This whole thread is interesting because I suspect that many hams that would love to experiment with digital radio like DMR, YSF, D-Star, P-25 etc are frustrated by the complexity that various products, write-ups, forums and products expose.
It's hard to "get" the big picture of where each component of the overall system fits in.
Once you start working with the technology and become familiar with it, it is almost "magic" how powerful it is. I've been helping with one of the projects (OpenGD77) and the authors and testing community regularly get together via ham radio in spite of world-wide geographic distance (Australia and France for some of the principle authors).
The magic is based on digital voice switching network technology implemented over the Internet. This voice switching works on very highly compressed low data rate digital packets of voice data and the meta data needed to route it from user to user. These voice switching networks support both private contacts and one to many / many to many group contacts.
There are more than one DMR ham voice switching networks. My preference is one that is built with open source software and almost completely self configured and maintained by users - Brandmeister, but there are others. https://brandmeister.network/
It all started out with Motorola's DMR IP Site Connect technology. The Motorola products are proprietary technology, and defined the actual behavior of the voice switching technology which eventually was standardised by an organization in Europe. The amateur DMR-MARC voice infrastructure is an extended version of Motorola's technology, and was one of the first to become widespread. https://www.dmr-marc.net/
You'll also see other DMR voice switching networks. TGIF comes to mind. http://tgif.network/
D-Star has it's own voice switching network, as does Yaesu YSF. These are deployed with proprietary technology in the voice network servers.
Through a series of bridges and gateways between these voice infrastructures, users of the different over the air digital technologies can communicate with each other. There are differences between how the private conversations and group conversations are structured ("Rooms" in YSF versus "TalkGroups" in DMR, for example) but techniques have emerged for getting them to work together.
A system like Brandmeister is particularly powerful, which is why it's become popular for DMR users. It is really a second generation approach to building and maintaining one of these voice infrastructures.
So how do the different components fit in? First, Pi-Star
PiStar does a couple of things. It connects your installation to the Internet, switches voice packets between the Internet and local radio equipment (duplex repeaters or simplex hot spots) and displays a dashboard full of status messages and indicators about the state of conversations it's working on.
This is all built in layers. I've just discussed some of the voice switching layers. Below that is the component that provides an interface between users and the Voice Over Internet (VOIP) networks. The VOIP networks that directly attach to your device are radio technology specific. Yaesu products only connect to YSF compatible servers and use a closed proprietary technology. The behavior of that technology has been analyzed and cloned, and this allowed open versions to be built as gateways between VOIP networks. D-Star is a different closed proprietary technology that has been analyzed and cloned too.
DMR is a standards based technology that was first implemented in a proprietary product family by Motorola. It has been analyzed and cloned like the others, but the work on this has been much more deeply explored. There are components within Pi-Star that talk directly with DMR VOIP networks via the Internet using Motorola's standardized IP Site Connect protocols. The VOIP networks themselves support various functional capacities deployed by Motorola. For example, DMR-MARC is built partially out of Motorola components, and it supports the Remote Management technologies for Motorola repeaters. Brandmeister makes limited use of the Remote Management protocols, and doesn't expose them to users.
The various bridges and gateways in the VOIP infrastructures allow connection to other VOIP connection managers. In the case of Brandmeister, you connect to a "Master" from your home PiStar Raspberry Pi computer via IP, and the masters connect to each other worldwide. They take care of switching your voice packets to others, and their voice packets to you. The masters can switch voice packets to other masters which then connect other networks of the same radio technology (DMR to DMR for example) or gateways to networks of other technologies (DMR to YSF for examlple).
PiStar does some of it's magic by being able to connect to local radio devices to create local repeaters and hot-spots. The MMDVMHOST component of PiStar runs inside the Raspberry Pi and knows how to talk to local radios in several radio technologies. That's what the "MM" stands for "Multi-Mode". It can handle voice packets and control meta data in DMR. It can do it in YSF. It can do it in D-Star. It can do it in P.25 (a public safety protocol) and it can do it in NXDN (Kenwood's version of digital radio).
The Raspberry Pi runs Linux and the software is executed on it's ARM 32 bit processor. There are several versions of these single board computers. Even the smallest Raspberry Pi Zero W has enough power and storage to run PiStar efficiently.
MMDVM is the firmware that controls a local device (sometimes packaged as a "hat" that plugs directly into a Raspberry Pi board's connector pins) that implements the radio controller and often a low power software defined radio chip.
