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 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.