Balanced vs Unbalanced
Definition
A balanced connection is an interface in which the two signal conductors present equal impedances to ground, so that interference induced equally on both (common-mode noise) is rejected by the differential input of the receiver. It uses two signal conductors plus a shield, typically on XLR or TRS connectors. Many balanced outputs also drive the two conductors with opposite-polarity copies of the signal, but according to Bill Whitlock (Jensen Transformers), whose CMRR test with realistic source-impedance imbalances was adopted in IEC 60268-3, the noise rejection depends only on the impedance balance and not on this signal symmetry. The quality of the rejection is expressed as the common-mode rejection ratio (CMRR), in dB. An unbalanced connection (TS, RCA) has a single signal conductor with the shield as the return, so noise induced on the shield adds directly to the signal.
In the field
Balanced lines are therefore used for long runs, such as stage-box and multicore cables, while unbalanced runs are kept as short as practical and away from interference sources such as dimmer racks and power cables. Hum on an unbalanced link often comes from a ground loop, which a transformer-isolated DI box can break.
Related lessons
Sources
- Jensen Transformers (B. Whitlock), « Understanding, Finding & Eliminating Ground Loops » : la réjection dépend de l'égalité des impédances à la masse, pas de la symétrie du signal · jensen-transformers.com
- Whitlock, « CMRR in Balanced Interfaces », SVC : la réjection dépend de l'interaction entre émetteur, câble et récepteur · svconline.com
- Balanced audio (Wikipedia) : conducteurs, connecteurs XLR et TRS, réjection de mode commun · en.wikipedia.org
- Radial Engineering, Comparing DIs : DI passive à transformateur et isolation galvanique · radialeng.com
- Whitlock, « The Truth About Balanced Audio Interfaces In Systems », ProSoundWeb : test CMRR avec déséquilibre d'impédance adopté par l'IEC 60268-3 (3e éd., 2000) · prosoundweb.com