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Old 30th July 2007, 11:07 PM   #1
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Default Can I convert 200ohm analog audio into 75ohm?

I am hooking up an A/V distribution system and, for simplicity's sake, used the 3 component video connections available in the video distribution component to carry composite video and analog audio to each display. So far it has been working fine, but I recently talked to a member of tech support for the company that makes the distribution equipment who said that the analog outputs of my components (3 satellite receivers,1 media server) are rated at 200 ohms and the video distribution equipment is 75ohms, and that this might fry the audio outputs of the components. First of all, is this really something I should worry about, or more of a "just so you can't say we didn't warn you" kind of thing, since it has been working for a few weeks so far, and second, is there a way to convert these analog signals from the components into 75 ohm so I can avoid any possibility of problems?
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Old 30th July 2007, 11:29 PM   #2
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Due to lack of interest in anything "video" i have no idea or experience but plenty of common sense

Sounds like the distribution system terminates all video inputs into 75R. A lot of audio sources will not appreciate working into such an impedance and will exhibit lower output/higher distortion but unless very poorly designed won't blow.

What i don't quite get is the 200R for audio sources - way too low.
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Old 31st July 2007, 08:20 AM   #3
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Thanks for the reply. I hope you're right. I'm gonna just leave it all the way it is and hope for the best.
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Old 31st July 2007, 08:44 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

This is all RF (radio frequency stuff), I'm no expert ......

The impedance of a phono plug at RF is around 180 ohms.
The impedance of a BNC connector is around 50 ohms.
the impedance of a "aerial" connector is around 75 ohms.
Typical "fat" coaxial cable is 50 to 75 ohms.
simple spaced pairs and 2pin plugs are 300ohms

So for low cost video interconnect 200 ohm cable and phono plugs
and sockets are used. A video switching unit should maintain the
impedance matches. Generally speaking a simple resistive switching
unit has resistors in series, e.g. a a simple 75 ohm resistive splitter /
mixer (works both ways) has a V type circuit with 75R in each leg.

None of the above has anything to do with audio frequencies and
basically all basic video switching equipment should read open
circuit across any input and output at DC and audio frequencies.

In the above case audio signals should be fine.

When you move on to video / r.f. switching splitting / mixing via
baluns (r.f. transformers) and the like then audio performance
would be terrible, consequently I assume this would be at the
least immediately obviously unsuitable.

Nevertheless you can measure the DCR of the inputs to check the
case. Open = simple switching, near short = transformers etc.

/sreten.
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Old 31st July 2007, 08:58 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
trying to read between the lines, so I'm guessing here.
the video is using the standard 75ohm send/receive impedances.
the audio is using a 200ohm source impedance for feeding into a >>1k receiver impedance.

The standard (component video) phono plugs cannot properly transfer 75ohm signals. I'm guessing that scart is even worse!
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Old 31st July 2007, 10:23 AM   #6
OzMikeH is offline OzMikeH  Australia
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200 ohms sounds wrong.
I would expect 600 ohms or 120 ohms. 600 ohms is a standard telephone type circuit, 120 ohms is the characteristic impedance of cat 5.

you can get 600 ohm to 75 ohm baluns (transformers) used for 2 megabit tributary streams (a standard data comms T1 channel size)

They should work well with audio. I'd only bother if you are having problems with noise.
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Old 2nd August 2007, 08:00 PM   #7
EC8010 is offline EC8010  United Kingdom
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Default No worries...

600 Ohms is dead. It was dying in the mid-80s and died once ISDN arrived in the 90s.

Analogue video is sourced and terminated in 75 Ohms. Your analogue audio could well have a source impedance of 200 Ohms but this will expect to see 2k Ohms or more. I think your technical support chaps were talking about two separate issues. So long as you plug audio into the audio hole and hear sound you can rest assured it isn't being terminated in 75 Ohms. Enjoy your music and pictures and stop worrying.
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Old 3rd August 2007, 12:28 AM   #8
jlsem is offline jlsem  United States
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Quote:
600 Ohms is dead.
Like a lot of other things, it still lives in DIY audio.

John
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Old 3rd August 2007, 09:08 AM   #9
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Strangely enough, the audio from the 3 el-cheapo satellite receivers passes through just fine, but the audio from the $3000 Escient media server is distorted enough to make it unlistenable. Actually, it sounds ok when the dynamic range of the music is at a minimum, but when all of the instruments kick in and the singer belts something out, it sounds like a bad radio station. It's too bad I can't put some kind of attenuator in line, because it sounds like it is somehow overdriving one of the inputs in the series. Any ideas?
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Old 3rd August 2007, 09:22 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by The Paulinator
Strangely enough, the audio from the 3 el-cheapo satellite receivers passes through just fine, but the audio from the $3000 Escient media server is distorted enough to make it unlistenable. Actually, it sounds ok when the dynamic range of the music is at a minimum, but when all of the instruments kick in and the singer belts something out, it sounds like a bad radio station. It's too bad I can't put some kind of attenuator in line, because it sounds like it is somehow overdriving one of the inputs in the series. Any ideas?
This description could indicate an impedance mismatch.
For all normal audio signals the receiver impedance is usually greater than 5times the transmitter impedance. 20times is probably better.

You can measure the input resistance of the receiver very easily with a multimeter IF the DC blocking cap is after the Zin setting resistor. But unfortunately the cap is usually located before the resistor. You then need to open up the box and look at the input end to identify the various components. there could be as few as one and as many as 6 setting the input conditions of the receiver. Draw out the circuit and then measure the various resistors in the front end of the circuit board. Much easier if the manufacturer will tell you the truth.

Similarly one can measure the output impedance of the transmitter. You need more equipment for this. signal source to drive the transmitter, DMM that operates well at the signal source frequency, some loading resistors (1k0, 10k, 100k), some plugs to solder the dummy loads into.
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