Basic RTTY Interface

Please note that the Basic RTTY Interface is deprecated. It is kept around but no new functions will be added directly to it. As new demodulators are developed, the Basic RTTY Interface may not be able to use them. Please use the Wideband RTTY Interface instead.

The basic RTTY Interface shown in Figure 3 is loosely separated into five sections. Just below the tabs for selecting cocoaModem interfaces is the RTTY Control Panel, below that is the Receive text view, followed by the Transmit text view, then the row of macro buttons and finally the QSO Info bar.

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Figure 3 - Basic RTTY Interface



RTTY Control Panel

The RTTY Control Panel (Figure 4) contains the control and display functions that are found in a typical HF Modem.

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Figure 4 - RTTY Control panel


On the left of the control panel is the crossed ellipse tuning indicator. To the right of that are the tone pair control and the input attenuator and a VU meter.

Currently, cocoaModem only supports an AM demodulator in the form for a matched filter, the button for the other modulators are grayed out.

The mode buttons allow you to select Mark-only (MO), Space-only (SO), strong Multipath (MP+), weak Multipath (MP-) and Mark-space (M/S) demodulation. Mark-space is the demodulation mode that should work best in almost all cases. Sometimes, it is possible to copy using Mark-only if there is an interfering signal covering up the Space tone, or to copy using Space-only when there is an interfering signal covering up the Mark tone. Similarly when there is multipath present, copy can sometimes be improved using the Multipath modes.

The “normal” filter is the optimal filter to use under normal circumstances (an RTTY signal with band noise but no interfering signal close by). When there is a nearby interfering signal, the narrow filter might provide better copy. Likewise, a weak signal but clear of any interference might give slightly better copy using a wider filter.

Finally, there are two polarity buttons on the far right of the control panel whose use is described in the section of the manual describing RTTY tone pairs.


Operating RTTY

After configuring the interface (see main RTTY documentation), you are ready for RTTY operations. Be sure to remember to turn on the Active button in the Config panel.

RTTY is transmitted over the air as a Frequency Shift Keyed (FSK) signal. A "0" bit is transmitted as one of the two FSK carrier frequencies (called the space frequency) and a "1" is transmitted as the other carrier frequency (called the mark frequency). When you send a character in RTTY, you will be switching between 1's and 0's, and the FSK signal will shift between these two frequencies and generate corresponding sidebands. On the spectrum, an active RTTY thus occupies not just the locations of the two carriers, but also the spectrum between them and some spectrum beyond these two carriers.

By convention, the mark frequency is the higher of the two carriers on the RF spectrum. Another convention in RTTY is to use a 170 Hz separation between the mark and space carriers. The amount of this separation is called the FSK shift.

A remaining RTTY convention is to transmit using a bit duration of 22 miilisecond. The symbol rate is thus 45.45 bits per second, with one bit per symbol. Usually, the baud rate is just rounded to "45 baud".

An RTTY signal is therefore uniquely decodable if the mark RF frequency is higher than the space, the shift is 170 Hz and the baud rate is 45.


Tuning an RTTY signal

When an RTTY signal is properly tuned in, you should see the pattern shown below in Figure 5. Tune your transceiver so that one of the ellipses is as vertical as possible and the other ellipse is as horizontal as you can make it.

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Figure 5 - Crossed Ellipse Indicator


One of the ellipses tracks the mark signal and the other ellipse tracks the space signal.

If the signal is far from the selected mark and space frequencies, it may be difficult to judge from the Crossed Ellipse display which direction to tune the transceiver to center the FSK signal. As an aid, there is a small display inside the the indicator in Figure 5 under the ellipses. You will find a small vertical red line that indicates the center frequency between the selected tone pair. If the RTTY signal is strong, you will see two flashing yellow dots. One of the dots corresponds to the received mark signal and the other yellow dot shows the space signal. Tune the transceiver so that the two yellow dots straddles the red line. When you are close enough, the crossed ellipse indicator will provide the indicator to accurately center an RTTY signal.


RTTY Tone Pairs

When receiving an FSK signal, the transceiver translates the mark and space carriers to an audio frequency tone pair. Refer to your transceiver's manual for the preferred tone pair frequencies.

The tone pair portion of the RTTY control panel (repeated below in Figure 6) just to the left of the Crossed Ellipse indicator allows you to customize the mark frequency to match your transceiver's.

As seen in the figure below, cocoaModem represents the tone pair by the Mark frequency and the Shift (the shift, as explained earlier, is the difference between the mark and the space frequencies).

