FELDHELL – Software MS-DOS per operare in hellschreiber

[INFORADIO]

FELDHELL – Software MS-DOS per operare in hellschreiber

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Provisional Instructions for Using The G3PPT Feldhell Program
*************************************************************

This program enables a PC equipped with an SB16 or compatible soundcard
to send and receive the 122 baud Feldhell mode of Hellschreiber. Full SB16
compatibility is required as the 16 bit functions of DSP version 4.XX are
used and many "clones" will not play ball.

Connections
***********

The audio input from the receiver is fed into the mic or line inputs of
the SB16. The mic is best as the AGC function of the soundcard can be
deployed and the input level set from within the program. The audio output
to an SSB transmitter is taken from either of the line out connections.

Running the Program
*******************

The program is very much a DOS product but has been found to run OK on a
Pentium running Windows95. For this run from the MSDos prompt and expand
to full screen using the button on the toolbar. Note the program can be
reduced back to a window by entering alt/enter from the keyboard. There is
no good reason for doing this under normal running except for getting
temporary access to other Windows functions.

On running the program, check that the input level is properly set by
choosing menu option 1 which will give a crude 'oscilloscope' display.
The gain can be adjusted with the left/right arrows and the AGC toggled
on/off.

Note that soundcards will not limit in the way that an op-amp does and if
overdriven then all sorts of strange effects will result.

Exit from the 'oscilloscope' option via <esc> and choose option 2 which is
the main part of the program. The way that the program operates will now
become apparent. The incoming audio is passed through an FIR bandpass
filter covering the range 800 to 950Hz and uses 100 coefficients. The
detected signal is displayed in the mini oscilloscope in the top left hand
corner of the screen. Simply tune the Feldhell signal in to maximum on the
display and the resulting product is displayed across the screen. In the
classic Hellschreiber way the signal is displayed twice with one row over
the other and if transmit and receive speeds are out of synchronisation then
the signal will scroll off the top display on to the lower one or vice versa.

Various parameters may be altered from the recive option, the gain may be
altered and the AGC toggled on/off in the 'oscilloscope' display, and a
threshold function deployed below which display is suppressed, this being
is shown by a red line across the mini-display. The display may be toggled
between 16 grey or colour scales.

Note that the sampling frequency of the SB16 is set to 7844Hz which is
divided by two in the program to 3922Hz thius giving a period of nearly 4.08
ms which is the length of one feldhell pixel. Due to the way DSP works, this
means that the Nyquist frequency is 1961Hz and any frequency above this
will appear on the display as an 'alias'. SO some external filtering MAY be
required and I use the pass band tuning of my IC735 as to yield a low pass
filter in the LSB position. Any convenient low or band pass filter that
gives a bandpass of 200Hz or so in the region 800 to 1000Hz will improve
reception enormously.


To go to transmit hit the spacebar. The transmit type ahead buffer is shown
in a cyan band across the screen and anything typed in is shown and clocked
out to the left as it is transmitted, some error correction being possible by
hitting backspace. Note that anything more than what can be displayed is lost.
Hitting F3 invokes a tune up procedure whereby a special signal of all pixels
on is sent to set up the transmit output level. Simply key SLOWLY left or
right arrows to lower/raise the output level whilst watching the TX output
or ALC levels. The audio is output through the SB16 DSP function from
synthesised sine waves and the frequency has been chosen such that code units
have a sine wave starting and finishing at the zero cross over point in an
effort to avoid switching transients.