Nowadays the word “BFO” does not come across our mind very often unless you are talking about homebrew equipment or are restoring vintage gear. But you always need a BFO, or its mathematically equivalent things, to receive a CW signal, because our auditory perception accepts neither RF/IF signals nor DC signals.
Here is a (simulated) example of a beat, which I encountered while trying a K3NG Arduino CW Keyer. When I started manipulating the paddle, it seemed OK at first, but I soon found that if I keep sending dots the sound will change into a continuous tone.
It was more than several months ago when I last used the keyer, so I thought something is wrong with the hardware, an Arduino Leonard board. It took me a while to notice that the paddle is connected to both the K3NG Keyer and the internal keyer of the rig.
The “wpm” was almost the same for the two, so there was a beat with the phase offset initially zero and increasing gradually to reach 180 degrees.