[RTTY] AFSK Tips

[Joe Subich, W4TV]

> Someone had a tip the other day (W4TV?) recommending setting the
> soundcard output level on the high side, thereby minimizing mic gain
> on the radio side, keeping unwanted hum and distortion at a minimum.
> That sounded like a good idea to me.

That was Chen's (W7AY) recommendation.  The only caveat is to keep the
soundcard "slider" to no more than 80% since a great number of sound
cards start to distort once one gets past that level

> On the K3, apparently there is a special 400 Hz DSP filter that can
> be engaged in the AFSK transmit audio path.

CONFIG: AFSK TX (FIL ON).  The Elecraft filter works wonders for hum
due to bad connections and overdrive.  However, one would still want to
take care in the connections and audio levels.  Set the K3 for 4-5 bars
of ALC action only as that is the point at which the ALC starts to act
and the inputs stages are neither expanding (added gain) or compressing
(reducing gain) on the AFSK input.

 > The MMTTY high order transmit filter also got good reviews.

If you don't have a K3, Enable the TxBPF (Options -> Setup MMTTY ->
TX tab -> TxBPF) and set it to 512 tap /300 Hz.  Be sure to take very
good care not to overdrive the transceiver - use the "Line In" if one
exists as it will generally have lower gain meaning less attenuation
between sound card and transceiver with a decreased chance of hum.

*NEVER* use a sound device that has automatic (audio based) PTT unless
that PTT circuit is buffered (operated from the right channel of the
sound chip or has an isolation amplifier between the sound chip and
the detector diode).  The inexpensive sound devices hang a diode off
the audio line (they tell you to run the sound card at max level)
and that will result in AFSK (and PSK) that is clipped on one half of
the side wave resulting in both a DC offset *and* significant harmonic
content.  Since these devices *follow* the MMTTY AFSK filter, these
devices can not be helped by MMTTY filtering.

73,

    ... Joe, W4TV


On 1/27/2013 11:17 AM, Dave Hachadorian wrote:
> Someone had a tip the other day (W4TV?) recommending setting the
> soundcard output level on the high side, thereby minimizing mic gain on
> the radio side, keeping unwanted hum and distortion at a minimum.  That
> sounded like a good idea to me.
>
> On the K3, apparently there is a special 400 Hz DSP filter that can be
> engaged in the AFSK transmit audio path.  That filter has also gotten
> good reviews from the guys who recently did the FSK/AFSK comparison
> study.  The MMTTY high order transmit filter also got good reviews.
>
> I am also considering going to AFSK, just to get rid of the K3's whacky
> power transients on FSK.  I am tired of seeing amplifier screen current
> jump all over the place.  It makes me nervous.
>
> Dave Hachadorian, K6LL
> Yuma, Arizona
>
>
>
> -----Original Message----- From: Dennis
> Sent: Sunday, January 27, 2013 8:57 AM
> To: rtty at contesting.com
> Subject: [RTTY] AFSK Tips
>
> Having read some of the issues about AFSK vs FSK, I'm thinking of
> changing over
> from FSK to AFSK operation.  I'll be using a K3, if that matters. I
> already know about
> not overdriving the mike input, but other than that, are there any tips
> for AFSK
> operation in general, or K3 specific?
>
> Dennis W1UE
>
> _______________________________________________
> RTTY mailing list
> RTTY at contesting.com
> http://lists.contesting.com/mailman/listinfo/rtty
> _______________________________________________
> RTTY mailing list
> RTTY at contesting.com
> http://lists.contesting.com/mailman/listinfo/rtty
>