[RTTY] [MMTTY] The Problem with AFSK ...

[Joe Subich, W4TV]

 > I do take exception to one of your admonitions:
 >    "*or* cases where the soundcard output is run wide open"
> That might be an issue for some sound cards, I don't know. HOWEVER,
> such erroneous advice may lead to problems among hams who use some of
> the most popular sound card technologies, the Signalink-USB.

It is certainly a problem for generic sound cards which can produce
signals of several volts P-P which are easily capable of driving the
dynamic mic preamps into hard clipping.  However, it is also an issue
even with your Signalink-USB if the user sets the jumper for high gain
and cranks the tramsit level "full right".

 > What you heard is no doubt a very poorly adjusted transmitting system
 > and an example of poor operator practice.

What I observed is the lack of system level control - where FSK prevents
gross garbage.

> Both "old tried and true FSK" (i.e., native RTTY), and modern "AFSK"
> (in actuality this is also FSK) are needed in ham radio. Not all
> radios can generate native RTTY, even very modern ones.

Other than the Flex-pseudo-Radio and some "trail" radios, I'd be hard
pressed to name more than one or two radios built in the last 20 or 25
years that did not support FSK (the Elecraft K2 and Yaesu FT-8x9 series
being among the very few).  For the majority of manufacturers FSK is
the *preferred* means for RTTY operation and I can't think of any new
design in the last 10 years (since the FT-8x9) other than Flex that
does not include FSK.

Given the ready availability of FSK, to misuse AFSK the way we've all
heard is simply unacceptable.

73,

    ... Joe, W4TV


On 1/19/2013 2:01 PM, Kai Siwiak wrote:
> Joe,
> Good observations, and I share your frustration at observing poor ham
> radio practices. You could say the same thing about a lot of SSB
> operation especially when compression and voice processing are
> involved.  I've observed similar signal atrocities in the PSK frequency
> segments as well. A mal-adjusted transmitter is the mark of a careless
> ham operator. I've also observed lot of extremely poor RTTY operating
> practices not related to the specific technology used.
>
> I do take exception to one of your admonitions:
>    "*or* cases where the soundcard output is run wide open"
> That might be an issue for some sound cards, I don't know. HOWEVER, such
> erroneous advice may lead to problems among hams who use some of the
> most popular sound card technologies, the Signalink-USB.
>
> The Signalink-USB soundcard is specifically designed so that the
> transmit "level" as defined by the computer soundcard audio properties
> MUST be set to maximum. That is, slider all the way up at 100%.  This is
> clearly stated in the Signalink-USB manual. If the level is set lower on
> that slider, then the transmitter will not key up!
>
> The appropriated transmit audio level should be set with the
> Signalink-USB front panel control.  The Signalink-USB front panel audio
> level control must be set correctly so as to keep the transmitter in the
> linear range - just like you should for SSB voice.  Furthermore, there
> should be NO compression or signal processing at the radio on the RTTY
> tones.
>
> What you heard is no doubt a very poorly adjusted transmitting system
> and an example of poor operator practice.
>
> Both "old tried and true FSK" (i.e., native RTTY), and modern "AFSK" (in
> actuality this is also FSK) are needed in ham radio.  Not all radios can
> generate native RTTY, even very modern ones.
>
> Cheers and 73,
> Kai, KE4PT
>
>
>
> On 1/19/2013 11:32 AM, Joe Subich, W4TV wrote:
>> With all the recent talk that AFSK is cleaner than FSK, this morning
>> on 12 meters I watched a classic example of the problems with using
>> AFSK and the reason I would rather put up with the "hard" keying of
>> some FSK implementations rather than supposedly "cleaner" AFSK.
>>
>> I started tuning the band and noticed what initially sounded like fast
>> keyboard CW but it had a familiar RTTY cadence.  When I could not make
>> any sense of the "CW" I turned to the Elecraft P3 to look more closely.
>> Then I noticed the "CW" signal was appearing and disappearing at the
>> same time as an RTTY signal which I decoded as a YV4.  I then watched
>> as the YV4 clicked up and down his waterfall ... as he moved down in
>> frequency the "CW" signal began to show both tones - when he went up
>> in frequency the "CW" signal (now obviously OOK space) would disappear
>> as the harmonics of his AFSK input reached the upper skirt of the IF
>> filter in his rig.
>>
>> This behavior is all to typical of AFSK signals where the audio is
>> applied to the mic input at such a level that it overdrives the fixed
>> gain mic preamp**or* cases where the soundcard output is run wide open*
>> and the ADC is clipping.  There is another possibility ... devices
>> that hang a diode on the sound card output to drive a "PTT" circuit
>> based on the presence of tone may also introduce clipping if the
>> diode is not carefully isolated either by operating on the "other"
>> output channel or by using a buffer amplifier.
>>
>> Yes, a properly filtered AFSK generator may be cleaner at the source
>> but that does not account for the 3KHz+ wide phase noise (synthesizer)
>> platform at -20 to - 30dB on many rigs in SSB mode, it does not account
>> for hum and digital noise at -40 dB on many "inexpensive" commercial
>> and home made interfaces, it does not account for common mode RFI
>> (feedback) on many AFSK interfaces, and it does not account for the
>> discrete "off channel" interference from rigs with carrier leakage,
>> poor opposite sideband suppression and audio harmonics due to overdrive
>> or improper set-up.
>>
>