Misunderstandings in SW transmitter design -- CODFM (as DRM usesit) in the SW bands is a PSUDO-CODFM
 

On Aug 18, 8:52*pm, "Max Power" wrote:

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"

Nowhere in the paper you pointed out does it state that DRM is
transmitted "over AM." Check it out again, paying attention to the
discussion on amplitude *AND* phase signals that must be generated.

-- 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.

Come now. Think of the ideal transmitter as being a box that generates
a perfectly flat RF spectrum within a well defined channel bandwidth.
Whatever input you feed that box modulates the RF carrier to create an
exact replica, as high power RF emission, of the input signal to the
transmitter. And nothing is emitted outside the desired frequency
channel.

COFDM exciters are what create all the subcarriers of COFDM, and which
modulate the signal in phase and in amplitude. The transmitter must
simply pass this on through, perfectly unchanged (ideally). The paper
describes problems that transmitters optimized for analog AM have in
transferring that COFDM waveform, and work-arounds. The main issue is
that AM transmitters are made more efficient by being designed to be
non-linear. This is because for analog AM, they don't need to be
perfectly linear in phase.

Nowhere does the paper claim you aren't transmitting amplitude and
phase variations, multple subcarriers, or anything else required by
DRM. The main thrust is, if you want to use your current AM
transmitter, here are a set of compromises and workarounds. The
underlying message being, the next tranmsmitter you buy should be
linear.

Ultimately, sure, you can describe the COFDM subcarriers as being
audio tones. Just as the telephone industry does, when they call their
version of COFDM "discrete multitone" (DMT). Calling it "multitone" in
no way changes the fact that these are multiple subcarriers of COFDM.

http://en.wikipedia.org/wiki/Discret...one_modulation

Bert