Re: How does QDX implement the raised cosine ramp in the FT8 spec? #qdx
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Yes. Constant amplitude. But my understanding of section 5, on pages 9 and 10, is that this is achieved by applying a Gaussian shape to the individual rectangular pulse making up symbol. The transmitted signal is the summation of these pulses. At the transition between symbols (audio tones) there is a summation of the preceding and succeeding tone, which are Gaussian shaped such that the summed amplitude is in fact constant.
So each symbol (tone) is a shaped pulse; but adding adjacent Gaussian smoothed pulses gives in effect a constant amplitude. It is also equivalent to a gradual frequency slide between the two tones. So there is no conflict with the "single tone FSK" theory, which is why we don't need a linear modulator and amplifier.
And then on top of that, a raised cosine at start and end.
That was my understanding but it's quite possible I misunderstood.
Either way - QDX doesn't implement shaping at the start and end, but it does track the frequency slides so it minimizes trnamsitted bandwidth during the transmission itself.
73 Hans G0UPL
-------- Original message --------
From: Paul Christensen <w9ac@...>
Date: Sun, Mar 27, 2022, 6:37 PM
Subject: Re: [QRPLabs] How does QDX implement the raised cosine ramp in the FT8 spec? #qdx