FreeDV Digital HF Voice QSO Party Apr 27/28th

Discussion in 'Amateur Radio News' started by VK5GR, Mar 2, 2019.

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  1. W0PV

    W0PV Ham Member QRZ Page

    No. SSTV is an IMAGE mode. Allowed in USA on that frequency, as per band chart previously posted.
     
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  2. W0PV

    W0PV Ham Member QRZ Page

    I am all in favor of experimenting with HF digital voice.

    However, IMO, as it is today, even with FreeDV, I think it is doomed to a very small niche.

    Claims of being more sensitive are arguable. I predict the DYNAMIC effects of HF propagation may play more havoc on digitized voice received signal results then anticipated from more simple steady-state demonstrations.

    Also I am curious if simply applying more advanced digital enhancement and noise reduction AFTER detection of analog signals at or just below the noise threshold would achieve similar or perhaps even better results then trying to modulate and xmit the bits first.

    However, the biggest factor that will doom DV is this - it brings back xmitting of a continuous carrier(s). That is a step backwards from SSB for most HF ops.

    Like FM, or legacy AM with carrier, when you press the PTT on DV, even without speaking, the xmitter immediately transmits significant QRO.

    Compared to SSB the increase of xmitter duty cycle and therefore decrease in overall efficiency is obvious. But the real negative effect is on the receiver end.

    DV will produce a "capture effect", just like FM.

    Only one signal, the loudest, at a time, will be capable of being decoded. And if signals are nearly the same, the resulting distortion may make none capable of being successfully understood. This is why FM (and DV) has not made inroads into aviation comms where AM and SSB still reign.

    While the resulting quality of clear channel DV may be acceptable for casual rag-chewing or round-table ham operations, this will not fare too well for other HF operating activities where multiple stations may call at the same time, ie, nets, special events, DX'ing, contests. And right now the bulk of operation on HF is the latter.

    Nevertheless, again, I think this planned event to play with FreeDV is worthwhile. I am setting the date to observe and perhaps participate too.

    Best of luck.

    73, John, WØPV
     
    Last edited: Mar 11, 2019
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  3. W0PV

    W0PV Ham Member QRZ Page

    Go even further back ....

    IMO, with the vast improvements in digital techniques, DSP and PC, if the worthy objective is to increase range and capabilities for detecting weak signal VOICE comms (like FT8 was for data) enterprising hams perhaps ought to more heavily explore bringing back the concept of amplitude-companded single-sideband or ACSSB, or sometimes called "linear modulation".

    This was developed long ago and got some traction in commercial services, mostly VHF. Hams experimented too, but it was a bit too complex (hardware) and unnecessary for most then because it was still the boom-time build-out of them using normal SSB (and we had sunspots ;)).

    A key point in the link provided above is, "It offers improved effective range over standard SSB modulation while simultaneously retaining backwards compatibility with standard SSB radios." (and SSB op practices - WØPV).

    There is quite a bit about ACSSB in the literature. Start with this nine year old thread here on the Zed.
     
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  4. WB5WPA

    WB5WPA Ham Member QRZ Page

    I think FreeDV (using OFDM, which uses many little 'carriers' to carry the digital voice info) represents a sea change over ACSSB, as it isn't backwards compatible.

    BTW, the use of ACSSB on VHF (notably the 220 MHz version by Securicor Radiocomms, previously SECURICOR PMR) did a voice shift on the upper voice band and and inserted a pilot tone or carrier; they called it TTIB (Transparent Tone In Band). The receiver likewise removed the tone, and shifted the audio back into position.

    From the manual for the Linear Modulation Mobile Radio System for said equipment:


    Clipboard_TransparentToneInBand.jpg
     
    Last edited: Mar 11, 2019
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  5. W0PV

    W0PV Ham Member QRZ Page

    I'm not suggesting to go back strictly to the original ACSSB methods any more then a supporter of FreeDV would prefer a throw-back to the AOR G4GUO open protocol. Just that the ACSSB modulation concept should be re-explored as well, especially for HF, using todays SDR & PC sound-card app capabilities.

    Fox example, the use of a pilot tone in ham ACSSB to precisely sync the channel could probably be eliminated considering the higher frequency precision of today's typical amateur gear, and the understanding and tolerance of the ham operator and their ability to "tune" to cope with any off-freq distortion, compared to the lack of that in a commercial user that needs precise station-to-station channelization. A different domain, yet a concept similar to why and how FT8 does away with xmsn leading edge detection for timing and instead requires a relatively precisely set clock.
     
  6. K3RLD

    K3RLD Ham Member QRZ Page

    Just an update: I manually compiled FreeDV 1.3 last night, as the various modes are NOT compatible (I had initially thought that perhaps they were). A large majority of the QSO's are using 700D, but via qso.freedv.org you can request that the regular ops use different modes in order to test your rig + software (Thanks to K5WH for the help last night, and for my first FreeDV QSO!).
     
