Pi-Star: Sharing your public hotspot

Have you noticed that HVDN has not posted much about DMR or even things relating to the different digital voice modes for a while?    Did you look closely at the photo above for some laughs?

Our focus has been on community growth and awareness via the massive June Hudson River Radio Relay event, a mini QRP HVDN field day, helping to support the USMA at West Point triathlon via APRStt this past August and most recently, spending the day at the Old Rhinebeck Aerodrome to support the OMARC K2A effort.

Plus, with a new weekly commitment to support UNDR Net, there has not been much free time to do much else this summer as HVDN prepares for a series of fall season events. 

A reminder from Brian W2BRM today that we have not chatted in a while, made me think I should go find something interesting to write about related to Pi-Star as I try to find the time to do a short video about a duplex hotspot to be included in Pete N2MCI's upcoming presentation about the basics of hotspots to the OMARC club coming up in a few weeks time.

Public Hotspot

Many people are afraid to touch certain buttons in Pi-Star, but I am not one of those.  Most people use a hotspot to just provide communications for themselves which means your hotspot is in private mode and only lets a radio programmed with your DMR ID to use your hotspot.

If you click the button under "General Configuration" from private to public, here is what will happen.

Up until recently, changing to a "public" hotspot would allow for anyone to use your hotspot.  This is helpful at public events as an example if many people have a DMR radio and it makes sense for them to all be on one hotspot for internet communications, while local use can take place via a standard simplex channel.

If you want anyone to be able to use your hotspot, leave the "DMR Access List" box blank.

Alternatively, you can add a list of DMR ID's separated by a comma which would allow only your closest trusted friends to also use your hotspot.

Be sure to remember to hit "apply changes" when adding or deleting DMR ID's. 

Bigger picture use case

Now, lets say your club has a DMR repeater that is managed by a Pi-Star interface on your favorite DMR network like Brandmeister or TGIF.  Using this feature could be helpful to ensure that only members of your club are permitted to use the DMR repeater instead of anyone else.  

While this can be considered exclusionary and not the best way to portray amateur radio, this feature may be helpful to help promote the use of certain talk groups on a repeater rather than a hotspot. Should an issue happen where preventing a specific user is needed, this feature can permit that also.

There are many ways this feature can be used, so feel free to share some comments below on how you may wish to use this or not.

-Steve K2GOG & HVDN Team

What happens when: Multiple DMR signals & SDRangel

The below video shows native DMR decoding with SDRangel software. On 427.505 MHz is an OpenSpot V1 and on 427.565 MHz is a duplex "China" Spot. 

It is very easy to leave the SDR, in this case the inexpensive NooElec Nano3 tuned to 427.565 and then toggle in the channel down to 427.505 MHz.  Both hotspots were tuned to the same talk group of 31630 during the Tuesday evening Northstar Digital Net.

One thing to notice is that the carrier time keys up quicker and also "hangs" when a discussion drops compared to the quicker times on the duplex "China" Spot.

With SDRangel, you can "run" multiple decoders for different (or the same modes) at the same time. 

You would leave SDRangel "tuned" to one frequency and you can then tune up or down from that frequency in each decoder. This will work within the SDR's capable bandwidth. In the case of the NooElec Nano3, this is 2.8 MHz. 

Also, you can "run" multiple decoders (Analog FM Narrow, DSD for digital voice, Channel Analyzer, etc) all at the same time on the same frequency to just "hear" for example the buzzzz sound of DMR via the analog FM Narrow decoder PLUS the actual DMR demodulated audio AT the same time.

There is A LOT of capability with SDRangel. Give it a try. You will like it and the ability to customize the layout to your liking for certain tasks.

Wireless Nextion Display You Say?

When I started tinkering with the JumboSpot and small OLED displays, it got me interested in larger and informative display options, so I quickly picked up a few Nextion style displays as covered a few months ago in the "Bigger Display for Jumbo/China/Covert" article.

