Look closely at a quarter-inch plug and you'll see thin black bands separating the metal into segments. Two bands means three contacts: tip, ring, and sleeve (hence TRS). What those three contacts actually carry depends entirely on what's plugged into the other end, which is the slightly confusing part.
Two completely different jobs, same connector
- Balanced mono: tip and ring carry the same signal (one inverted), sleeve is ground, exactly like an XLR's three pins squeezed into a smaller plug. This is what a balanced TRS connection on a mixer insert or line input is doing.
- Unbalanced stereo: tip carries the left channel, ring carries the right channel, sleeve is the shared ground. This is exactly what your headphone jack is doing, and it has nothing to do with noise cancellation at all.
A balanced TRS connection and a stereo headphone connection use the exact same physical plug and carry completely different kinds of signal. The cable doesn't know or care which job it's doing; that's entirely decided by what's wired up on each end.
How to tell it apart from TS
A standard quarter-inch instrument cable (TS, Tip-Sleeve) has only one black band, meaning two contacts instead of three. TS is unbalanced mono, full stop, no second job. If you're holding a quarter-inch plug and trying to figure out what it does, count the bands: one band is TS (simple, unbalanced), two bands is TRS (could be balanced mono or stereo, context-dependent).
Where you'll find it in a DJ rig
- Headphones (stereo, unbalanced, the most common TRS use case you'll touch every set).
- Balanced line inputs and inserts on some mixers and outboard gear.
- Booth monitor outputs on certain mixers, depending on the manufacturer's design.
In the context of Headroom
TRS comes up in Module 2: Signal Flow alongside balanced vs. unbalanced wiring, mostly to clear up the genuinely confusing part: the same plug shape doing two unrelated jobs depending on context.