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12 votes
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Understanding WFM modulation

Your “B” is the stereo difference signal of broadcast FM stereo. It is placed at twice the pilot frequency so that it can be recovered by having the receiver lock onto the pilot signal and frequency-...
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11 votes

Why is Morse code still in use?

Because it's fun to send and receive messages by Morse code! It's something that can be done without any specialized equipment for sending and receiving. It's a shared historic experience. It's a ...
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10 votes
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What should be the modulation when transmitting APRS signal?

APRS is a data transmission protocol and is independent of the underlying connection details. So there is no required modulation for the protocol. That said, the common implementation of APRS is FM ...
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10 votes

Why is Morse code still in use?

I love Morse code for these reasons: It's fun. There is something satisfying about pulling a really weak signal out from the noise just 10s of Hz away from another really strong semi-local signal, ...
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9 votes

Why is Morse code still in use?

I would echo the point mentioned by N8WRL: "Why do I operate CW? It is a blast!" I'm actually re-learning morse and realising I've been missing out on a lot of fun over the years. In terms of ...
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7 votes
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1200 baud packet radio can't communicate

If you're used to dealing with protocols between chips, like UART, SPI and I2C, then APRS will feel quite strange. Whereas if you've been involved with CAN, Ethernet, USB at the low level, then APRS ...
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7 votes

Why is Morse code still in use?

It was all I could afford when I was a kid. AM and SSB gear was expensive and still is! Stuck with it because: Signals are stronger, More of a challenge, and a CQ gets answered faster. Great people on ...
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7 votes
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Why there is no AFSK in FLDIGI

Fldigi is largely intended for HF operation. From the beginner's guide: Fldigi is a computer program intended for Amateur Radio Digital Modes operation using a PC (Personal Computer). Fldigi ...
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7 votes
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Is frequency hopping a viable solution for privacy and security without encryption?

Others do not gain access to the content of the message In the US amateur radio service, transmitting “messages encoded for the purpose of obscuring their meaning” is prohibited by §97.113 and ...
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7 votes
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Why does AM have sidebands?

In the picture there is only ONE frequency and the amplitude changes Ah, but this actually causes a second frequency component, the one that you wish to transmit: (Please excuse my rudimentary paint ...
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7 votes
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What are the technical reasons that there are no FM transmissions on the long, medium, or shortwave?

So, the bandwidth argument is the dominant one. You need to incorporate the full Carson Bandwidth in your channel spacing, not only the frequency deviation! As explained in my answer to your previous ...
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7 votes

Is double amplitude modulation useful?

One case where this might happen would be in the world of RF over fiber. In this technique, a radio-frequency signal is used to modulate an optical (usually infrared) signal for transmission over ...
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7 votes
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Why can I hear Morse when not in CW mode?

You don't say where you are hearing the Morse code. You say "on the same channels", but don't specify where you are listening. If you send Morse code using a buzzer, then that generates an ...
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6 votes

Why doesn't GNU Radio Companion's wx Constellation Plot doesn't give me the clean QAM constellation I get from its Qt sibling?

There are a lot of things wrong here. Neither plot looks correct. There is no way that Qt plot is realistic for anything but a signal generator. Where's the noise? Where are the three missing ...
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6 votes

Is FM modulation still more resistant to noise when given the same bandwidth as an AM signal?

FM modulated radio is much more resistant to noise and can deliver better sound quality than AM radio. Under certain conditions, that is! Namely, additive noise, and a fading channel. or is this ...
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6 votes

What exactly is a "balanced modulator"?

To put it simply, a balanced modulator is a mixer which has two inputs and two outputs. The outputs are (input1 + input2) and (input1 - input2). So, if the inputs are 100kHz and 1kHz, the outputs are ...
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6 votes
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Is double amplitude modulation useful?

Sure, it's technically possible. But I can't think of a reason it would be useful. The resulting modulation would be more complicated to implement, and occupy more bandwidth, than ordinary AM. ...
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6 votes

Practical limits to maintaining coherence in an HF channel

Doppler spreading is the change in received frequency from a distance transmitter due to the rise and fall of the ionosphere along the signal path. When the effective height of the ionosphere rises, ...
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6 votes
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Why do PSK modes have "bandwidth"?

Binary PSK with instantaneous phase shifts would be equivalent to multiplying a sine wave (the carrier) with a square wave with values at 1 or -1. When two signals are multiplied, this forms a ...
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5 votes

Air gapped digital communications mode on VHF?

Suitable, I suppose, though not especially efficient in terms of spectrum or power. Morse code was never really designed for machine demodulation, so unless you also need the ability to decode by ear, ...
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5 votes

How does rpitx generate arbitrary SSB data with a clock peripheral?

This is an incomplete and theoretical answer as I haven't looked at rpitx in particular. Rather than controlling the phase of a sine wave, my understanding is that with the Raspberry Pi "hack" the ...
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5 votes
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How does a transceiver work?

The article does in illustrate things rather nicely. Let's look at the receive side first: ...
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5 votes
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Why are carrier waves necessary?

The simplest answer is that the modulating signal, while it could be electromagnetic, will not have the desired propagation characteristics to carry out effective communications. By mixing the ...
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5 votes

What exactly is a "balanced modulator"?

Partial answer, because I don't know the actual electronics theory, but I hope which will help make progress towards a complete answer: A balanced modulator is a mixer with a particular feature. ...
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5 votes

Would Orbital Angular Momentum (OAM) multiplexing work over HF bands?

This is a physicist level experiment, and its benefit over established methods disputed, and even Wikipedia says that it's performance benefit over MIMO are nonexistent to negligible. So, no. Not in ...
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4 votes
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Video transmission between ATV and SSTV

So, a teaser first: With GNU Radio, you can transmit various DVB standards. Here's Alexandru Csete demoing his DVB-S2 transmission: As you can see from the FFT Plot generated by the flow graph in ...
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4 votes

Is frequency hopping a viable solution for privacy and security without encryption?

Frequency hopping is more typically considered an ECCM measure. That is to say that a transciever that continuously cycles frequencies in accordance with some "hop set" is more difficult to jam than a ...
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4 votes

Up converted audio transmission

Yes, this is a workable modulation. In fact, it's used all the time on HF; what you have described is precisely single-sideband (SSB) modulation. The usual choice of filtering is for an occupied ...
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4 votes
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SSB Modulation and Differential Signaling

My current understanding from reading and looking at spectrum plots from SDR software is that the sound is modulated with the carrier, but only half the carrier wave. That's — not really accurate. A ...
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4 votes
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How to identify type of modulation based on constellation diagram?

In general, any modulation that makes sense to plot on a constellation diagram could be described as a kind of QAM. However, we generally use the term QAM only for modulations that use both degrees of ...
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