Analog Audio Interface Contract
Definition
An analog-audio interface contract is the documented agreement among the source, cable, connector, receiver, shield, chassis, signal reference, power state, and measurement method. It includes topology, pinout, maximum clean differential level, common-mode voltage and range, source and input impedances, loading, bandwidth, headroom, noise, shield termination, mute and transient behavior, environmental limits, and pass/fail evidence.
Connector Type Is Only One Field
An RCA connector often carries a single-ended interface, but it can also appear on equipment with differential sensing or unusual reference arrangements. XLR, TRS, terminal blocks, and proprietary connectors likewise do not prove that the source is balanced, symmetrical, floating, isolated, or compatible with a particular adapter.
| Layer | Record |
|---|---|
| source | single-ended, differential, active-balanced, impedance-balanced, transformer, BTL, floating, output impedance, clean level |
| receiver | input topology, differential and common-mode impedances, common-mode range, clipping, bias and protection |
| reference | signal return, circuit common, chassis, cable shield, vehicle return, protective earth |
| cable | conductors, shield, pair geometry, capacitance, resistance, length, connector and routing |
| state | off, boot, mute, active, source change, shutdown, sleep, wake, fault, connect and disconnect |
| evidence | loaded level, bandwidth, distortion, noise, CMRR, transients, isolation and installed-state results |
Level And Loading
The source must remain clean into the actual receiver and cable. The receiver must accept both the differential signal and the common-mode voltage without clipping. Nominal level, maximum clean level, user maximum, source impedance, receiver input impedance, cable capacitance, gain, and headroom belong in the same record.
Vdiff = Vplus - Vminus
Vcommon = (Vplus + Vminus) / 2
For a single-ended interface, one conductor may be the source reference, but its voltage can still move because of current and impedance. For a differential interface, a small Vdiff does not guarantee the receiver is within its allowed Vcommon range.
State And Transient Contract
Document which device powers first, when the DSP is valid, how outputs remain muted, when amplifiers enable, what happens during source changes, and how shutdown occurs. Capture both steady-state performance and transitions. A quiet active state can still produce a damaging or objectionable pop during boot, cable insertion, brownout, sleep, or wake.
Acceptance Record
- Identify source and receiver hardware and software revisions.
- Confirm pinout and every shield, reference, chassis, and power-return connection.
- Measure the source unloaded and under the intended load.
- Measure differential and common-mode voltage at the receiver.
- Verify bandwidth, noise, distortion, rejection, clipping, mute and transients.
- Repeat in the final cable route and every required vehicle state.
- Link the result to the approved system release record.