio level a one point  you will see that center spike of the signal start to go down and then disappear entirely. At this point, while injecting your 1247.4 Hz signal, you adjust the TX pot until the carrier is completely gone, your deviation should be at exactly 3 KHz. Now you perform this same procedure on the other transmitters and they should all be in alignment now. 
Here is in excellent video on measuring FM deviation using this method

You can inject that 1247.4 Hz tone in one of two ways that I know of. One is using a function generator and injecting it in the receive input of the RTCM/Voter Board to go out the transmitter. If you are doing this, make sure you insert the audio tone input the RTCM/Voter board and NOT generate it using a service monitor as RF going into the radio receiver. You will find if you do this, it will cause the null to be incorrect.  The other which is easier and works just as well in my experience, is using the Allstar software to generate that tone for us. This is again done in the Asterisk command line using this command:

 rpt cmd your_node_number cop 48 !1247.4/20000

This command as written will generate a 1247.4 Hz tone for a period of 20 seconds and then stop. The breakdown of this command is: 
cop48 - the call to generate a tone or tone sequence
! - This prefix says that these are single audio tones and not DTMF
First Number - Frequency in Hz of the tone(s) you want to generate
/ - Separation between the first and second numbers
Second Number - The Duration in Milliseconds you want the tone to transmit for

For more information on generating tones and tone sequences in Allstar, go HERE.

Do not mix the methods of generating tones between the transmitters otherwise you may get poor results. The point is for all the transmitters to be the same and in order to do that, the same tone generation method must be employed for every transmitter in the system using the same equipment.  
Voting Receiver RX Adjustment

This adjustment is not as critical and all that is needed is a signal generator that can produce a 1000 Hz tone at 3 KHz of deviation. If this isn't available a good quality radio that can produce DTMF tones can also be used. Most radios produce their DTMF tones at between 3.0 and 3.3 KHz of deviation on wide band. A good DTMF digit to use would be the "A" digit because this produces the highest and lowest tones being 697 Hz and 1633 Hz. 
The RTCM/Voter Board makes this adjustment pretty easy, in the menu under option #97 you can view the audio input level in real time.

Using a signal generator you can adjust the display using the RX Audio pot to 3 KHz or using the a DTMF tone you would adjust it to just over 3 KHz on the display. This level needs to be the same across all your receivers otherwise you will notice odd voting behavior between the receivers as well as differences in the audio level when the system switches from one receiver to another.