differential versus common-mode noise?

Whether digital (or other) noise is common mode or differential has to do with each of the places it gets to, not with where it originates.

If it appears equally on two associated conductors, to a close approximation, then it is common mode at that location. If it appears significantly differently on two associated conductors, then it is differential at that location. "Associated conductors" could refer to the two signal conductors in a balanced signal pair, or to unbalanced signal and return conductors, or to two related internal circuit points, or to AC "hot" and "neutral" conductors, etc.

So the answer is it could be either. It might even be both at once at a single location, as digital noise consists of a great many individual frequency components that are simultaneously present.

Regards,
-- Al

There are several sources for noise, including:

1) components - if the outputs are balanced or differential, then they will generate both differential and common-mode noise. If the outputs are single-ended RCA, then common-mode only.

2) ground-loops create common-mode noise by themselves. If you can isolate inputs and outputs of each component from cable ground returns, this is the best solution. Almost as good is putting RF filters in the AC cords earth grounds for each component.

Steve N.
Empirical Audio

Thanks Al and Steve.
I take away that both common and diff noise emanate; therefore common and differential noise isolation is required to properly isolate digital components?
Which noise from the incoming ac line is most disturbing to the digital circuits; ie incites noise within my combination CD/DVD, SACD player-a Lexicon RT 20 from about 2006?