Comment by crote | Hacker News Reader

Comment by crote | Hacker News Reader

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The problem is the speed. S/PDIF doesn't have a DC component at the S/PDIF bit rate, but to an SFP+ transceiver that S/PDIF signal is a lot closer to DC than to its expected signal. A single S/PDIF bit viewed as if it were a 10Gbps signal looks like thousands of 1s followed by thousands of 0s. Yes, they all balance out in the end, but you can still develop quite a large drift within a single sub-S/PDIF-bit sequence.

A thought experiment to clarify it: let's say you are hoisting a bucket with a DC motor. You're feeding it with a 50Hz AC power source. It's obviously not going anywhere, because it's just oscillating rapidly. You'd need for the motor to run in a single direction for a few minutes to actually lift the bucket. Now drive it with a 0.0000001Hz AC power source (which starts at peak voltage). The motor is going to reverse after 58 days, but does that actually matter? For any practical purposes, how is it different from a DC power source?

That's why you get problems around 10Gbps, but simple 10Gbps optics and afaik all 1Gbps or slower optics don't use the "fancy" kind of signal processing because it wasn't needed. Their lower cut-off frequency should be around 100kHz.

Does SFP+ not have a scrambler/descrambler to make this a non issue, like almost all other phy?

https://en.m.wikipedia.org/wiki/Scrambler

This is done before the SFP+ module sees the signal, but the module makes the design assumption that it is being done. It is right for 10G ethernet, it is wrong (at a certain time scale) for SPDIF.

I also think that https://en.wikipedia.org/wiki/Line_code is the term you're looking for.

Ahh, I'm only familiar with the copper side of things. But, that explains why the search came up with nothing for SFP+!

> I also think that https://en.wikipedia.org/wiki/Line_code is the term you're looking for.

In 10G ethernet phy's, it's a multiplicative (self-synchronizing) scramblers [1] and does not use line code. From what I remember, it's statistically fine, and plays into LDPC correction easier.

[1] https://www.iol.unh.edu/sites/default/files/knowledgebase/10...

Thanks for the explanation!

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