Even though having a larger and more informative display to be used with the MMDVM based hot spot is great to show callsigns, name, location, IP addresse, temperature, etc, that means the hotspot can not be put where I want the hotspot for maximum RF range and still see the display.

Be sure to purchase a Nextion display and not a TJC branded one. Only the Nextion version has English development software. TJC screens are designed for the China market or those that can read Chinese software. Otherwise, they are the same thing and would work fine with any MMDVM Pi-Star Hot Spot

So, what about making the display itself remote from the actual hot spot?  Seemed like a good idea and here is what I have done to make this a reality.

Wireless Nextion Display

Getting the correct signal from the hot spot was easier than I thought after some research and decided instead of trying to send the signal over wireless serial blue-tooth connection to a remote receiver that feeds a Nextion display that using 70cm amateur radio spectrum would be just be a bit more fun to experiment with.

Silicon Labs is just one of the companies with components
that make this small data transceiver module a reality

Inexpensive HC-12 modules allow serial based communication over a wide range of speeds and can be configured to operate on a number of predefined channels set by the manufacturer or configured by the end user. Here is a great user manual I found on the Elecrow website that will fill in some of the blanks on its capabilities.

The HC-12 can be driven from from the TXD, GND and VCC pins on the duplex or semi duplex hot spot boards easily.  All I needed to add was a 1N4007 diode to drop the voltage a little in order to not over drive the HC-12 module since its sensitive to over voltage.

Since I was not interested in providing a two way wireless connection, there was no need for the RXD connection between the hotspot and HC-12.

Steve K2GOG SDR capture of MMDVM DMR audio and HC-12 pulses

After getting everything connected, crossing my fingers and apply power, I started to see pulsed signals being sent on the default HC-12 module frequency of (3) 433.400 MHz as shown on the screen capture by using an inexpensive SDR dongle sch as the NooElec Nano3 and the popular SDR# software package.

The other two signals shown is the output or transmit signal generated by my duplex hotspot that was connected to the HC-12 and was set to operate on (1) 432.525 MHz, only 875 kHz away.

The very narrow continuous unmodulated signal at (2) 433.063 MHz seems to be coming either from my cable modem or cable line, but does not seem to be an issue I can easily fix, so just ignore that.

Within the Pi-Star dashboard, I selected screen type as "Nextion", port as "Modem" and Nextion Layout as "ON7LDS L3" since that is what I use with my 4.3 Inch Nextion display that runs a modified version of one of the nice layouts found in the file section of the Nextion Ham Radio Screens Facebook Group

Connecting the HC-12 module to the Nextion display was a little more tricky to receive signals though.  I connected the RXD and GND from another HC-12 to the Nextion display and ran +5V to power up the display.

I needed an isolated power supply with another 1N4007 diode to power up the HC-12.   Everything worked on the second try!   The first try was when I accidentally forgot to use a diode to drop the voltage and "smoked" an HC-12, but that is why I ordered a few spare just in case!

On the second try I thought I had a different issue, but as it turns out the default setting of the HC-12 is pretty high transmit power and I recall the manual saying to separate modules by a few feet.

Once I moved the contraption across the room, my display was updating with users chatting away on TG 91 worldwide DMR talk group!

An Additional Future Experiment

Here is where I hope to get some people interested and assist with an extra added feature/benefit.  What about decoding the data packets that the HC-12 transmits by using the SDR?

Its possible to decode weather stations, door bells and other 433 MHz range devices with the rtl_433 application when used with an SDR dongle, so maybe someone can find a way to translate the pulses and view those on a computer or smartphone connected to an SDR?

There are likely many protocols that an HC-12 can send and receive, so why not figure out a way to add another to read what the Nextion display expects to see and render that in some sort of application.

It is easy to record and playback "spectrum" with SDR#. To play back the file related to this article, instead of  selecting your own SDR device in the menu drop down, select "IQ File (*.wav) and navigate to where the base-band recording downloaded here was saved to. Then press "Play" and be amazed! 

