Interconnect Geometry

Geometry as Engineering

The geometry of a cable — the spatial arrangement of its conductors, dielectric, and shield — determines its electrical behavior as completely as the materials from which it is made. Two cables using identical copper and identical insulation will measure and sound differently if their geometries differ. This is not a subtle point. It is the central engineering variable in cable design.

Geometry determines capacitance, inductance, characteristic impedance, and shielding effectiveness. It also influences how the cable interacts with source and load impedances. Understanding the principal geometries is the first step toward making sense of cable specifications.

Coaxial Geometry

A coaxial cable places a central conductor inside a cylindrical shield, separated by a dielectric. The geometry is rotationally symmetric, providing consistent capacitance per unit length and excellent shielding.

Coaxial geometry was developed for RF transmission. Its primary virtue is a precisely defined characteristic impedance maintained consistently along the full cable length. In audio, it is the correct choice for S/PDIF at 75Ω, word clock at 50 or 75Ω, and unbalanced analogue connections.

Its limitation: the shield carries both return current and shielding function simultaneously. Interference induced in the shield appears in the signal return path — which is why unbalanced coaxial interconnects are more susceptible to ground loops than balanced alternatives.

Twisted-Pair Geometry

A twisted pair places two conductors in close proximity and twists them at a consistent rate. The twist is not decorative — it serves a precise function. As the pair rotates, interference induced in one conductor is cancelled by equal and opposite interference in the other conductor half a twist later.

Twisted-pair is the foundation of balanced audio interconnection. The signal is carried as equal and opposite voltages. The differential input stage subtracts one from the other, recovering signal while rejecting common-mode interference.

Ethernet and digital audio cables apply the same principle with the additional requirement of precise characteristic impedance. Twist rate, conductor diameter, and dielectric must maintain 100Ω differential impedance along the full length.

Star-Quad Geometry

Star-quad places four conductors at the corners of a square cross-section, twisted together. Diagonally opposite conductors are connected to form the two signal paths. This doubles common-mode rejection compared to simple twisted pair.

The interference rejection can be substantially higher, particularly at low frequencies where twisted-pair rejection diminishes. This makes star-quad well suited to microphone cables and balanced interconnects in noisy environments.

The trade-off is capacitance. Four conductors in close proximity produce higher capacitance per unit length. For high-impedance sources, this can cause measurable high-frequency roll-off. For low-impedance professional sources, the effect is negligible.

The Dielectric: The Overlooked Variable

The insulating material separating conductors is as consequential as the geometry itself. Every dielectric absorbs and releases a small amount of energy as the signal passes through — described by the dielectric absorption coefficient. In practical terms, the dielectric stores a trace of the signal and releases it slightly delayed, smearing fine transient detail.

PTFE (Teflon) is the standard reference. Its dielectric absorption is among the lowest of any practical insulator, and its permittivity is close to air. Foamed PTFE introduces air voids that bring effective permittivity even closer to unity.

Polyethylene and PVC offer adequate performance at lower cost, but their higher absorption becomes audible in high-resolution systems and long runs. The choice of dielectric is a considered engineering decision with measurable consequences for timing accuracy and low-level resolution.