ted:
On the drawing frequency is not changing but presence of the pulse, 0/1
"Re: Serial1.begin(57600)"
If we go back to your original post (the title of this thread), then ..... the 57600 refers to serial communications signalling rate. And, to understand serial communications signalling rate, it's necessary to learn about serial communications.
As for your question about whether or not '57600' can be linked (somehow) to a measurable frequency........ then, yes..... it could be linked to 57.6 kHz ........ that's if you assume that the serial communications signalling rate is maintained (sustained).
And..... USB serial communications (itself) does not involve PSK.