1. Read the format and direction
“Optical” describes a physical path, not necessarily the digital audio format. TOSLINK is used for both ADAT and optical S/PDIF. Sending one into an input set for the other is not solved by another passive cable. Read the active port mode and connect OUT to IN. A switchable optical port may expose a different set of inputs after its mode changes.
Use the Optical Audio Format Checker for a declared pair. It does not select a TV DAC, decode Dolby/DTS or guarantee that a converter supports the required direction/rate. A device that changes connector/transport is not automatically an ADAT/S/PDIF format translator.
2. Count channels at the intended audio rate
| Audio sample rate | Documented mode required | Channels per optical link | Links for 8 target channels |
|---|---|---|---|
| 44.1 / 48 kHz | Base ADAT | 8 | 1 |
| 88.2 / 96 kHz | S/MUX2 at both ends | 4 | 2 |
| 176.4 / 192 kHz | S/MUX4 at both ends, not universal | 2 | 4 |
The last row is conditional arithmetic, not a statement that your interface has those ports or modes. A unit may support 192 kHz analogue operation while its optical expansion stops at a lower rate. Check its exact manual and firmware.
Example: two expander OUTs into one receiver IN at 96 kHz gives one optical link’s four slots, even if both boxes advertise eight analogue inputs. Buying a second cable cannot add the missing receiving port. Two real IN ports still do not override a receiver’s smaller per-rate channel ceiling. Enter both ceilings in the ADAT Expansion Planner; zero means unknown, not unlimited. Its bundle worksheet is logical, not a universal DAW channel-number map.
3. Draw timing separately from audio
In one synchronous two-device domain, choose a supported timing leader and a follower input that actually receives that leader’s reference. With only expander B OUT → interface A IN, embedded timing travels B → A. If A must remain leader, B needs another supported timing path from A. Depending on the devices this could be a return ADAT connection or a documented Word Clock OUT → IN connection.
A dedicated word-clock cable supplies timing only; it is not the missing audio link. Not every digital receiver can follow its optical input. Apogee Quartet’s fixed-leader rule is a useful counterexample to universal setup recipes. The Clock Route Planner preserves capability and rate unknowns instead of choosing a leader from a brand name.
BNC base/double/quad-rate modes and termination are exact-model matters. The selected audio sample rate is not a universal prescription for BNC clock frequency. Multi-unit distribution, passive T chains and asynchronous sample-rate conversion are outside this worksheet. Check current manufacturer firmware notes as well as the manual.
4. If it locks but stays silent, follow one channel
- Observe a named source input meter.
- Confirm that channel is routed to the intended digital output.
- Observe the matching receiver digital-input meter.
- Observe the corresponding DAW input, not a local analogue or playback meter.
- Only then inspect its monitor/output destination at a low normal listening level.
A lock indicator is timing evidence, not proof of these routes. Clicks can be intermittent even if one snapshot says locked. Missing channels after a rate change can be a port-map or capacity change rather than a failed cable. The Digital Audio Failure Isolator keeps unobserved points unknown and ranks a next test.
Compare one documented known-good external cable with modes, ports and clock unchanged. If the outcome changes, the original path is implicated; reseating may also matter. Do not reset all software, modify electrical wiring or look into optical emitters to follow this guide. Use equipment status indicators and manufacturer-approved external checks.
What these tools deliberately do not claim
No audio permission, hardware scan, exact product compatibility list, negotiated-rate readout or prediction of jitter/sound quality is provided. The numerical budget is conditional on your declarations. Independent USB audio-driver aggregation, networked audio and multichannel TV decoding are different problems.
Primary evidence and next steps
The rules are grounded in Audient’s dual-optical example, PreSonus S/MUX, Focusrite clock-source guidance and standalone routing documentation. The source Reference records scope and exceptions instead of extending one model’s behavior to all devices.