Digital Voice on HF (FreeDV 700B Mode)

Discussion in 'Amateur Radio Software' started by K5OKC, Oct 5, 2015.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: L-Geochron
ad: MessiPaoloni-1
ad: Left-2
ad: abrind-2
ad: HRDLLC-2
ad: Left-3
  1. K5OKC

    K5OKC Ham Member QRZ Page

    An interesting Ham communication mode is Digital Voice on High Frequency (HF) shortwave. Many users have experimented with the FreeDV program, which has been the vehicle for experimentation on Windows and Linux.

    There is also a hardware device now, that implements the modem, and can be held in your hand like a microphone, or jacks are provided for interfacing it with your shack.

    I've always liked experimenting with the modem, not so much operating it over the air. I don't have a lot to say over voice, but I am impressed by the fancy signal processing.

    The first modem used Differential Quadrature Phase-Shift Keying (DQPSK) at 1300 bit/s with 300 bit/s FEC. This is the old standby PSK mode, and not only works well, but is much easier to mechanize. This is the modem that is duplicated the hardware above.

    The FreeDV software program has implemented the newer Coherent PSK mode called 700B. This is a higher order of difficulty as far as modems go, but this is the cats meow for weak signal work. It operates at 700 bit/s with no FEC, but a load of synchronization pilot data (about 2/3 speech data, 1/3 pilot data).

    That's right, the modem works down into the dirt. Users have reported they can't even see the signal on their SDR radio screens, and are still decoding voice. Still though, you do need some sort of propagation, there's no magic here.

    But let's also be clear here, this voice is not telephone quality, nor is it meant to be. If you are interested in 6 kHz wide-band voice with a studio pre-amp, this modem will not be desired under your Christmas tree this year, or any year.

    Nope, this little sucker works with 1.4 kHz bandwidth (the ITU designation is 1K40J2E) and must be used in the phone allocations in the USA (sorry I didn't vote for allocation by mode, call Sumner, he was there when the last Choo-Choo left the station).

    The signal appears as 14 sub-carriers. 7 sub-carriers are used to transport the 700 bit/s digital speech vocoder frames. The other 7 are used as a Diversity channel, and are a duplicate of the other 7. So as the ionosphere does its thing all over the left half, the right half still makes it, and vice-versa.

    The modem operates at 75 baud (symbols/s) with a 2100 bit/s effective data rate, so all this poop to transport two 28 bit voice frames (56 bits) seems like a lot of overhead. The reason we shouldn't care, is the bandwidth used is about what we'd use on SSB, and it works down in the dirt. SSB sounds like poop down in the dirt, this one sounds a bit more robotic, but actually better than analog SSB. Actually, analog SSB can't usually be understood, at this noise level (I've tried), while the noise free vocoder will get you the information needed. You might need to say your callsign twice, but at least you'll get your Worked All States diploma in record time.

    Being a software hacker, I took the FreeDV code and chopped it up with a axe. I want to get it down to the bare minimum code so maybe I can put it in firmware (like the previous modem), or more likely convert it to Java SE and put it in my PC-Arduino with its touch-screen. If you're interested in hatchet work, have a cup of morning coffee and browse the code. You might think of a way to use it.

    ...and have fun, Steve
    K5YVY likes this.
  2. KK4YWN

    KK4YWN Ham Member QRZ Page

    N4JS and myself have been testing it on VHF ssb links. Its truly impressive. We're finding that the average transmit power is much higher with freedv that with voice, so the SNR is greatly increased over marginal links.

    I found the audio to be fatiguing at first, but I pipe the receive audio into an effects-chain. An eq and reverb help a great deal.

Share This Page