FreeDV 2400A

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 FM sample and DV sample for comparison. At the same power even SSB would be a scratchy 6dB SNR copy (noise measured in a 3000Hz bandwidth).

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
Frame Period 40ms
Bits/Frame 96
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!

9 thoughts on “FreeDV 2400A”

  1. Hi All,

    10 db better. TRULY AMAZING. And open source!! And incoherent at that!!

    I still wonder about CDMA? It seems that might go even much lower?

    [I notice my SMS phone seems to ‘just work’ everywhere even when voice is impossible.]

    And then one could add gear shifting. More average signal, better quality.

    Lots of fun :).

    John

  2. Since you have the spare bits, maybe the 1600 vocoder may up your voice quality. Which is the highest speed 40 ms vocoder.

    Unless something else is planned for the bits…

    1. Right now we’re leaving the spare bits open for protocol experimentation and as padding for future TDMA work. Two of them are currently set up for use with the varicode text message stream and 20 of them (overlapping the varicode bits) are set aside as ‘protocol’ bits accessible from the FreeDV API.

  3. A couple thoughts:

    I got this idea looking at the protocol specs for the other ham digital standards: In terms of buying back some bits, you could only sync every N frames or so, say N=5, leaving more extra bits.

    eg:
    16 bit sync + 5*[64 bit voice] + 144 spare bits = 480 bits

    You’d be resyncing every 200msec, which might make for more of a dropout if you miss a sync, but gives the bonus bits to play with. My math uses 1600 bits/sec voice so you as to match what’s on HF and thus includes the FEC bits.

    This is probably only useful for Mode B since you’ve still got the TDMA stuff in mind for mode A (which I’m really excited about btw :)

    The second thing is more of a question: What is the advantage of using 4FSK over QPSK? Both give you a single frequency at a time so you can use a Class C amp, but QPSK would use less bandwidth. Would multipath end up killing any gains from QPSK?

    1. Yes other frame formats are possible – easy to try too if your keen.

      Actually QPSK tends to have amplitude variations so you need a linear-ish PA. If QPSK is coherently demodulated it is about 2dB better than 4FSK at a given Eb/No (SNR) however the demods are far more complex and it’s hard to get that theoretical performance in the real world. Multipath would be worse on QPSK as multipath messes with phase, and incoherently demodulated 4FSK doesn’t care about phase. However for VHF it’s flat fading so the entire signal gets wiped out by fading, not just a part of it like on HF.

  4. Incredible work indeed. I think you’ve got the sensitivity exactly spot on. Is there scope to improve the voice quality? Sadly, I can’t copy anything out of the DV sample other than the callsign and the word “testing”. Having said that, the FM sample is completely unreadable but I guess it would improve with signal strength whilst the DV would not.

    This is a fascinating radio project that I’m following with interest. Keep up the good work.

  5. Just wanted to say that I’ve been following the Codec2/FreeDV project for a while now and you guys are doing great work!

    I can’t wait to see what FreeDV 2400B will bring. 2400A looks truly revolutionary, which is good and bad in amateur radio. Requiring a completely new and separate infrastructure makes a barrier to entry that is hard to overcome. No one wants to be the first to invest in a new mode de jure (not that I think FreeDV is one) that will fail and leave them with unusable equipment.

    The idea of a FreeDV mode that’s usable with existing hardware is extremely appealing to me. I feel I probably represent a lot of amateurs in my age group in that I’d love to try new things and do something different, but I don’t have a lot of cash to tie up in the digital voice wars.

    I’d love to see something like what you have planned for 2400B implemented into a speaker-mic that could be attached to a standard FM HT. That way you could try out the mode with a minimum investment and with permission from a local repeater operator, you could try it out on existing infrastructure. Might be a great thing to do with the idle 220Mhz repeaters in the area!

    As for the spare bits in 2400A, my vote is for FEC. The best way to sell a new mode is to have it be more robust than what is currently available.

    1. Yes it was a conscious choice to push beyond “re use of existing equipment” for 2400A. In particular the use of analog FM demodulators. That philosophy has been holding back VHF DV (both Ham and commercial) for too long.

      Cost won’t be an issue. The high cost of DSTAR/C4FM is not related to the actual cost of implementing digital modes. Think $50 HTs and $20 SDR dongles. The SM2000 will be in the few hundred $ range.

      Hope to have 2400B in the SM1000 speaker-mic.

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