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HDMI Signal Quality


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#1 Ian Hedley Wakefield

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Posted 19 September 2011 - 06:13 AM

I'm trying to work out how to determine the quality of signal coming down a HDMI cable.

I have time to invest in this little bit of a personal research project, I just lack the know how on how to go about it.

I've read over and over that the 7D doesn't put out a uncompressed signal via HDMI. I m curious as to how this is determined as I have a FS100 and a 7D and I'd like to do some comparisons.

Eventually the FS100 will be running into a Atomos Samurai via a HDMI -> SDI Atomos connect which is essentially why I'm embarking on this project.

I realise there are many variables like cable quality and length that can affect the quality of a signal. I'd like to begin eliminating these variables.

Thanks in advance

Ian
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#2 Phil Rhodes

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Posted 19 September 2011 - 11:09 AM

It's digital. It either works or it doesn't - or, more completely, it might be unreliable, but it's likely to be unreliable due to termination issues and physical build quality rather than anything else.

Recording the 7D uncompressed is a nonstarter, I've tried.

The only time the cable issue may be a problem is if you want to go a very long distance. There is no maximum distance spec on HDMI; it simply relies on the manufacturer testing their cables and being happy with the result. While this is in some ways a realist agenda, since things like ethernet and SDI will, practically, go much farther than they were designed to do, it also means that long runs - over five meters, say - are very much up to you to test. If you are doing this, test by doing a lot of manipulation of the cable, bending and flexing it, which will also manipulate the insertion loss and effective impedance of the cableform, and test the automatic equalisation that's built into every HDMI/DVI receiver. These cabling systems use twisted pair cables, as opposed to coaxial, and they are not designed to go as far or as reliably. This is why HD-SDI goes down coax.

But on a short cable, from a camera to a recorder, it's irrelevant. If it works it works.

P
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#3 Ian Hedley Wakefield

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Posted 19 September 2011 - 04:41 PM

So on a 20cm/6-8inch cable camera out --> recorder. If I'm getting a signal that's as good as it will be?

See picture --> thank appropriate deity --> remember to press record :P

Cheers Phil.
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#4 Phil Rhodes

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Posted 19 September 2011 - 04:53 PM

There are edge cases where you can get individual pixel noise if a ilnk is really close to failing, but that's very much the exception. Generally if it works it works.
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