Recently we saw newly created a very interesting project. The idea of creating a repeater on the basis of a sound card ended in partial success. Problematic was the selection of the reference voltage level in different sound cards.
In addition, with regard to the sound cards you have to be selected such models, which have a flat characteristic. For these purposes, the most suitable are models based on the chip CM family.
First image shows graphs at sampling frequency of 44kHz.
Here's an example repeater scheme based on the sound card plugged into a PC
Above solutions, as I said, have some problems (noise floor, reference voltage).
So the bright idea was to change sound card with multi mode modem.
Jonathan Naylor G4KLX said:
The Multi-Mode Digital Voice Modem is a combined hardware and software development of a modem to handle all amateur digital voice modes. Initially it will support D-Star and DMR, with System Fusion and P.25 coming later, as well as a built-in FM repeater controller. For all modes other than DMR and FM, the modem can be used in simplex or duplex mode, while for DMR and FM full duplex must be used. The protocol between the modem and the host is new.
The hardware will typically be ARM based, with initial developments being aimed at the Arduino Due and Teensy 3.1, the STM32 range is also looking like a suitable chip, with the more advanced Cortex M4 architecture and higher clock speeds. The interfacing hardware will consist of op-amp low pass filters and level shifters.
Here we comes to the point where we understand idea. Some of modern chips like Atmel SAM3X8E (Cortex M3), have built-in ADC and DAC.
If so when we have input/output, enough power RAM and Flash in the chip, there we only need library to obtain DSP and then we can decoding
various analog signal ( up to about 5kHz) attached to input port (RX) from radio (RF). The same is for encoding - from output comes from chip
(TX) we attaching signal to radio . This may be done in many ways using passive elements (resistors) or by active elements usng OP-AMP's.
At Pacificon 2015 KI6ZUM presented first version od MMDVM based on his GMSK board (CMX589 not mounted on it). Next his board (PCB with white solder mask) was redesigned and using another pinouts of arduino. MMDVM board is mounted on top arduino as a shield.