[App_rpt-users] RTCM and Quantar Interfacing

Brett Woollum brett at woollum.com
Fri Mar 18 18:31:27 EDT 2016

Hi Everyone! 

I wanted to share my experiencing setting up DIAL with RTCM's in hopes it will help the next person who searches for similar information. 

I have a Motorola Quantar (Quantro technically) repeater that has been set up for in-cabinet repeat. We've been having issues with handhelds not being able to talk into the system very well, especially in certain areas such as inside buildings. Our solution to this is to add remote voting receivers and feed them back to the repeater for transmission. I thought about doing it the traditional way using RF links and a central voter, but found app_rpt and RTCM's to be a more flexible solution. Plus, we already have an IP network to use as the transport between sites. 

At the moment I have an RTCM connected to the Quantar's 50-pin telco connector. The Quantar has a 4-wire wireline board installed, though I've configured the Quantar for an enhanced 8-wire card using the drop-down option in the software. 

We have one remote RTCM for testing. This is currently connected to a Motorola CDM1250 which is configured for receive-only. 

Some things I learned that I would like to share: 
Use the right pins on the Quantar! 
My first mistake was connecting the RTCM's audio to pins 1 and 2 on the 50-pin telco connector, thinking I would just go into the software and configure the repeater to put audio on those pins. Of course this didn't work. Use pins 5 and 30 on the 50-pin connector for your RTCM audio! The other pins for COR/PL and TX were easy (don't forget to add the wildcard entries as necessary to make these work!). 

Set up a VoIP Phone 
Setting up a remote SIP phone to listen in is a must! I would have continued to struggle if I hadn't set up a VoIP phone reverse-autopatch style. It was super easy using the instructions found in another thread on this list. Being able to call in and listen directly to the server was immensely helpful in diagnosing issues. I was able to figure out if the audio problems were on the receive or transmit side. I have a physical VoIP phone and a iPhone VoIP phone app connected. 

Recording is a must! 
I've configured app_rpt to record sound files on the server's hard drive. This allows me to re-play audio files and hear what the audio sounded like from the server's perspective. Again, this has been extremely helpful in allowing me to diagnose where in the system audio problems are coming from. A recent example of this was choppy transmit audio. I was able to go back and listen to the audio files, which sounded fine. This helped me narrow down on the problem, which turned out to be an issue between the server and the RTCM at the transmitter site (lack of QoS on a network link feeding the RTCM at the transmitter site). 

Crappy Transmit Audio 
I ran into an issue where the repeated audio sounded really bassy, muffled, and not very understandable. Compared to a typical simplex radio-to-radio transmission, the audio through the repeater (RTCM and Dial) was unacceptable. What I discovered was that the RTCM has an internal pre-emphasis function that was disabled. See, I had intentionally disabled "txctsss" and "txctcsslevel" in voter.conf because I didn't want the RTCM transmitting CTCSS (the Quantar was doing that for me already). I ultimately found a post online that said enabling txctcss = some valid tone (114.8 in our case) and setting txctcsslevel = 0 would turn on the pre-emphasis function in the RTCM without transmitting CTCSS tones. I did, and it worked like a charm! Audio now had more treble and was less bassy/muffled. So, future RTCM users, be sure not to comment out txctcss and txctcsslevel in voter.conf! Just enable and set the level to 0. You'll thank me later. 

Network Quality 
Your IP Network quality is important here. In our case I had to manually ensure we had a rule in our routers to prioritize traffic on UDP port 667 so that RTCM audio was given priority over other traffic. I recommend the same. 

RX/TX Buffers are NOT Both Millisecond Values 
I assumed an RX buffer (in voter.conf) of 120ms would be equivalent of a TX buffer (in the RTCM) of 120ms. Even though I followed the directions and set the buffers correctly, I only later realized the TX buffer was the number of 125 microsecond intervals, where the RX buffer is in milliseconds. I ended with an RX buffer of 160ms and TX buffer of 900 (high just to be safe). 

Don't Disable COR on the RTCM 
At one point I tried to disable COR on the RTCM since I was feeding it a PL signal from the Quantar. While it worked, it seemed to have disabled the RTCM's ability to measure RSSI, and therefore always reported a full 255 RSSI. Not good when trying to vote. 

Hopefully future RTCM's will have softpots for adjustments instead of physical adjustment screws. This would be great because the pots could be adjusted remotely without a trip to the remote site. 

So far the system seems to be working well! I need to make some minor adjustments to audio and squelch levels, but other than that everything is working. I'll probably end up picking up another 1-3 RTCM's and setting them up at remote sites to enhance our coverage even further. 

I hope someone finds this useful down the road! 

PS: For those of you interested in MDC1200, check out my MDC1200 paging system which works through this Dial/RTCM/Quantar system: https://www.youtube.com/watch?v=8QQa4krAxQI 

Brett Woollum 
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