Several products have been produced that run MMDVM, the Multi Mode Digital Voice Modem. It's completely open source - the hardware and firmware that implements the radio modem and MMDVMHOST software that runs in the Raspberry Pi, and is being continuously improved.
There are a number of products that run MMDVM or it's equivalent. Open projects like the MMDVM Hotspot Hat "JumboSpot" board project have commercial competitors like the similar ZumSpot hat board, DVMega, OpenSpot products and others. These boards and hats either provide the modem and radio or interface to radios, and can even also include the VOIP interface too (OpenSpot).
Firmware has to be written to interface between PiStar's MMDVM Host and the software defined radio components that put voice packets out over the air and receive them. This firmware, that knows how to actually transmit packets through various technologies, is specialized for controlling the radio chip on devices like the "MMDVM" Board. This separate circuit board has a controller that prepares the voice packet transmissions, deals with receiving incoming packets and turns the radio chip on and off. It's implemented in boards like the "JumboSpot HotSpot Hat" in a self contained processor. From the description:
"It runs on the Arduino Due, the ST-Micro STM32F1xxx, STM32F4xxx and STM32F7xxx processors, as well as the Teensy 3.1/3.2/3.5/3.6. What these platforms have in common is the use of an ARM Cortex-M3 or M4 processor with a clock speed greater than 70 MHz, and access to at least one analogue to digital converter and one digital to analogue converter."
Here's the Analog Devices ADF7021 chip, a high quality RF transciever:
https://www.analog.com/media/en/technic ... DF7021.pdf
This processor is used to control the radio chip. On the MMDVM Hotspot Hat board it is a ADF7021 (or RF7021SE module) radio transciever chip. The duplex version of this board has two of them. This chip implements the radio receiver and transmitter on those boards. Other boards used with external radios don't have this chip since they send and receive signals through the external radio. This software defined radio operates at one of two clock speeds, and the quality and precision of the clock chip has a lot to do with the quality of an individual board. A high quality clock chip will be stable and accurate. A cheap one will probably not be.
This radio transciever chip generates a very low power signal that is directly transmitted by the hotspot hat board through a small antenna. A few milliwatts, it's suitable for local use within a few hundred to thousand feet.
Together the firmware, Arduino Due microprocessor, A to D and D to A converter and ADF7021 transciever chip are a software defined radio with the flexibility to transmit and receive all the major digital mobile radio technologies. DMR, D-Star, YSF, NXDN and P-25. Very inexpensive and pretty neat.
By using these low cost components and the free open source software, the open source hardware JumboSpot HotSpot Radio Hats (simplex and duplex) are both powerful and affordable. Unfortunately, there are a couple of issues to be aware of. Most digital mobile radio modulation technologies require fairly precise frequency centering for transmission and reception. Every radio varies a few cycles to few hundred cycles based on component tolerances. This means that one portable handheld radio may be a few hundred cycles off the exact center RF frequency. In more expensive radio implementations, receivers include "Automatic Frequency Control" that will steer the receiver dynamically to center the incoming frequency. On the low cost JumboSpot, the ADF7021 processor controlled by MMDVM does not implement AFC. This means that when the signal it's receiving is off a bit, it will generate "Bit Error Rate" errors. There is an expert setting for MMDVMHost on PiStar that lets you adjust both a receive "RXOffset" and transmit "TXOffset" to minimize these errors with a specific radio. With adjustment, you can get the BER below 1%. There is a "MMDVMCAL" utility for doing this within PiStar.
The packaging and implementation of these components varies from different designers and different manufacturers. Most that implement MMDVM can communicate in all the technologies supported by the MMDVMHOST software. There are a couple of projects that don't. I've been working on the OpenGD77 project. This is DMR only since the radios it run on only communicate via DMR protocols. So far, this project has implemented the DMR hotspot portion of MMDVM within Radioddity GD-77, GD-77, and Baofeng DM-1801 radios on their internal processors. This is in addition to providing a much more amateur radio friendly firmware for these radios. These have the advantage of operating with higher power, and being implemented on radios that include automatic frequency control, which deal with bit demodulation errors on receive. In this project, the PiStar running MMDVMHOST plugs directly into the portable handheld radio via USB (the same attachment used to program the radios). Thanks to the 50 percent RF duty cycle of DMR, it even appears that these can run long periods of time at moderate power output (adjustable from 1 watt to 5 watts). These are ideal for creating event hotspots with some range.
I hope that this overview can help you understand the relationship between all the components and new unfamiliar names you encounter when first working with digital mobile amateur radio.