How-To: Scope operation

    1. Introduction
    2. Character recognition without an oscilloscope
    3. Character recognition with an oscilloscope
1. Introduction

The oscilloscope in HOERWACHE (HW) was developed to better assess the quality of the input signal. Without this graphic representation, it is often difficult to analyze a problem if the decoding in HW does not work so well.
HOERWACHE receives the input signal via the audio interface of the web browser, which in turn reads in the signal from the sound card.

The audio signal is first passed through a bandpass with the bandwidth
400 – 1500 Hz. This increases this frequency range a little. The connected Morse code key or the associated tone generator should work in this frequency range. After the band pass, the signal is sent to a demodulator, which consists of a diode and a low pass. The signal behind the low pass is the input signal (InSig) as it is displayed in the oscilloscope. Bandpass, diode and lowpass are of course purely software-based (DSV: digital signal processing).
All values above an adjustable threshold value are raised to a maximum before decoding, all values below the threshold value are set to zero. The resulting square-wave signal (OutSig) is then decoded and sent character by character to the HOERWACHE server via the websocket interface. From there it is distributed to all connected stations.

2. Character recognition without an oscilloscope

Of course, the signal decoding also works without an oscilloscope. To do this, simply switch on the transmitter, select ‘Key’ on the mode selector and turn the MIC control up almost to the stop. This is how HOERWACHE receives the signal from the sound card and can decode it. Before the first transmission you should give a few V-groups until the characters are recognized and correctly displayed in the window for Receiver # 1. If everything goes correctly in the further course of the transmission, nothing further needs to be done. If too many ‘dirt marks’ are generated, it is time to turn on the oscilloscope and investigate the cause.

Sender für Key eingestellt

Morsetaste abstimmen

Picture 1: V-groups are already recognized, but there are still too many ‘dirt marks’.

3. Character recognition with an oscilloscope

To switch on the oscilloscope, we click on the scope switch at the top right under the activity indicator in the radio station.

Scope-Schalter

The oscilloscope appears in the activity monitor. So that the oscilloscope signal is not disturbed by the activity display, we temporarily switch on a band where usually little happens, here the 27M band.

Scope-Anzeige-Grundeinstellung

We now see the oscilloscope basic setting. The threshold is at the value 128, which stands for a zero signal and the signal value is at 223. This is the value that we previously set with the MIC control.

We saw in the text output above that some V-groups were interpreted as <SN>, a character string that is often used in practice to initiate a transmission, but which does not officially exist. In the following picture, which shows the output signal OutSig, we can see that HOERWACHE has actually not received any V-groups, but something like <SN>. The short filled rectangle after <SN> was interpreted by HW as a short fault and therefore was ignored:

OutSig

Now let’s look at the input signal InSig and then finally get to the point, i.e. I will then describe how the detection error can be cleared up.

InSig

We immediately recognize that the InSig signal is a V-group: The fact that the dots and lines are slightly flattened or dented does not bother us, because everything above the threshold value is raised to the maximum value. What should bother us, however, are the trailing ripple or spikes,  which is above the threshold value and is therefore interpreted as a point if the width is sufficient.

So let’s go to and use the slider to raise the threshold so that the peak value of these spikes is below the threshold value. In the previous picture (InSig) you can see that the peak is just slightly above the threshold value of 128. We set a threshold above 128 with the controller (the thick blue point at the top in the picture with the operating elements of the oscilloscope). An estimated value of about 140 is necessary to be safely above the spikes.

OutSig2

A new test with V-groups confirms this: After the threshold value has been raised to 143, we receive square-wave signals at the output (as a reminder: the square-wave signal OutSig is the input signal for the HW decoder) in which we can easily recognize V-groups.
By the way, don’t be confused: The threshold value regulator is displayed differently in the various web browsers: I use the Google Chrome web browser, which shows the handle for the regulator as a blue point.

Also important: The threshold value must never be above the signal value (the value that is set with MIC), otherwise the entire signal is suppressed and there is no signal at the output.

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