I see a lot of comments about the MMDVM not supporting various hardware with the new FM work. I think I need to point some things out:
1. The developers don't have access to all of the hardware that is out there. We can only support what we have, and even then it isn't easy. Ultimately if your hardware was created by G9BF, then it is G9BFs responsibility to support it. If we can make sure it runs correctly then we'll try to do so, but we make no guarantees. The most common quality modem hardware is provided by KI6ZUM, N3XCC (Repeater Builder), BI7JTA, and DF2ET. This list is not exhaustive, but these represent good quality boards and you can reasonably expect them to work. However see note 2 below.
2. With the additional work on the MMDVM, we are finding that some of the older MMDVM hardware no longer has enough processing power to run it. This first came up with NXDN mode, and this is now showing itself with the new FM mode. Geoffrey F4FXL had to do some amazing optimisations in order to get the standalone FM mode to run on the Arduino Due, but there is no way that this platform will support the use of external audio (i.e. Echolink). This comment also applies to other boards, typically ARM M4 boards with clock speeds lower than 180 MHz. This unfortunately applies to Arduino Due based interfaces, Repeater Builder V1 and V2 boards, SP6POG boards, and many no-name boards that are sourced from China via eBay.
If you are buying a new MMDVM modem board and you aren't sure what processor that it uses, ask the seller. If it's less than a 180 MHz ARM M4 and the seller claims that it supports FM or NXDN mode, don't believe them. Better, buy your board from a reputable seller, you may be paying more, but you'll be getting guaranteed hardware with a good chance that it will work with future upgrades.