[pskmail] Re: Comparing digimodes and ssb voice...what is good/bad result?
- From: John Douyere <vk2eta@xxxxxxxxx>
- To: pskmail@xxxxxxxxxxxxx
- Date: Wed, 15 May 2013 09:33:48 +1000
Hello Tarmo,
I did similar test than you a few years back. I tested on VHF some data
modes transmission both in SSB and in FM from the same locations. I
progressively increased the distance until I had no reply under any mode /
modulation from the Pskmail server. Using remote desktop control I was
checking the decoding on the Pskmail server side as well, mainly for
frequency alignment (explanation below). Here is what I noticed:
1. The FSK modes like Thor and MFSK will work almost the same under SSB
than FM modulation, right down into the noise, to the point that it is very
difficult to say if there is modulation or not being transmitted
(especially with THOR8).
So I deduce from my tests that digital modes were going through when not
voice contact could be made at all, both under SSB and FM modulation.
On the other hand the PSK modes were much less sensitive under FM than
under SSB.
2. At VHF and I suspect even more so at UHF frequencies, the VFO accuracy
is poor for digital modes. I was getting 100Hz+ of VFO mis-alignment with
an FT-817 and an FT-857 in my tests. Therefore the BIG advantage of FM
modulation is that there is negligible audio frequency shift even with a
significant VFO shift, and therefore sensitive modes like MFSK16 or THOR8
can be used quite effectively.
3. There was an article in QST some years ago from Skip Teller about
digital modes under FM modulation and he had the same conclusion. In fact
his article triggered my tests.
So, assuming we get the same or close sensitivity with FSK modes under SSB
and FM, then you should get a sensitivity advantage using digital modes
over voice regardless of the underlying modulation (SSB/FM) since an SSB
voice communication requires a minimum of about +6dB of s/n to be
understandable while digital modes like THOR22 can be decoded at about -8
to -10 dB of s/n (using a white noise test).
From memory THOR8 was readable down to -13dBs in my bench tests so it
should be usable further than voice regardless of the modulation.
So if distance/sensitivity is the target I would definitely stay with FM
modulation and use the sensitive FSK modes. Based on what I saw you
"should" get a better result than under SSB voice.
Let us know what you find. Hope that helps. All the best.
Regards,
John
On Wed, May 15, 2013 at 4:54 AM, Tarmo Huttunen <tarmopaha@xxxxxxxxx> wrote:
> Hello all!
>
> I need a little advice from more experienced pskmailers.
> Last night I send some aprs beacons with albrecht2990 handheld/AndPskmail
> to my own base with thor22.
> Distance was about 20km. Beacons were received mostly ok, about 80% right.
> In that distance/path voice communication is easy, it is in about the
> border of practical fm work, still usable (I have tried that place before)
> and phone ssb is easy and clear.
>
> So queston is: what range should I expect from digimodes like thor22?.
> Should thor22 have "longer" or "shorter" maximum distance on line-of-sight
> communicatons as phone ssb?
>
> I know there can not be accurate answer for this question. What I am
> looking for is for example experiences like " I was able to work ssb when
> psk500 didnt get throw...but thor22 did..." or something like that.
>
> If we consider pskmail digimodes starting from most robust (thor8?) to
> less robust (psk500?) so where we should consider limit for phone ssb
> usually to be? Is it somewhere on thor-modes or somewhere on psk-modes or
> how?
>
> Without arq, is this thor22-80% right bad or good result on path where
> phone ssb is mostly easy?
>
> Tarmo
>
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