cocoaModem
cocoaModem Hellschreiber Interface

Kok Chen, W7AY [w7ay (at) arrl (dot) net]
Last updated: July 1, 2006


Index (User's Manual - Hellschreiber Interface)

General Information
Aural Monitor
Accessibility (Incremental Speak and Voice Assist)
Macros
RTTY Interfaces
PSK Interface
MFSK Interface
Hellschreiber Interface
CW Interface
ASCII Interface
SITOR-B Receiver
HF-FAX Receiver
Synchronous AM Receiver
Versions
Part II



Hellschreiber Interface

cocoaModem implements two Hellschreiber modes, the more common Feld-Hell mode and also two variants of the more robust FM-Hell mode.

Feld-Hell is a low duty cycle mode that transmits the Hellschreiber dots as an on-off keyed CW signal. FM-Hell is a 100% duty cycle mode that transmits in FM.


Fig 1 shows the cocoaModem Hellschreiber interface printing a Feld-Hell signal.

smallhell
Figure 1 - Hellschreiber Interface


The top section of the interface consists of the Hellschreiber control panel, the middle section is the receiver section , the next section down is the transmitter section with macro buttons below it, and the usual QSO Info strip at the bottom most.


Hellschreiber Control Panel

The most prominent object in the control panel is the familiar waterfall display. Like the other modes, the scale below the waterfall changes depends on the sideband (upper or lower) and VFO offset that you have set in the Hellschreiber configuration. You can get to the Hellschreiber config panel by selecting the Config item in the Window menu in the menu bar while the Hellschreiber interface is selected. Read the section on VFO Offset for the PSK Interface to get a better idea of the sideband and VFO offset settings in the Config panel.

The input signal strength bars and input attenuator are on the right side of the waterfall display.

Below the waterfall display is a slider called “slope.” A Hellschreiber signal is sent as on-off carrier signals at the rate of 122.5 baud (8.163 ms/bit). If the sender’s and the receiver’s clocks don’t perfectly agree with one another, the printed lines of Hellschreiber will tilt slightly upwards or slightly downwards, depending on the relationship between the two clocks. The slope slider compensates for this clock difference.

Hellschreiber does not send a sync signal for each character. Each column of a text character that is displayed can have any vertical phase position relative to the transmitted text columns. Even when the slope is correct, it is sometimes desirable to realign the test rows so that the text is centered in the “tape”. The Position buttons allow you to do that.

A Hellschreiber receiver has no control of what the received font looks like. As a facsimile mode, the received font is an image of the sender’s transmitted font.

For transmission, cocoaModem’s Hellschreiber transmitter can choose between five fonts. One font, cm bitmap, strictly follows the original Hellschreiber specifications -- each bit in the font is either fully on or fully off. The font fit nominally into a 14 high by 7 wide grid, with the vertical scale of the CTM of one half.

The second font, cm bitmap aa, is a grayscale anti-aliased version of cm bitmap. The pixels of the font can have a value that is neither fully on and fully off (i.e., grayscale). You have to make very sure that your transceiver is operating as linear as possible (with absolutely no ALC, just as when operating PSK31) when you use the anti-aliased fonts.

The third is a double wide font which Jeff KC4FOX had created. When signals are weak, a double wide font can provide better readability.

All of the above fonts have the two-bit per pixel spatial anti-aliasing characteristics. These fonts do not work with the FM Hell 105 mode, which require a one-bit per pixel font that fits into a 5 high by 7 wide grid. For this reason, cocoaModem has two additional fonts, cm small and cm small aa, the latter is a grayscale anti-aliased font. When the FM Hell 105 mode is selected, only these two fonts are enabled in the font menu. When the Feld Hell and FM Hell 245 modes are selected, all fonts are enabled, although the first three font will provide better appearance.

As with the MacOS X text display, a grayscale anti-aliased font can give a more pleasing and less jagged look when the signal is clean and noise free. However, when the transmission path is noisy, the regular font may be more readable since it has higher contrast.

A popup menu to select between Feld-Hell, FM-Hell 105 or FM-Hell 245 modes is next to the font menu.

Finally, there is a dynamic range slider for the waterfall display.


Receiver Section

Click on a signal in the waterfall to tune to a Hellschreiber signal. The waterfall in Figure 1 shows a typical Hellschreiber spectrum operating in Feld-Hell (the most common Hellschreiber mode). Unlike a PSK31 or an RTTY signal, it is harder to identify a Feld-Hell signal from its spectrum. It is easier to identify a signal from the sound on the transceiver’s loudspeaker. To hear what Feld-Hell sounds like, select the computer’s speakers as the output device, start the cocoaModem Hellschreiber transmitter and start typing into the transmit view.

A Feld-Hell signal is more than 100 Hz wide, so tuning is not very critical. Because of that, no AFC is used in cocoaModem. The receiver is tuned to where you have clicked on the waterfall. As with the other modes, you can fine tune by using the scroll wheel of a mouse while the cursor is positioned inside the waterfall area.

To stop the receiver from "printing," apply a Shift-Click (hold down shift key and apply a mouse click) anywhere within the waterfall with cocoaModem's window selected as the currently active window (click on the title bar of cocoaModem window once, if you are unsure if it is the active window).

