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Re: [Amps] Audio BW -- Please just let us operate!!

To: Paul Christensen <w9ac@arrl.net>
Subject: Re: [Amps] Audio BW -- Please just let us operate!!
From: Vic Rosenthal <vic@rakefet.com>
Date: Fri, 27 Aug 2004 13:03:39 -0700
List-post: <mailto:amps@contesting.com>
Paul Christensen wrote:

http://www.aorusa.com/ard9800.html

With a bit of data augmentation, such a product has real promise for those
wanting to explore high fidelity audio on the HF bands.  The modulation
technique in this unit is DPQSK.

Since this is an AMPS mail group after all, I am interested in knowing more
about the peak-to-average ratio of the data and how it fits within the
acceptable duty-cycle of today's commercially-manufactured amplifiers.

I just looked at the web site, and since this device uses a form of PSK modulation, I get the impression that the duty cycle is 100%. So you would use RTTY ratings.


It's important to distinguish between the method of ENCODING a digital signal (which is what I was talking about) and the method of MODULATING an RF signal to transmit the digital information. The encoding sets the minimum theoretical bandwidth that is required to transmit a given signal.

Modulation is the process of converting the bits in the digital stream to RF that varies in some way, so that a receiver can recover the original digital stream which it will then convert to audio. Various modulation methods will be able to approach the theoretical minimum bandwidth more or less closely. Other things that depend on the modulation scheme are the ability to recover signal in the presence of interference, and duty cycle.

Normal AM and SSB are ways of modulating RF directly from an audio signal. Adding a digital encoding step (and corresponding decoding in the rx) allows us to use what we know about voice signals and how they are perceived by the brain to selectively remove information from the signal to reduce bandwidth without damaging (perceived) fidelity.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco


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