Re: LF: SSB - why not go digital?

[Markus Vester <markusvester@aol.com>]

Hi Geri, all,

 

I like the idea of trying voice on MF or even LF,  and I was fascinated by actually hearing Gus talking on 507 kHz, and Geri on LF slow voice. I admit that I even tried (low-power daytime) SSB above 135.7 kHz once, and Stefan was actually able to hear and record my voice, albeit at marginal SNR at 180 km. As my LF TX antenna is fairly narrowband (Q ~ 200), I had inserted a phase shifter to transforme the low output impedance of the PA into a current source at the feedpoint.

 

The good thing about slow voice is that we also get the SNR gain associated with the narrower bandwidth. But the non-realtime operating procedure is sort of difficult. We did use "roger beeps" to mark the end of message., but there is always an inconvenient delay for recording and replaying the messages. So some sort of realtime narrowband voice transmission would indeed be desirable.

 

However I would NOT fancy digital voice modes all that. For one, like all digital modes its it's all-or-nothing. Either the message decodes well, or you get garbage, and you don't know what has been going on in the channel. In linear analog SSB, you hear if someone else is calling on frequency, or what type of interference came up, or whether there is selective fading. The other thing I don't like about digital modes is that they tend to occupy the whole channel permanently with high average power - just look at the spectra of DRM vs AM modulated BC transmissions. It may well turn out that a 2 kHz linear SSB transmission is much more friendly than a 1 kHz digital channel. 

 

I have been making some attempts on analog quasi-realtime narrowband voice transmissions. The principle was is much the same that has long been used for changing audio speed without changing pitch, ie cut the audio into ~ 20 ms grains. To accellerate or reduce bandwidth, you either leave out or average several grains, using a sliding window. On replay, each grain is repeated to extend it in time. This is not difficult to implement. In my own trials, I could maintain speech readability with a factor four bandwidth reduction, but it did sound very "robotic" because the fixed timing for the grains impressed itself onto the voice pitch. I think that there are better ways of adapting the timing by tracking the fundamental frequency to preserve the pitch modulation. Using Windows media player for accelerating and decelerating seemed to provide quite natural speech quality at a reduction factor of four.

 

Regarding the frequency allocations, like Graham I would think that on MF the narrowband beacons should best be placed in narrow slots near the band edges. To avoid blocking, we would preferably again use split band for west-east vs east-west intercontinental work, as we do on LF. Parts of these slots should also be utilized for the slow versions of Opera or WSPR. The wider modes would better be placed in the in the middle, perhaps with 2 kHz each for CW, middle-range digital modes, and one full-width SSB channel.

Best 73,

Markus (DF6NM)

 

-----Ursprüngliche Mitteilung----- 
Von: geri <geri@dk8kw.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Do, 3 Jan 2013 10:09 am
Betreff: LF: SSB - why not go digital? Have a look at FDMNV

Hi,

 

I just did some research and obviously there are people who already have thought about low bandwidth voice communications. There is a mode called FDMNV, based on a common CODEC that transmits voice in a 1100 Hz bandwidt (see http://n1su.com/fdmdv/).

 

In the describtion of the software it says "it caters to high quality digital voice under poor band conditions, in only 1100Hz bandwidth" which probably is the kind of features we are looking for. The software is open source and surely worth to have acloser look at. 

 

Also, i am sure we (in Germany) can negotiate with our authorities to get an extension of our 800 Hz bandwidth limitation at least under certain conditions. For everyone without limitation this seems to be the software to just go ahead and try it!

 

 

Best 73

 

Geri, DK8KW

 

 

> > ----- Original Message -----
> > From: M0FMT
> > To: rsgb_lf_group@blacksheep.org
> > Sent: Thursday, January 03, 2013 12:30 AM
> > Subject: Re: LF: 477 A local SSB chat band ?
> >
> >
> > Hi Graham Geri and all
> >
> > Warren has seen SSB first hand on the MF QRG and we have this side also.There was an SM station (call sign forgotten?) in the early days of 500 that added an SSB anouncement on his beacon and it worked well.
> >
> > To my spec. :- "One SSB channel at the high edge of the band only used during daylight hours no DXing!" add " Use VOX, no monologuing allowed" to be a laissez faire operation no band plan, but fair play!.73 es GL es HNY Pete M0FMT IO91UX
> >
> > From: Graham <g8fzk@g8fzk.fsnet.co.uk>
> > To: rsgb_lf_group@blacksheep.org
> > Sent: Wednesday, 2 January 2013, 20:48
> > Subject: Re: LF: 477 A local SSB chat band ?
> >
> >
> >
> > Geri
> >
> > 800 Hz B/W ... May be this is something 'Wolf' can code for you ? a b/w compression / expansion module in SL , to limit the tx b/w to 800 Hz must be possible to divide by 3 and mult by 3 with a linear shift as well ?
> >
> > G..
> >
> >
> > From: Holger 'Geri', DK8KW - DI2BO - W1KW
> > Sent: Wednesday, January 02, 2013 8:36 PM
> > To: rsgb_lf_group@blacksheep.org
> > Subject: Re: LF: 477 A local SSB chat band ?
> >
> >
> > Hi,
> >
> > I kind of like the idea of an SSB channel. I just pulled out the conditions of my first secial license for 160m from 1973 or 1974. That said 1815 to 1835 kHz with 10 Watts in CW, additionally one SSB channel for 1832 to 1835 kHz, so why not trying this on 630m, too? Here in Germany we are currently limited to 800 Hz bandwidt but I am sure we can sork towards a special license under certain conditions such as daylight operation only .. sounds good to me!
> >
> > Vy 73
> >
> > Geri, DK8KW
> >