[RTTY] Distortion

[Bill Turner]

On Tue, 24 Aug 2004 11:27:09 -0700, Kok Chen wrote:

>Secondly, you can apply a steep bandpass filter around the AFSK signal 
>(I have no idea how many software modems actually do that, but I know 
>of at least one that does) to reduce the modulation sidebands -- you 
>can get modulating sidebands that are 300 Hz from the center frequency 
>(600 Hz total bandwidth) to fall off to nothing.
>
>This AFSK "roofing"  does not practically degrade the printability of 
>the signal, as observed by people being able to print RTTY perfectly 
>fine using 500 Hz and even 250 Hz receive filters.  But it helps reduce 
>your modulation sidebands (and allow you to apply that just little bit 
>more of the transmit power to where it counts -- but don't tell any of 
>the "QRO forever" guys :-).

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I would be wary of trying to narrow the RTTY signal by reducing the sidebands.
RTTY is a form of FM (think of a carrier modulated by a square wave) and as
such, sidebands are necessary to permit the receiver to recreate the original
signal.  Reducing the sidebands causes rounding of the square wave and thereby
reduces the number of milliseconds the output wave spends at full power.

A 170 Hz shift signal requires at least 250 Hz of bandwidth for good linearity,
and 300 Hz is probably better.

Any edition of the ARRL handbook has a good explanation of the FM sideband
phenomenon and all RTTY ops should read same.

--
Bill, W6WRT
QSLs via LoTW