On Wed, 30 Apr 2008 21:16:04 -0800, glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>Jeff Liebermann wrote:
>(snip)
>
>> I'll skip all the intermediate modulation schemes and jump directly to
>> the present. Microprocessors have become so cheap, that it's now
>> economical to do DSP (digital signal processing) in every radio.
>> Digital has the huge advantage of offering error correction, noise
>> immunity, bandwidth compression, and high spectral efficiency. Schemes
>> have been devised that will extract useful audio or data from signals
>> that are well below the thermal noise floor. You can't do that with
>> AM (or FM). If you attempted to do Wi-Fi using pure AM technology,
>> the data thruput would be horrible and/or the error rate would be
>> hideous.

>Somehow this reminds me that there have been discussions on
>using digital techniques to demodulate standard AM and FM radio
>signals.

Ah, topic drift...

SDR (software defined radio) uses digital techniques to demodulate
just about anything. Even CW (Morse code) can be demodulated and
decoded. For example:
<http://www.rfspace.com/SDR-IQ.html>
<http://www.amqrp.org/kits/softrock40/>
You can even do it with a PC sound card using Linrad, SD-Radio, and
other software.

>Now that we have digital radio, what seems to be
>called HD radio (radio stations seem to advertise it pretty often).
>http://en.wikipedia.org/wiki/HD_Radio

Yep. It's the new name for Ibiquity (IBOC) radio:
<http://www.ibiquity.com>
I've played with commercial products. It's nifty. Good quality
stereo crammed into a 9KHz AM broadcast channel.

In Canada and Europe, they have a different system called DRM (digital
radio mondiale).
<http://www.drm.org>
I have a downconverter (simple mixer from 455KHz to about 12Khz) hung
on my HF ham rig and use a sound card to do the demodulation. The
catch is that I had to pay $50 for a codec license. Grrr....
<http://www.drmrx.org>
<http://www.drmrx.org/receiver_mods.html>
I couldn't get the open source DRM software to work for me:
<http://drm.sourceforge.net>

Anyway, the point is that it's easy and common enough to demodulate AM
with digital techniques.

>I don't remember much discussion here about it.

Where is here? I'm in the Santa Cruz, CA area, and we discuss just
about anything.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS