Hans,
Your anti-vax anti-social analogy is quite apt. I don't understand the
allure of DSB (or the superstition against vaccines, for that matter). If
one is going to suppress the carrier, why not suppress the superfluous
sideband as well? As you point out, it's not that hard, but I suppose it's
the crystal filters that seem daunting to some, and the phasing method of
generating SSB is near rocket science to the uninitiated.
OTOH, your anti-social critique of DSB-SC also applies to the traditional
AM mode. It's not used much anymore--only a small number of boat-anchor
fans, and even they probably use SSB a lot more than AM. So when they're
hogging 6+ kHz of bandwidth it's not a big deal (except, perhaps, during a
contest) because most everyone else is on SSB.
Of course, your wasted-power argument applies even more to AM than
suppressed-carrier DSB. But unless you're battery dependent, that doesn't
matter that much. A nice old boat anchor pumping wasted watts into the
atmosphere at least helps keep the shack warm on a cold winter night.
73,
Todd K7TFC
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On Wed, Oct 30, 2019, 12:17 AM Hans Summers <hans.summers@...> wrote:
Hi all
In my opinion... I don't care about the boring legal aspects... purely
technical:
1) DSB is something of an abomination in that it is difficult or impossible
for two DSB stations to work each other. It's the only mode I can think of,
where you cannot QSO with someone else using the mode. The other side needs
to be using SSB. On FT8, if they were a few Hz apart it would probably not
decode, as far as I can see. You could offset the two stations by more than
the 50Hz FT8 bandwidth, then both sets of transmissions would appear twice,
to the other side. But that would get very confusing, and hardly be
practical... particularly considering the transceiver is not frequency
agile, it is crystal controlled, without an adjustment. Even if an offset
could be arranged between two DSB stations the next DSB station worked
would have to go through the hole rigmarole again.
So DSB relies on the fact that only a small proportion of operators will be
using it. Everyone else will use SSB. In that regard it is like an
anti-vaccinationist. You don't vaccinate your child because you want to
avoid the tiny risk of potentially very serious side effects. But this only
works if most other people DO vaccinate their children, so that yours is
unlikely to come into contact with a disease bearer. But if a significant
proportion of people don't vaccinate their kids then preventable diseases
return and infant mortality starts to go back to pre-vaccination levels. So
NOT vaccinating your kid could be viewed as a rather antisocial action. By
this somewhat tenuous analogy, so could operating DSB...
2) Inefficiency... we consider that the output power, say 2W, is divided
between a wanted Upper Sideband and an unwanted Lower Sideband. So 50% of
the RF output power is wasted. In fact in one view, it is worse than this
because a larger proportion of the available DC *input* power is wasted
too. The fact that it is DSB (two tones), means you must use a Linear. In
this case, Class-A. If FT8 was modulated as SSB then it would be a single
tone and a linear would not be needed; the power amplifier could be Class
C/D/E etc and that would offer higher efficiency. This is the reason that a
triplet of BS170s are here producing around HALF the power of an equivalent
triplet of paralleled BS170s in a CW transceiver, operating at similar
supply voltage and current.
3) Receiver SNR degradation... the fact you are receiving BOTH sidebands
gives a 3dB penalty on the Signal to Noise Ratio, even if there are NO
other interfering stations on the unwanted sideband, only noise.
4) Interference... you are interfering with anyone using the unwanted
sideband. That 3kHz of band below the FT8 is therefore unusable to other
stations. Yes a single DSB is a 50Hz slice in that band. Harmless? Maybe,
if that band segment is also normally used for narrow band modes. But a few
watts of carrier in the middle of an SSB QSO would not be so much fun...
and even a CW QSO will typically be using a wider bandwidth than 50Hz so
the potential for interference is greater. Also bear in mind that the
FS8Call band is 4kHz above FT8. So anyone operating DSB on FS8Call is
likely to be directly stepping on FT8 traffic.
I gave this a lot of thought. Overall I concluded, in my personal opinion,
I just don't think there's much of an excuse for using DSB in 2019. In
1960, maybe. But in 2019... it just isn't that much harder to make a nice
single sideband signal. Just my view.
73 Hans G0UPL
