[hpsdr] Is DRM the only free Digital AM waveform?

[Tony Langdon]

At 01:55 PM 4/7/2007, Chris Albertson wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>
>DRM is "AM"?   No, I think it uses OFDM where every
>carrier is modulated with QAM

Correct, DRM is OFDM with QAM4 - QAM64 on each carrier.

>One thing ham radio CAN do that DRM can't is use a
>"back channel" that sends error statistics back to the
>transmitter from the receiving end.  With this data
>the transmitter can redistributed power between
>the carriers and even "tune" the inter-symbol timing.
>(opening a hole around QRM, so to speak)
>I really think with this kind of tuning you can do very
>close to the Shannon Limit.

Well, doing this in real time requires a full duplex radio (DRM is 
continuous, not packetized), but it's certainly possible to send a 
time delayed set of statistics when different people 
transmit.  Therein lies another piece of complexity.  ham radio is 
more about one - many communications, as opposed to one-one, and in a 
one-many setup, there are N - 1 paths in use simultaneously (N being 
the number of stations on the air).  And as different people 
transmit, all those paths change dramatically...


>DRM's bandwidth is to wide to be used on ham HF bands.
>I think you can do this on UHF.   But there was an effort
>called "ham DREAM" or something like that where they
>modified DRM to get it down to under 3Khz bandwidth.

In VK, you would have to limit bandwidth to 8 kHz to use shortwave 
DRM below 28 MHz.  You could use the standard shortwave DRM mode on 
10m and above.


>"DREAM" is an Open Source implementation of the full
>DRM standard.
>
>But "Narrow DRM" is already been done.  I ran across
>this the other day
>http://www.linuxinsider.com/story/56710.html

WinDRM is another implementation of narrow DRM.  And there is DRMDV, 
which uses a lower bitrate (1200 - 1400 bps) to carry voice.


>Even the full DRM is still narrow enough that a sound
>card would work.  So only simple SDR hardware is
>needed
>
>I'd like to try things like "TCP/IP over ham-DRM"  this
>ham-DRM could work like or replace AX25

Hmm, you'd probably have to set it up in duplex pairs and run PPP 
over the data channels. :) There is a significant training time, so 
simplex packet operation wouldn't be feasible (turnaround time too long).

73 de VK3JED
http://vkradio.com


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