C4FM stands for Continuous Four Level Frequency Modulation, which is a special type of 4FSK, which is used in conjunction with FDMA – Frequency Division Multiple Access. FDMA is the same mode that is used in P25 Phase 1 by public service groups, including Emergency Responders, but its not compatible.
What sets this new technology apart from other digital voice technologies, such as D-STAR, DMR, P25, and NXDN, is its backwards compatibility with analog FM.
Here’s a look at bandwidths used by different analog and digital modes, including FDMA:
Automatic Mode Select
Every Fusion radio and repeater is aware of the current QSO and it’s mode, using a feature called Automatic Mode Select. This function instantly recognizes whether the received signal is C4FM digital or conventional FM. The communication mode automatically switches to match the received mode. Even if a digital signal is being used, you can switch to FM communication if radio signals are received from an FM station. This function enables stress-free operation by removing the need to manually switch the communication method each time.
AMS function breaks down into the following operating modes that are fully selectable by the radio user:
- AUTO: The both RX/ TX mode is automatically selected from one of the four operating modes (DN, VW, DW and FM) to match the characteristics of the received signal. This is the current AMS operation.
- TX MANUAL: The both RX/ TX mode is automatically selected from four operating modes to match the characteristics of the received signal. And if desired to change the TX mode, by pressing the Microphone PTT switch momentarily, the TX mode switch between DN and FM modes.
- TX FM FIXED: The RX mode is automatically selected from one of the four operating modes, but the TX mode is fixed to FM.
- TX DN FIXED: The RX mode is automatically selected from one of the four operating modes, but the TX mode is fixed to the ‘DN’ (Digital Voice Narrow) mode.
- TX VW FIXED: The RX mode is automatically selected from one of the four operating modes, but the TX mode is fixed to the ‘VW’ (Digital Voice Wide) mode.
System Fusion uses a similar DVSI AMBE DSP chip as used in DMR & P25, but newer than what’s used in D-STAR.
System Fusion’s Four Digital Modes
V/D (Digital Narrow) – Voice+Digital or “V/D” mode
Voice FR (VW) Mode – Utilizes all available bandwidth for high-fidelity voice operation, providing the most crystal clear of voice communications.
Transfer data such as images or text messages at full rate with speeds up to 9600 Bits-per-second
Maintains backwards compatibility with existing Analog FM Equipment, allowing a wide range of users to experiment with System Fusion Digital.
Capabilities such as Automatic Mode Select (AMS) on the DR-1X Repeater allow an even wider range of users to communicate, by running the repeater in “Fixed FM” mode on Transmit, and “Automatic Mode Select” on receive the repeater will automatically detect the incoming signal and convert it to an Analog FM Transmission. This mode allows digital users to communicate with existing Analog FM users without the need to switch their radios into FM Mode, allowing crystal clear Digital reception into the repeater that is converted into a conventional FM Signal.