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Author Topic: MADI over 50ohm BNC for short runs.  (Read 936 times)

Helge Dr. Bentsen

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MADI over 50ohm BNC for short runs.
« on: April 10, 2017, 03:28:05 am »

I have a Digico SD8 that I sometimes use with the stagerack besides the surface.
It's a bit cumbersome to use the whole multicore (95m) when I need 5m.

Is it possible to use 50ohm cables for these short runs?
I have those available from my wireless system.
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Jean-Pierre Coetzee

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Re: MADI over 50ohm BNC for short runs.
« Reply #1 on: April 10, 2017, 05:55:13 am »

According to this sticky your post should be in another forums.

Please read things that are at the top of the page when they have names like How this forum works.
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Riley Casey

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Re: MADI over 50ohm BNC for short runs.
« Reply #2 on: April 10, 2017, 12:48:46 pm »

Seems like an easy thing to test.  Certainly works with Dante connections but thats a different kettle of fish.

Geoff Doane

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Re: MADI over 50ohm BNC for short runs.
« Reply #3 on: April 10, 2017, 02:18:57 pm »

Is it possible to use 50ohm cables for these short runs?
I have those available from my wireless system.

The fundamental frequency for a 64 channel MADI stream at 48 kHz sampling rate is somewhere around 75 MHz.  That has a wavelength (λ) of about 4m.  If you follow the "rule" that transmission line principles should be followed once you're going farther than 1/8λ, then you should be using 75Ω cable, even for a relatively short run of only 5m. 

I'm making some other assumptions here, including that the velocity of propagation  in the wire is 100%, which it never really is.  A 5m 50Ω cable might work, but I would not count on it for anything mission critical (anything other than testing in the shop when you're the only one listening).

If you know anybody who works in television, you could probably get your hands on a short cable very easily.  That's what I do at my day job (TV and radio), and we have stashes of  short jumpers all over the place, and the tools and supplies to make up any length we need on short notice.  Even the old Belden 8281 cable ("Belden Blue") with the 50Ω connectors would work for what you want to do, although it has been culled from our plant since we went DTV.

Going the other direction, there are length limitations with copper cable (75Ω HD-SDI coax).  I have a run of something more than 100m from one studio to the loading dock where the mobile is parked, and I can't get a MADI stream over it with any reliability.  Fibre is fine of course, and AES (stereo) will work on the trunks, as will NTSC video sync, but we have to convert to fibre to get an HD-SDI signal down there.

GTD
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Helge Dr. Bentsen

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Re: MADI over 50ohm BNC for short runs.
« Reply #4 on: April 10, 2017, 04:37:10 pm »

Geoff,

I tried the 50ohm solution earlier today in the warehouse, it worked and passed audio, but I'm not entirely comfortable with it. I ordered some 75ohm cables last week but they are delayed untill after easter. Upon reading your post I'm going to call some fellow engineers and see if I can borrow a pair of cables untill mine arrives.

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