Why a web page cannot receive radio — and what this is instead
Let us be straight about it, because plenty of sites are not: a browser has no receiver in it. There is no antenna, no front end, no way for JavaScript to reach the electromagnetic spectrum. Any page claiming to tune the bands is either playing you recordings or streaming somebody else's radio over the internet.
So this page does two honest things instead. The band above is a simulator — generated signals laid out across a real CW segment, behaving the way real ones do. Underneath it is a decoder, which reads genuine morse out of any audio your computer can hear: a public receiver in another browser tab, a recording, or a key and an oscillator in front of your microphone.
The thing nobody explains: tuning CW changes the pitch
This is the single most confusing thing for anyone who has only used broadcast radio, and it is the reason this simulator exists at all.
A CW transmission carries no sound. It is a bare carrier switched on and off — silence, interrupted. If you fed it straight to a speaker you would hear nothing whatsoever. To make it audible, a receiver in CW mode generates its own tone and mixes it with the incoming signal, and what you hear is the difference between the two frequencies.
Which means the pitch you hear is not a property of the transmitter. It is the gap between where they are and where you are tuned. Move your dial 200 Hz and the note moves 200 Hz with it. Tune exactly onto them and the tone drops to nothing at all — a beginner's classic, tuning a signal into silence and assuming it stopped.
So a crowded CW band is not a queue of stations you step between. It is a chord. Five operators within a kilohertz of each other arrive as five different musical notes at the same time, and picking one out is much closer to following a single instrument in an orchestra than to changing channels. That is a real skill, it is nothing like the skill of copying morse from a clean recording, and it is what the band above is for.
What the filter does
The filter width is the other half of it. Wide open at 2.4 kHz you hear everything within earshot at once, which is how you find out what is on the band. Narrow it to 250 Hz and everything disappears except the note you have tuned to — the others are still transmitting, but they now fall outside the window.
The trade is that a narrow filter makes a crowded band easy and makes finding anything difficult, because you can sweep straight past a station and never know. Real operators work wide to search and narrow to copy, and switching between the two is a habit worth building here where it costs nothing.
What the simulator models, and what it does not
Signals sit at fixed places in the segment, so the same band always looks the same and you can come back to a station you were working on. Each has its own pitch offset, strength and speed, and some drift slowly the way an old transmitter does when it warms up. Fading is there, because a signal that is perfectly readable one minute and gone the next is the normal condition of shortwave rather than a fault.
What it does not model is propagation — which bands are open, at what hour, to where. That depends on the sun, the season and the sunspot cycle, and no simulator will teach it. For that you need the real thing, and the receivers listed above cost nothing to use.
The decoder, and why your ear will beat it
The decoder listens through your microphone, reads an audio file, or listens to another tab, and prints what it hears. It is genuinely useful for checking your own sending and for confirming a callsign you nearly got.
It is not a substitute for learning, and the reason is worth understanding. Machine decoding works well on a clean, machine-sent signal and falls apart on everything else: a slightly irregular fist, a second station in the passband, deep fading, or static crashes. A human operator copes with all four without noticing, because the ear is filling in from context — you know a callsign is coming, you know what the missing letter probably was, you heard the rhythm even though the signal dropped.
This is why morse survived as long as it did. It is the mode that gets through when nothing else will, and the reason is largely that there is a person on the end of it, not because the signal is strong.
If the band is still too fast, the course builds the characters up one at a time, and the practice library has clean signals with no interference at all. When you can copy a whole message without stopping, the mill is where that goes next.