differential versus common-mode noise?

1. Is it correct to assume most noise emanating from digital products is differential noise. 2. Would there be any common mode noise from digital products? Thanks for the help.
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.

-- 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?
Pt - I recommend using USB interfaces with isolation or filtering.

Analog cables can be isolated using transformers such as:

S/PDIF interfaces should use transformers at the DAC input preferably.

Reclockers can be used to both reduce digital jitter as well as isolation, such as:


Its important to break every ground-loop. There should be only earth grounds to each component, not ground connections between components (with cables).

Steve N.
Empirical Audio
Thanks Steve. I recognize and appreciate the value of the units suggested. However they are a much bigger investment than I'm able to make.Your thoughtful consideration of my question is much appreciated. I feel you are an exceptional asset to those of us posting here. Top of mind for me.
Pt - you can find lower cost alternatives. Just do your research.

First thing if you use cable TV or internet is to get a Jensen isolation transformer for the cable.

Steve N.
Empirical Audio
Thx Steve

Thanks Steve. I'm in my 60's and not tech savvy.
So far I don't use cable TV or internet for
music. Will look into.