"AM" vs. DSB

[Keith Rowland]

Joel:  DSB sounds exactly like SSB on the air.  The only difference is
that it transmits both sidebands, the upper and the lower, and there is
no carrier.  Obviously, if there is no carrier, it must be reinserted by
the BFO or product detector at the receiver, just like any other kind of
sideband.  It is not compatible with conventional double-sideband AM
using full carrier (plate or otherwise modulated).

On the screen control grid, cathode or other types of modulation:  Only
a plate modulated rig can be modulated 100%. It is the audio impressed
upon a carrier that gives it its intelligibility, obviously, therefore
the higher the percentage of modulation of a carrier, up to 100% (on
AM), the more solid the signal will be.  There are few things more
frustrating than having a carrier of, say 40 over 9 on your meter, full
quieting in the receiver, and this thin little voice coming over at 65%
modulation or less.  The only reason for means of modulation on AM other
than plate modulation was price.  For a kilowatt of AM carrier, you need
to impress 500 watts of audio to modulate it 100%.  That's a lot of iron
in power supplies and a lot of energy consumed in modulator tubes.
Plate modulation is the best form of AM modulation, but also the most
expensive.

I hope that clears the modulation question up somewhat!

73, Keith



Joel Govostes wrote:
>
> Thanks for all of the replies to my inquiry about low power 60's phone
> transmitters (screen mod. vs. plate mod.).  The info was both helpful and
> interesting.
>
> Now -- DSB.  Several years ago, I owned a Hallicrafters HT-(I think)32,
> which I ran in conjunction with a SX111 rcvr.  The tx ran either sideband,
> CW, or or "DSB", as it appeared on the mode switch.  This, as I was told by
> the ham I obtained the gear from, was compatible with AM, and I did get the
> rig going on 3885 for a while, and had many enjoyable QSO's with "real"
> am-ers.  It also performed very nicely on SSB.
>
> I never did monitor my own signal, (and the rig operated fine,) but now I
> wonder - was the rig only generating the sidebands (both); i.e. was there
> no carrier generated in that case (true DSB)?  Or would there have actually
> been a carrier, perhaps reduced, compared to a "normal" AM  transmitter -
> signal.
>
> Perhaps the DSB was merely a feature to allow for AM compatibility on the
> air(?) -- but I never asked -- would the other stations have had to use a
> BFO to copy me?
>
> I also have some older radio handbooks which feature DSB transmitter projects.
>
> 73...& thanks for any info on this
> Joel   N1AEP
> Freeville, NY
>
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