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Figure 6 - RTTY Tone-Pair Selection


cocoaModem comes preset with the following tone mark tones and shifts in the four memory locations:

2125 Hz Mark with 170 Hz shift (“North American” standard),
2100 Hz Mark with 200 Hz shift (200 Hz shift TNCs),
1415 Hz mark with 170 Hz shift (for receivers whose filters are centered at 1500 Hz) and
1275 Hz mark with 170 Hz shift (“European” low-tone or mid-tone standard).

You can select one of these using the memory popup menu.

Changing the Mark, Shift and Baud rate fields will immediately change the tone pair and data rate that is currently used in cocoaModem (as usual, remember to type a tab or a carriage return when you have finished changing a Cocoa text field). These values are not permanently saved until the Store button is clicked; the next time you change the memory menu, the pairs that are save to the plist will reappear.

If you want to replace one of the above pairs by a different tone pair, simply select a memory address of the tone pair which you want to overwrite, then replace the Mark and Shift fields with your own custom frequencies. Finally, click on the Store button. This should replace an existing tone pair with your custom tone pair.

All “stored” tone pair sets are saved to your preference file when you quit cocoaModem and will be restored the next time you launch cocoaModem. The most recent menu selection is also saved to the plist when you quit cocoaModem.

Notice that I have included a 2100/2300 tone pair (2100 mark and 200 Hz shift) in the cocoaModem default set.

If the shift of a transmitted FSK signal is different from the Shift that is used by the demodulator, the two ellipses in Figure 5 will not be at right angles to one another. many ops are still using hardware TNCs as modems. To reduce costs, some TNCs would use the 200 Hz Pactor shift even when operating in RTTY. There are also FSK transmitters where the shift can drift with time.

Most modems, likewise cocoaModem, will print signals whose shift is off from the standard. However, under marginal conditions, you can obtain slightly better print (and they from you) if the two shifts match exactly.


Reversing the RTTY Tone Pairs

By convention, the Mark tone is the lower of the two audio frequencies of a tone pair. When this is passed through a LSB transmitter, the relation of the tone pairs reverses and the Mark carrier becomes the higher of the two RF carriers of the RTTY signal.

(Notice that the same can be achieved by making the Mark tone the lower of the two audio tone pair and using a USB transmitter instead of an LSB transmitter.)

There are times when the station you are copying is not following the standard convention but transmit “inverted” or “reversed” tones. (A good RTTY operator will switch the tones back to normal polarity when they are informed that they are transmitting “inverted” or “in reverse.”)

If a signal is strong and yet not printing anything meaningful, and the crossed ellipse indicator is spending more time on the average drawing the vertical oval (the Space oval) than in the horizontal oval (the Mark oval), then there is a good likelihood the received signal is inverted. It is easy to quickly switch cocoaModem’s demodulator to copy an inverted signal by clicking on the Rx Polarity button that can be seen on the far right of the RTTY Control panel in Figure 4. Remember to click it back to normal when you are through with that station, or you will have problems with copying normal stations.

There are times when the operator on the other end both transmits and receives in reverse. (It happens surprisingly often when it is some rare DX that you need!) This is a case where you too will have to swap your transmitted mark and space tones. This can be done by turning the Tx Polarity button to Reversed.


RTTY Baud Rate

The baud rate used by radio amateurs is typically 45.45 baud, usually call simply “45 baud.” Occasionally, someone will use other baud rates such as 50 or 75 baud. To switch to these rates, just enter the number into the Baud field (Figure 6).


RTTY Squelch

cocoaModem does not use a traditional audio squelch that is keyed off of the signal strength.

cocoaModem has a unique demodulator in which the Mark and Space signals are passed through 15 separate automatic threshold correction (ATC) functions. Traditional FSK modems use a single ATC.

Each of cocoaModem’s ATC has a slightly different AGC time constant or a different Mark/Space time delay (equalizers).

The resultant 5 data bits from each of the ATC function are sent to a voting unit before it is passed on to the Baudot decoder. The voting unit determines not just what bits come from the majority of the ATC, but also how many of them disagree with the majority.

After a vote is taken, the character can be rejected if there are too many disagreements. cocoaModem’s squelch control (Figure 7) selects the error threshold for when a character is rejected from printing. When the squelch slider is all the way to the right, all characters are printed no matter how many ATC are in disagreement. With the squelch slider all the way to the left, only characters where every ATC is in agreement will be printed.

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Figure 7 - RTTY Squelch


The squelch slider in the basic RTTY interface is located on the right of the receive text view (see Figure 3).

Bear in mind that this does not mean that every printed character is guaranteed to be correct when the squelch slider is pushed all the way to the left. Large noise pulses could cause all ATC to demodulate into the same bit pattern, for example – their outputs may agree and they could all be wrong. All that is guaranteed is that with the slider moved all the way to the left, characters that are printed have a greater likelihood of being correct than a printed character with the slider to the right.

