[RTTY] RTTY spectrum analysis article

[Joe Subich, W4TV]

> It looks like several radios have built-in "sound cards" that are
> available by a USB connection. In principle this should be
> essentially the same thing as the Flex flex is doing--no analog audio
> line to the radio. I have no idea what the interface is like, what
> the quality is, or to what extent you can mess it up, but a bit of
> internet searching and browsing through manuals indicates that this
> is something that people really haven't looked into very much, even
> in the world of PSK31 and other digimodes.

The USB "sound cards" in the current crop of rigs are nothing more than
a standard USB CODEC like one would find in any external USB sound card
fed into the balanced modulator.  Some rigs use a switch to select the
mic "chain" or USB sound card, others simply parallel the two inputs
with nothing more than a pair of "summing" resistors.

Using an internal USB sound card is certainly *not* an "add digital"
solution.  It is just as important with the built-in sound cards as
with the external interfaces to use good proactive to keep drive
levels to the proper level and avoid external sources of common mode
RF/hum/etc.

73,

    ... Joe, W4TV


On 1/15/2013 2:02 PM, aflowers at frontiernet.net wrote:
>
> Salvo brings up an interesting point when it comes to generating FSK.
> (Incidentally, one of the "narrow" examples on the webpage happens to
> be a Flex 5000 and with an Alpha 8410 running about 1000W, using
> MMTTY.)  To my knowledge, the Flex 1500/3000/5000 architecture has no
> on-off keyed FSK line--if you want to send FSK you can do it with
> software-defined audio tones and a "virtual audio cable" through the
> firewire connection.  The interesting thing here is that the "AFSK"
> is generated in the radio and there is no analog link between the
> software defining the modulation waveform and the conversion to RF.
> The entire "audio" chain from PC to RF is digital, so you won't get
> RF distortion in the audio and it becomes much harder to "overdrive"
> the radio, particularly if the Flex/PowerSDR is smart enough prescale
> the levels (I have no idea if it actually does this).  In some sense
> it's really "faux-AFSK".
>
> I have not been the transceiver market recently, but I decided to
> look around to see what has changed in the last 5-10 years as far as
> interfacing for digital modes.  The Flex architecture is not unique
> in having the potential for this "faux-AFSK" model.  It looks like
> several radios have built-in "sound cards" that are available by a
> USB connection.  In principle this should be essentially the same
> thing as the Flex flex is doing--no analog audio line to the radio.
> I have no idea what the interface is like, what the quality is, or to
> what extent you can mess it up, but a bit of internet searching and
> browsing through manuals indicates that this is something that people
> really haven't looked into very much, even in the world of PSK31 and
> other digimodes.  Perhaps its just so common that nobody mentions it?
> The higher-end Icoms even have S/PDIF in and out.  Not only can you
> not "overdrive" the S/PDIF input on the radio (I don't what is
> downstream in the radio that you could still mess up, however), but
> the RF isolation on a 3-ft TOSLINK cable is supurb below 300 THz :-)
>
> This is of course in addition to radios that provide filters and
> prescalers for (legacy?) analog audio inputs.  The K3 and some others
> already have features like this specifically for AFSK-RTTY.  In any
> event, line-level inputs are pretty much standard on every radio in
> the last 10 or 15 years.  (Why anyone would use a microphone input
> when there is a line-level alternative is crazy to me, but it doesn't
> help that the manufacturers are currently endorsing this in their
> manuals).  I guess my realization is that "AFSK", as we tend to call
> it, has the potential to be much safer than it used to be five or ten
> years ago, at least with some of the more recent inovations.  That
> said, I'm sure there are definately still some wrong things one can
> do.
>
> Anyway, these just some thoughts that Salvo triggered.  Is anyone out
> there using SPDIF or the radio's internal soundcards for RTTY or
> other digital modes?  I'd be interested in your experiences, be they
> good, bad, or ugly.
>
> Regards,
>
> Andy K0SM/2
>