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  7. WB5WPA

    WB5WPA Ham Member QRZ Page

    @W0PV

    I'm not suggesting to go back strictly to the original ACSSB methods any more then a supporter of FreeDV would prefer a throw-back to the AOR G4GUO open protocol. Just that the ACSSB modulation concept should be re-explored as well, especially for HF, using todays SDR & PC sound-card app capabilities.

    VHF/UHF 'path' environment different than HF WRT fading and muti-path. Picket fencing heard on weak signal FM results in noticeable voice amplitude going up and down with SSB with moderate signal strengths even. Reason why ACSSB was invented. Ever talk to a mobile on 2m SSB? Then you hear up and down flutter on voice. We are spoiled by FM ...

    Fox example, the use of a pilot tone in ham ACSSB to precisely sync the channel

    Pilot tone used to AMPLITUDE equalize the channel. Secondary function to acting as frequency 'sync' or reference for (effectively) rcvr fine tuning.

    could probably be eliminated considering the higher frequency precision of today's typical amateur gear,

    See above

    and the understanding and tolerance of the ham operator and their ability to "tune" to cope with any off-freq distortion,

    See above

    compared to the lack of that in a commercial user that needs precise station-to-station channelization.

    AMPLITUDE equalization. Probably even more important when DATA is passed (as the ACSSB system was also adverted to do DATA)

    A different domain, yet a concept similar to why and how FT8 does away with xmsn leading edge detection for timing and instead requires a relatively precisely set clock.

    Clock is really NOT that critical with FT8, or WSPR for that matter. Precise is wrong descriptor. Have you looked at typical "DT" values from users on FT8?
     
  8. W0PV

    W0PV Ham Member QRZ Page

    Again, not suggesting for HF amateur use to copy all the previous commercial ACSSB methods. As pointed out, the main intended use for that system, as a narrow band higher range alternative over VHF/UHF FM, has specific requirements and features.

    Of course, the FT8 clock is not critical to get some results, especially as an absolute reference. It is a relative parameter between a specific xmtr and rcvr. Sure, being exact is not always necessary. however, more precision between two parties does have a very noticeable effect on demodulator decoding success and ultimate sensitivity.

    What I am suggesting for a ham ACSSB v2.0 is similar to what was done with FT8, ie, strip out some of the features of previous modes that are perhaps not absolutely required to pass acceptably intelligible short payload ham HF weak signal communications.

    That user specification, ie, acceptably intelligible short payload HF weak signal voice communications, is far different then what a typical FM or DV user is anticipating (or AM ragchewer) For them I bet its mostly about achieving the higher fidelity aspect, and QSO's characterized by long monologues with no fast-break T/R interactions possible or convenient during pauses in speaking. (most HF FT8 QSO's these days are not between weak signals :() Copying by ear a fluttering 2m SSB mobile may not be fun but it can be accomplished at far lower signal levels then a similar level FM picket-fencer.

    Perhaps an ACSSB rcvr amplitude expansion method, good enough for the HF weak signal ham specs above, using modern DSP, can be accomplished using a "virtual" pilot tone, ie, info derived from the detected sideband signal and/or otherwise synthesized on the receiver end, like the BFO in a product detector.

    Again, for amateur use, unlike commercial, a degree of real-time operator intervention or "tuning" required for optimum results is a viable requirement (like getting the PC clock close enough). Parameters that could be op adjusted for best results, like the expander gain ratio, equalization, and/or other dynamics or characteristics.

    But to steer back to the thread topic, I am still curious to see how FreeDV works in real life in typical HF conditions; think 75 meters summertime at night, or a crowded 20m band. Outside of its weak signal potential over FM, I wonder if with adequate SNR it will produce a near totally "noise free" (and artifact free) vocal reproduction.

    That could be far better then ESSB, AM or FM, with all the crashing QRN and hetrodynes.

    Will it be as Steely Dan famously sang, "No static at all" ?

    Great for round-tables :D but not DX'ing :(.
     
    Last edited: Mar 13, 2019
  9. K2SDS

    K2SDS XML Subscriber QRZ Page

    As it stands it's a fun, interesting experimental mode. Not ready for mainstream. Almost all voices FreeDV sound the same to me, hard to tell the difference between OPs without knowing who it is in advance - imo. Requires excellent conditions for low noise and no dropped data. The exception on HF.
     
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  10. W0PV

    W0PV Ham Member QRZ Page

    Sounds like to get hi-fi DV in noise and QRM will require sacrificing weak-signal potential and narrow bandwidth and moving back toward a DRM-like derivative again. Like no "conversations" on FT8, it's hard to get the best of both worlds. :(
     

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