Building Ultra Long Range Toslink

Original Article ↗ Hacker News Discussion ↗

An experiment to send TOSLINK digital audio over long-distance fiber using SFP transceivers prompts broader exploration of how SPDIF, HDMI and other AV standards map onto modern optical and Ethernet hardware. Commenters unpack technical issues like Manchester coding, DC wander, latency, and DRM, contrasting “just works” consumer formats with pro‑audio solutions such as AES50, Dante, SDI and Audio over Ethernet. Many see the project as a charmingly over-engineered proof of concept that also hints at niche real‑world uses for low-latency, long-haul audio links.

DIY optical audio hacks (lasers, mirrors, robustness)

  • People link a video where TOSLINK LEDs are replaced with lasers for wireless surround.
  • Concerns raised about line-of-sight links breaking from vibration (subs, walls, cars with loud bass). Others note beam divergence and joke about “self-correcting” when bass drops the link and thus the vibration.
  • Critique of Manchester-encoded amplitude-modulated TOSLINK in free space: vulnerable to ambient light; suggestion to modulate onto a higher-frequency carrier like old IR headphones for robustness.
  • Some argue consumer IR (38 kHz carrier) is insufficient for good audio; others counter that IR headphones prove it works in practice.

SFP modules and low‑bitrate signals over high‑speed optics

  • Key observation: with SFPs the project is really “S/PDIF over SFP fiber” rather than extending classic plastic-fiber TOSLINK.
  • Discussion of AC coupling and DC wander: 10G optics expect high-rate, scrambled data; slow Manchester-coded S/PDIF looks almost DC, stressing coupling caps and retimers.
  • Participants note this explains why links only work above ~100–150 kHz effective transition rates.
  • Mention that SDI and AV-specific optics handle “pathological patterns” better; normal Ethernet optics assume pre-scrambled, line-coded signals.

S/PDIF, TOSLINK, HDMI, and formats/DRM

  • Some lament S/PDIF’s ≈1.5 Mbps cap limiting it to compressed 5.1 (DTS, Dolby), pushing people to HDMI for uncompressed surround.
  • Others contest the strict 1.5 Mbps limit, citing 24‑bit/96 kHz stereo specs (~5 Mbps). This remains unresolved in the thread.
  • Lack of bidirectional signaling and robust DRM is cited as a reason TOSLINK wasn’t extended for richer formats; HDMI won due to HDCP.
  • TOSLINK is seen as “boringly reliable” and still widely used (TV → amp, legacy CD/DVD), despite being old.

Audio over Ethernet and live‑sound latency

  • Live‑sound folks compare: AES50 (layer‑1, synchronous, ~62 µs per link) vs Dante/Audio-over-IP (1–10 ms typical, can be lower in some modes).
  • Very low latency over long fiber (≈11 µs) is seen as valuable for digital live audio paths, though venue speaker arrays still deliberately add delay for alignment.

Fiber physics, tools, and oddities

  • Clarification that light in fiber travels ~c/1.5, slower than in vacuum, which matters for latency and trading links.
  • Mention of OTDR launch fiber spools (100–200 km) as an easier way to test ultra-long optical paths.
  • Fiber tech anecdotes: talk sets, non‑intrusive fiber clamps that detect modulation, and the difficulty of mid‑run tapping without splicing.