The reason that this multi-ATC squelch also works as a general squelch is that when a signal is absent, the energy in the Mark and Space channels are about equal, and as such the relative delays between the Mark and Space channels will produce very different results. With the slider push all the way to the left, for example, will cause all the characters to be suppressed, since not all ATC will be in agreement.

If you are new to cocoaModem, it is best to push the squelch slider all the way to the right. This will cause all characters to print. Later, you can adjust the squelch control to suit your needs. With the control moved towards the left, fewer noise hits would “print.”


RTTY Reception

Once an RTTY signal is perfectly tuned in, characters should start “printing” into the receive text view. You can scroll the text view using the scroll knob, or if you have a scroll wheel mouse, using the scroll wheel of the mouse while the cursor is inside the text view. Be sure that cocoaModem is the active window by clicking on the brushed metal regions if you have selected the brushed metal interface, or by cicking on the title bar of a non-brushed metal interface.

If you do not see any print, check the squelch control that is mentioned above. If the print is garbled, it is also possible that the signal could be reversed. As described earlier, you can copy a reversed signal by engaging the Rx Polarity button. If you find that the majority of the signals can only be copied in the reversed position, it is quite likely that you are the one who is operating in reversed! Please check to make sure that the USB/LSB popup menu in the RTTY Config panel is set to the sideband that your transceiver is using.


RTTY Transmission

The transmit text scroll view is below the receive text view. This is where the text that you enter through the keyboard goes into the transmit buffer to be transmitted.


Transmit Buffer

When text is inserted into the transmit text view in the RTTY window, either from the keyboard or from macros, it enters a transmit buffer and stays there until every character has been transmitted. If the RTTY modem is not in the transmit state, the characters simply stay in the buffer until the next time the transmit state goes active (or until the buffer is flushed). When a character in the transmit buffer is transmitted, the character is also echoed to the receive text view. The color of the echoed text can be chosen to be different from the color of the other texts in the RTTY window (see Text Attributes and Copying Texts in Text Views in the General Information section).

Transmitting Text

To start transmitting the contents of the transmit buffer, press the Transmit button (Figure 8) to the right of the text views.

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Figure 8 - RTTY Transmit Indicator, Transmit Button and Flush Button


When the Transmit button is pressed, the caption of the button changes to Receive and the color of the small indicator above the button turns red, indicating that you are now transmitting (i.e., putting out a tone on the audio output device). Clicking on the button (that has turned into Receive) will return you to the receive mode.

Whenever you are in transmit mode (when the red indicator is on) and the transmit buffer is empty, cocoaModem will insert an RTTY diddle into the character stream. This is the beedle-beedle tone that you hear from RTTY stations when nothing is being typed. Some people do not send diddles, and the idle station would send a steady Mark tone instead. Diddles are preferred since it is a way for others to accurately tune you in when you are transmitting but not typing. Further, it uniquely identifies the transmission as an RTTY signal.

When characters are entered into the transmit scroll view, the new characters will replace the diddles and be transmitted instead of the diddles. Characters can be typed directly into the transmit view, or they can be entered through the use of macros.

Any text that is typed ahead that has not yet echoed to the receive view can be cancelled by the Flush button that is beneath the Transmit button. Once a character is echoed to the receive view, its Baudot representation has already been transmitted on the air and cannot be "erased." Unlike the Varicode that is used in PSK31, there is no backspace character in Baudot. RTTY ops often type XXX to indicate an error in sending much like a CW op uses dididididit.

The Flush button does not end the transmission. If you are in transmit mode, cocoaModem will resume sending diddles after a Flush. If you’re typing ahead when you are in the RTTY receive mode, cocoaModem simply wipes the entire transmit buffer clean. Flush will also remove any text macros you have inserted into the transmit buffer.


Exiting the Transmit Mode

To return to the receive mode, press the Receive button (which was the Transmit button before you started transmitting).

If there is still untransmitted text in the transmit buffer, cocoaModem remains in the transmit mode: the red indicator will turn yellow until the last character in the transmit buffer has been transmitted. cocoaModem has returns to receive mode when there are no more characters in the transmit buffer, and the indicator will also be grayed out.

You can also use keyboard shortcuts to switch between transmit and receive modes. And there is a keyboard shortcut that you can use when in transmit mode to flush all characters in the transmit buffer and return immediately to the receive mode. (See Using Keyboard Shortcuts to control Transmit/Receive in the General Information section of the manual.)




Back to the Main RTTY page.

Dual RTTY Interface page.

Wideband RTTY Interface page.