Brady O’Brien, KC9TPA, has been working hard on two new FreeDV modes for VHF/UHF radio. To the existing Codec 2 1300 bit/s mode, he has added framing/sync logic and our high performance 4FSK modem. This mode is designed to be “readability 5” at -132dBm, which is 10dB beyond the point where analog FM and 1st generation DV systems stop working.
Brady tested the system by setting up a low power transmitter using a HackRF connected directly to an antenna (tx power about 20mW). A GNU Radio system was used to play FreeDV 2400A and analog FM signals at the same transmit power:
He then went for a drive and found a spot 2.5km away where the signal was weak, but still decodable.
Here is a spectogram of the two signals, FM/2400A/FM/2400A.
SDR radios are required to reach the performance goals for this mode. FreeDV 2400A is not designed to be run on legacy FM radios, even those with data ports. The RF bandwidth is 5kHz, too wide for SSB radios. This represents a complete departure from “FM” friendly VHF DV modes – DStar/C4FM/DMR which pass through an analog FM modem, and suffer performance degradation because of it. The mode has been designed without compromise in the modem and to explore new ground. It is also completely open source – especially the codec.
However we are also developing FreeDV 2400B – which is designed to run though any FM radio, even a $40 HT. Some test results on that soon.
FreeDV 2400A is available now in the FreeDV API and can be tested using the FreeDV command line utilities, for example:
./freedv_tx 2400A ../../raw/ve9qrp_10s.raw - | ./freedv_rx 2400A - - | play -t raw -r 8000 -s -2 -
It requires a 48kHz interface to the SDR.
Some information on the FreeDV 2400A mode:
|Bit Rate||2400 bit/s|
|RF Bandwidth||5 kHz|
|Suggested Channel Spacing||6.25 kHz|
|Modulation||4FSK with non coherent demodulation|
|Symbol Rate||1200 symbols/s|
|Tone Spacing||1200 Hz|
|Unique Word||16 bits/frame|
|Codec 2 1300||52 bits/frame|
|Spare Bits||28 bits/frame|
The spare bits are currently undefined but could be used for data, routing information, or FEC. It’s early days but this is an important first step – well done Brady!