Getting Started on RTTY - Page 3
Hooking up the Sound Card to your radio to receive RTTY is pretty easy. You'll need to buy or make a cable in order to do this. Many operators use commercially made interfaces such as a West Mountain Radio RIGblaster, MFJ Sound Card Interface or Bux Comm Rascal to connect the sound card to the radio. These interfaces are covered in more detail on page Page 9.
The basic idea is to come from the audio output of your radio and go to the audio input of your sound card. You can go directly from radio to sound card, but it's always a good idea to put an audio transformer in line between your radio and sound card for isolation. Although this is not absolutely necessary, it is good practice. When we get to the transmit part of RTTY, and if you decide to use AFSK transmission, you will also want to put the same type audio transformer between the audio output of the Sound Card and the audio input of your radio. The most popular transformer used for this application is the Radio Shack audio isolation transformer part number 273-1374. Here is a diagram showing the connection.
Courtesy of W5BBR website http://www.w5bbr.com/
The computer end of the cable is pretty simple. Your
sound card will be either stereo or mono. More than likely, it will be a
stereo sound card. It doesn't matter. In most cases, the plug
required is a 1/8" (3.5mm) phone plug (Radio Shack p/n 274-284). This is a stereo plug with
tip, ring and sleeve. Use the tip pin for your audio and the sleeve for
your ground and shield. You can use the ring if you like, but if your
Sound Card is mono, it won't work. Normally, tip is left channel, ring is
right channel and sleeve is common or ground. You don't have to use a stereo
plug. You could use a mono plug regardless of whether your Sound Card is
stereo or mono. In any case, it's best to use shielded cable in an attempt
to keep RF off the audio line. In MMTTY you can use left channel, right
channel or mono. This is selected in MMTTY under Options, Setup MMTTY on
the Misc tab.
The radio end of the cable requires a little thought.
There are several ways to derive audio from your transceiver. You could
get the audio from an accessory jack or come straight from the PHONES plug or
from an external audio filter that you may already be using. Some
discussion is required here.
Getting the audio from an accessory jack
usual works well, especially if you can vary the level of that audio with a menu
command on your radio. If the audio coming from the accessory jack is at a
constant level , you can still adjust the level to the input of the sound card
through the Windows Recording control in most cases. Getting the audio
from an accessory jack may be advantageous since you will be able to adjust the
audio gain on the front on your transceiver to a comfortable listening level
while maintaining a constant level to the Sound Card. This is
important.
I've always used audio coming from the PHONES jack (via an
external audio filter). If you decide to get your audio from the PHONES
jack, you need to consider how you are going to listen to the RTTY tones coming
from your receiver as well. It's necessary to listen to the tones coming
from your radio when operating RTTY to assist in identifying and tuning the
signal (in the case of deaf operators, tuning could be done by using the tuning
indicators alone but it's difficult and takes practice). You could simply
split the audio coming from your PHONES jack with a Y cable or adapter. Or
better yet, as I do, you could use an external audio filter to further filter
the signal before sending it to your Sound Card and headphones. I use a
JPS NIR-12 on my radios. A JPS NIR-10 or Timewave DSP-599 will also
work. If you already have some sort of external filter, it's recommended
that you take the audio output from that filter and send it to your sound
card. In many cases, doing it this way, will not disrupt the audio
going to your headphones. There is a big difference in the quality of
audio coming from a commercial or home-brew audio filter and the audio coming
straight out of your PHONES jack. Cleaning up the audio before sending it
the Sound Card will prove wonders when copying RTTY, especially weak signals.
The nice thing about getting audio from your PHONES jack directly or via an
external audio filter is that you can vary the level with the audio gain control
of the radio to obtain a proper input level to your sound card and still
maintain a comfortable listening level to your headphones or speaker.
The
NIR-12 audio filter allows me have a constant level coming out of the filter and
still be able to adjust the volume control on the front of the unit for easy
listening in my headphones much like using audio from the radio's accessory jack
except I have an added advantage of adjusting the audio input to the filter with
the audio gain control of my transceiver. The NIR-12 is no longer
available on the market, but I'm willing to guess most of the other commercial
external audio filters have this same feature.
You can also obtain your
audio from a SPEAKERS jack, but if you do this, you definitely should have an
isolation transformer in line to get rid of any hum or voltage that might be
present and you may need to attenuate the audio using a resistive "pad".
To start out, it's not important where the receive audio comes from as long as
you can still listen to the RTTY tones and get the correct level of audio to the
input of your Sound Card.
The best source I know of for excellent
information on interfacing to a Sound Card is on W5BBR's web site at www.w5bbr.com/soundbd.html. Go there and study the
different ways of interfacing to your Sound Card. Make the necessary
connection between your radio's receiver and your PC's Sound Card input then
come back here and we'll see if we can print some RTTY.