Hellschreiber characters are typically sent as a 5x5 dot matrix inside a 7x7 matrix. Characters can be wider than 7 dots but cannot be taller than 7 dots. Each dot can be position at half dot vertical alignment as long as the dots and the spaces between the dots are no smaller than the height of a full dot (this keeps the bandwidth of a bilevel Hellschreiber signal from exceeding the normal bandwidth limits).

The characters are sent in column-wise rasters, starting at the lower left of a glyph and ending up at the top right of the glyph.

Since Hellschreiber does not send a synchronization bit in the data stream, the bits of the received character can be out of phase with the bits of the transmitted character. Because of that, Hellschreiber is traditionally displayed with two repeated columns. In this manner, at least one full height character is displayed. (See the images here on how the double hight characters are printed to a paper tape on a historical Hellschreiber mcahine.) A cocoaModem “tape” with the vertical misalignment is shown below in Fig. 2.

pastedGraphic
Figure 2 - Vertical Misalignment

As explained in the previous section, cocoaModem provides a pair of position buttons to align the characters so that they are centered on the “tape.”

As with other modes, you can select the background of the display view and both the received text color and the echoed transmit text color by using the color wells in the Config panel. Note that since the received display is not text, but a facsimile image -- you will not be able to cut and paste text from it. cocoaModem provides a short history that you can scroll back the receive view to read.

Figure 3 shows a typical FM-Hell 245 signal.

FMHell
Figure 3 - FM Hell 245 Signal

When it is idling, an FM Hell signal is a pure carrier. as can be seen on the top part of the waterfall display in Figure 3 above. The typical deviation that is used in FM Hell is 61.25 Hz. When there is modulation present, FM Hell looks like the lower part of the waterfall in Figure 3, and pretty much sounds to the ear like a 245 baud minimum shift keyed RTTY signal (i.e., a very fast RTTY signal with a 122.5 Hz shift).

The line that you see at 0 on the waterfall scale in Figure 3 is the black level of the text, and the white level is 122.5 Hz to the left of it. Most fonts have more black background than text stems, so the "black" level will usually be much more prominent on the waterfall.

Figure 4 shows a typical FM-Hell 105 signal. The white and black levels of FM Hell 105 are 52.5 Hz apart.

FMHELL 105
Figure 4 - FM Hell 105 Signal

cocoaModem is set up to allow you to copy FM-Hell by clicking on the more prominent "black tone," which is on the right side of the FM Hell signal if you make the sideband menu in the Receive config panel to be the same as the sideband selected on your transceiver.

Using the black level as the reference tone to click on in the waterfall allows you to tune to an FM Hell signal even when the transmission is idling. In practice, it is hard to miss which tone is the black tone even when an FM signal is not idling, since it is usually the strongest component of an FM Hell signal.


Transmitter Section

cocoaModem generates Feld-Hell by generating on-off pulses of an audio tone. These are converted by an SSB transmitter into an on-off carrier. On the air, a Hellschreiber signal is simply high speed synchronous CW.

Unlike PSK or RTTY signals, there is an absence of any audio when Feld-Hell is transmitting space characters, in the space between characters, and when you pause typing. Even the background portions of the Hellschreiber characters themselves carry no signal.

If you are using VOX, this means that the VOX must turn on very rapidly (in much less than 8 milliseconds). To allow VOX to work, cocoaModem has included a method similar to RTTY diddles. When nothing is being typed, cocoaModem will insert dots into the stream that will keep VOX asserted.

To insert these “diddles,” check the Diddles box in the Transmitter Config panel.

You should also include a VOX hang-time or delay so that PTT does not disengage between characters. For example, with the SignaLink SL1+, engage the Delay button in the front of the box. With a proper VOX delay dialed in, the PTT should remain engaged while the diddles are being transmitted.

If you are using hardware PTT, you need not include the diddles. The diddles are there just to keep the VOX engaged when you are not actively typing into Feld-Hell.

cocoaModem filters the output audio to limit the bandwidth of the Feld-Hell output signal to about 200 Hz. Be sure you operate your SSB transmitter in a linear mode. This means no ALC showing at all. This is especially important if you are using the anti-aliased (cm bitmap aa) font.

In the FM-Hell modes, cocoamodem generates a 100% duty cycle continuous phase audio FM signal that is used to modulate an SSB transmitter. The pulses are raised cosine pulses to keep the bandwidth minimal -- in FM Hell 105 mode, it produces a signal that is the same as a minimum shift keyed (MSK) signal. Since FM-Hell is a full duty cycle mode, you need not apply diddle when you are using VOX to key the transmitter. As long as cocoaModem is "transmitting," there will be tone, and the VOX will remain engaged. Be mindful, that like RTTY and PSK31, FM-Hell is a full duty cycle mode and you have to reduce the power of your transmitter to whatever the requirements are when operating full duty cycle modes to avoid "frying" the transmitter.

As usual, the command-T and command-R keyboard shortcut allows you to enter and leave transmit mode. You can engage in a conversational Hellschreiber QSO without taking your hands off the keyboard.

The Hellschreiber Interface has its independent set of macros. These macros are saved to your preference (property list or plist file) when you quite cocoaModem and read back in when you relaunch the application.


Next (CW Interface)