Misunderstandings in SW transmitter design -- CODFM (as DRM usesit) in the SW bands is a PSUDO-CODFM
Max Power wrote:
You misunderstand modern (post 1950) SW transmitter design.
Class-B, PDM and PSM (and hybrids of the two) modulators produce an AM
waveform, no matter what.
Some Russian Class-B transmitters can handle transmitting FM, but these
are very pure analog beasts.
Only the modern PDM / PSM / Hybrid transmitters are being put into
service for DRM, no Class-B modulators.
CODFM (as DRM uses it) in the SW bands is a PSUDO-CODFM; the audio of
the CODFM signal is imposed on an AM waveform for transmission. The BSEE
electronics is a lot simpler, and it is backwardly compatible with the
installed base.
Thankfully this DRM AM-CODFM design is so simple that it is totally
misunderstood, and you can't tell the difference at the receiver end.
PSUDO-CODFM aka AM-CODFM
-- CODFM signal is of Audio bandwidth, 10 kHz or 20 kHz
-- Most modern SW transmitters can accept 20 kHz audio with only minor
retrofits
-- Most modern SW transmitters are "linear" up to about 40 kHz, based on
90 kHz PDM / PSM clock rates
-- Most modern SW transmitters only know how to transmit AM +SSB (but
not FM)
-- SW DRM transmitters universally get their PDM / PSM clock rates
increased to 100 kHz + (Continental, retrofit infos)
-- A true wideband CODFM signal of 1 MHz would not work on SW, ITU
allocation issues aside ... the installed transmitters base cannot
handle it, only PSUDO-CODFM is workable
-- VHF / UHF CODFM transmitters are real CODFM transmitters, as there is
no PDM / PSM / etc involved.
There are no true CODFM transmitters in the SW band with powers above
2kw (or maybe 10kw), as the electrical engineering is just too hard to do.
DRM,
the new standard for DAB over SW uses AM modulation with CODFM content.
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DRM, Digital Radio Mondiale, uses COFDM, as does the European VHF and
L-band DAB system and the US HD Radio. COFDM, in its multiple
subcarriers, does not use AM. Instead, each subcarrier is modulated with
an orthogonal QAM. So that's a modulation in which both amplitude and
phase are varied at the same time, to carry twice as much information as
an AM signal of the same bandwidth can carry.
My head hurts.
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