On Aug 17, 8:46*pm, "Max Power" wrote:

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.

PSUDO-CODFM aka AM-CODFM
-- CODFM signal is of Audio bandwidth, 10 kHz or 20 kHz

DRM can use 9, 10, 20, 50, or 100 KHz bandwidth. Maybe more. The
bottom three are when it's used in the LW, MW, or SW bands. The upper
two are for the new extension of DRM into the VHF band (up to 120
MHz). Those bandwidths are chosen specifically so that DRM can coexist
with analog spectra in those RF bands.

The bandwidth of the signal is not directly related to the max audio
frequency here, as it is in AM. It's related to the max bit rate, at
different levels of robustness, like any other digital transmission.
In turn, that max bit rate can be related to the max audio frequency.
But with digital, you can trade off the audio frequency limit against
the bits used in quantizing each sample.

In short, DRM could have been used as the basis for the new HD Radio
service in the US, instead of the Ibiquity system we have here.
Functionally, they are similar.

Bert

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.

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