Shortwave band PDM, PSM and hybrid modulators can't modulate true CODFM --
only AM variants.
http://www.contelec.com/pdf%5CDRM_Requirements.pdf
"DRM Transmitter Requirements and Applying DRM Modulation to Existing
Transmitters"

High power SW transmitters are not linear, in spite of the advent of PSM &
PDM and Hybrid modulators.
High power MW transmitters have some of the same linearity problems.

A kit usually is needed to fool or coax most modern AM (be they on SW or MW)
transmitters into transmitting CODFM -- without it most AM transmitters
would not be able to handle CODFM with any linearity whatsoever. This is
CODFM limited to the audio band only, not wideband CODFM.

CODFM on SW (and MW too, same kind of technology) rides on top of a "primary
carrier wave" with sidebands that carry the other "pseudo carriers" at
energies nearly equal to the primary carrier wave. The pseudo-carriers are
just audio tones, not true carrier waves in the modulator.

DRM demodulators on SW and MW are based on PLL and Sync detection ... of a
primary carrier wave. The slightly increased power of the modulator's
primary AM carrier helps hold recover sync for non-ground wave reception.
The AM sidelobes are still there, it is just that they are at 100% duty
cycle.

With true VHF / UHF etc ... CODFM, there is no primary carrier wave at all
in the modulator -- and the modulator is not PDM or PSM or a hybrid of
them -- and the energy distribution of the carriers is linear. You can't
really do this on MW or SW, or no one has bothered to create a modulator to
do so.

With CODFM over SW (and MW), one of the carrier waves in the carrier group
rides on primary carrier wave (and it probably has ~1.5db ... 3db more
energy in it to help with the link margin, due to fading margins this does
not disturb linearity).

-- You can switch to analog "AM" mode service with a "flick of a switch", as
CODFM is imposed on an AM waveform. A true CODFM modulator could never be
switched into AM modulation service due to the electrical engineering
impossibility of such.
-- With AM modulators it is difficult (or even impossible) to make the
primary carrier have the same amount of energy as the sidelobes ... but this
is also a energy saving issue for analog transmission.

How you managed to confuse HD Radio and VHF DRM (more or less totally
unrelated technologies to DRM on MW and SW) is a mystery to me. Masking (and
simulating) is even not my DRM radar, albeit it works for VHF service. There
is no DRM for VHF yet, only SW and MW. DRM over VHF will probably resemble
HD Radio.

===============
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.
===============
Sorry, but that's simply not the case. It is true that DRM can be used
along with AM, or along with FM, exactly the same as HD Radio does. In
a simulcast, where the DRM COFDM spectra can be placed on either side
of the analog spectrum. But the DRM signal per se is COFDM, modulated
either QPSK, 16-QAM, or 64-QAM. Not AM.

All of this is readily available information:

http://www.drm.org/uploads/media/ETS...980_v2.3.1.pdf

Check out in particular Annex K, which I think is what's confusing
you.