he reason I ask is that a digital signal, by it's very nature, doesn't have frequency in the way analog signals do. As I've quoted above from a reputable book on electronics, a digital waveform is a DC signal that varies between zero volts and a max volts,
Transmission of digital data, as I quoted from the electronics text, is DC
You are probably confusing the way Digital is represented in a drawing....most encyclopedia's and layman reference material incorrectly represents digital. When you see a typical analog waveform drawn with a "digital" stair steps superimposed that look like DC levels.... this is WRONG it doesn't exist like this.
Sorry but I studied digital signal processing and analysis in college...the whole subject has been badly dumbed down and leads to much confusion and fear of digital ( obviously a stair case is nothing remotely close to an analog waveform and hence the source of some of the malicious rumours about digital )
Digital data is sent as analog waveforms. Toslink Light is an alternating waveform as is the signal on an RCA coax or an HDMI cable. Typically all electronic devices used some form of MODEM between digital devices (Modulator and Demodulator; the digital data is encoded into some form of alternating signal often with a separate or embedded clock alternating signal to help in decoding the "states" into discrete "bits").