To get anyone (and everyone) started, I did a base-band recording that can be easily played back in SDR#. You can play back the file as if you were in real time listening to the signals sent by the Nextion. Its even possible to use DSD+ to decode the active DMR audio signal too! 

Caution:  Using DSD+ can be daunting, so look elsewhere on getting that to work.

This way you can know who is speaking to match it against the pulses sent by the HC-12. The pulses will show call sign, talk group and DMR ID to whom ever can decode it so you know what sort of information to look for in the packets.

Just 37 seconds of 2 MHz wide of spectrum took up 225MB, so I have it hosted on Google Drive.

Download UHF Baseband File Here

The First HVDN Contest

If you can make some progress, share your tips in the comments below or send a private email instead via one of the HVDN contact forms.   

First maker, hacker, ham, etc to make MAJOR documented progress, Hudson Valley Digital Network will reward your efforts with an HC-12 and an RTL-SDR device, so ready.....set..... 01000111 01001111 00100001

A Tale of Two Time Slots

Not everyone who has been grabbed by the DMR evolution may understand what two time
slots accomplish.

DMR is based on TDMA technology which allows one 12.5 kHz wide channel to host two simultaneous discussions or data streams at the same time, on the same frequency, without interfering with each other.

Comparing FM to TDMA based DMR.
More details can be found here: https://hvdn.org/whatsdmr

There are also benefits of TDMA compared to standard FM that are not spectrum related. Things like better battery life on a handheld radio is just one. This is accomplished due to the fact that the transmit cycle is not full duty cycle, but rather keyed on and off rapidly, which benefits on less batter load and overall longer life between recharges.

Another benefit is reduction of the cost for repeaters since some components like antennas, feed lines, controllers and more can be shared. The cost for two traditional single channel FM repeaters is cut down greatly by replacing it with just one DMR repeater.

Dual Time Slot and Full Duplex Hotspots

Many repeater operators and owners are currently using a single time slot repeater, even though many specify to use a certain talk group on time slot and other talk groups on time slot 2 which may seem misleading.

Its currently not possible with 99% of amateur radio repeaters to take advantage of true dual time slot features. Part of this reason is due to cost.  A repeater would need two sets of RF equipment in order to make use of true two time slot capabilities. This prevents repeater operators from upgrading equipment. Often times, analog FM repeaters are re purposed as DMR repeaters with out board controllers and voice modems and its hard to do this to create a dual time slot system without great frustration.

Toronto based VA3XPR repeater system and its various components
showing how complex repeaters of any time can become

Since operating a repeater can be costly for both equipment and site related needs such as internet access and leases at great wide coverage area sites, a dual time slot hot spot fills a certain gap and need.

Enter Inexpensive Dual Time Slot Hotspots

Many hot spots are one time slot at a time and also semi-duplex.  This means that while the hotspot is transmitting, its not possible to receive signals.  This is often referred to as semi-duplex operation.

Its usually hard for most people to be involved in two discussions at the same time, so this is not usually an issue, but from a control perspective of a hotspot - a full duplex hot spot allows parameters to be changed while its in transmit since its receiver is on full time.

An example of this application would be using a TG 4000 disconnect to drop active users from the hotspot at any time, rather than waiting for a pause in transmission or by using the Brandmeister dashboard to drop active calls.

You can also have time slot 1 set to say, World Wide 91 and time slot two set for USA Nationwide 3100 at the same time, on the same frequency.  Here is a sample video of that.

As covered in the "DMR Code Plug Concepts" article, you can get very creative with code plugs thanks to a dual time slot hot spot.

Sound off in the comments on what you think a dual time slot hot spot or repeater can accomplish that a semi-duplex repeater or standard JumboSpot, OpenSpot, DVmega and others.

Welcome to 2018, the break through year for DMR and digital voice evolution in ham radio.

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