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Author Topic: Resister req'd across video (rca) connector to mimic connection to tv?  (Read 2736 times)

David Allred

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I have an odd need.  I have a USB media player ($25)  made in china, no brand name, etc.  So tech support is not an option.  It can be set up to auto play a selected audio or video format when powered up.  There is an odd problem.  I am using the supplied 4 conductor mini to component video + L&R audio cable.  The player will not boot and auto start unless the video plug is connected to a TV or monitor.  My use will not and cannot have a tv present.  I am playing an audio file.
I know by experimenting that shunting the video lead pin to the shield that the player boots properly.  Should I mimic the resistance of the tv video input by bridging the rca with a resistor?  What resistance?  My gut says 50-100 ohms, since that is the typical range of video cable used.  But, could a dead short work just as well.  While I won't be using the video while in operation, it is required for set-up and future changes, when a TV is used. 
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John Roberts {JR}

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The 75(?) ohm termination is to prevent ringing. It should work without a termination.

I would prefer to add a 75 ohm resistor over shorting it.

JR
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Tom Bourke

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I would do a 75 ohm resistor.

Typically an input's impedance is set by what ever resistor goes to ground from the signal connector.  This is then feeds some other high impedance chip input, some times coupled with a capacitor.
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David Allred

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Thanks, JR / Tom.
That confirm what I thought.  I can't figure out why an EE would require that the video output be "closed" for the devise to activate.  If it were a video (with audio) only player, I could comprehend it.  But since it is also designed to play wav, mp3, and many other formats, why?  Engineers, I guess.  Or perhaps this particular unit has a defect that requires a work around.  Maybe QA simply tests by hooking all 3 up and tests the various modes.  Some problems never arise unless someone like me uses the product outside their particular field of view.

Would 100 ohms be equally as beneficial?  I have 100's in stock, but would have to order 75's.
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John Roberts {JR}

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Thanks, JR / Tom.
That confirm what I thought.  I can't figure out why an EE would require that the video output be "closed" for the devise to activate.  If it were a video (with audio) only player, I could comprehend it.  But since it is also designed to play wav, mp3, and many other formats, why?  Engineers, I guess.  Or perhaps this particular unit has a defect that requires a work around.  Maybe QA simply tests by hooking all 3 up and tests the various modes.  Some problems never arise unless someone like me uses the product outside their particular field of view.

Would 100 ohms be equally as beneficial?  I have 100's in stock, but would have to order 75's.
It is unclear what exactly is going on... unterminated there will be ringing that reflects back into the send. If it expects 75 ohms, 100 ohms will be under damped but may reduce ringing enough to work. 50 ohms (two 100 ohms in parallel) will be over damped and probably not break anything.

But I do not recognize the fault mechanism.  Perhaps experiment with both ways.

JR
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David Allred

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It is unclear what exactly is going on... unterminated there will be ringing that reflects back into the send. If it expects 75 ohms, 100 ohms will be under damped but may reduce ringing enough to work. 50 ohms (two 100 ohms in parallel) will be over damped and probably not break anything.

But I do not recognize the fault mechanism.  Perhaps experiment with both ways.

JR

What is this ringing you refer to? 
 If you were to rank the effectiveness of these various loads, best to worst, what would they be?

Dead short
50 ohm
75 ohm
100 ohm
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John Roberts {JR}

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What is this ringing you refer to? 
 If you were to rank the effectiveness of these various loads, best to worst, what would they be?

Dead short
50 ohm
75 ohm
100 ohm

In a 75 ohm transmission line, if the end is terminated with the characteristic impedance it looks like an infinitely long transmission line and signal disappear off the end into infinity. If open or shorted they will reflect back off the discontinuity and can superimpose on top of the send end which is probably a low impedance driver with 75 ohms in series (guess).

IF it is a 75 ohm line (another guess) 75 ohms will be the ideal termination.

 I still do not understand why device is not booting. It may look at video out to confirm a proper termination. With no load, a passive source termination will be 6 dB hot.

JR 

Caveat Lector I do not design video gear.
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David Allred

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 I still do not understand why device is not booting. It may look at video out to confirm a proper termination.
JR 


My thoughts also, but who knows.
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Lee Douglas

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Have you tried a standard TRS it?  Some of the processors I use for touch screen control have a jack that is TRRS for control outputs, but will work fine with TRS.  Perhaps it is sensing the longer TRRS plug and waiting for it to do something to confirm it.  Just pondering and speculating.
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David Allred

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Have you tried a standard TRS it?  Some of the processors I use for touch screen control have a jack that is TRRS for control outputs, but will work fine with TRS.  Perhaps it is sensing the longer TRRS plug and waiting for it to do something to confirm it.  Just pondering and speculating.

This particular TRRS is nearly 3/16" longer than other TRRS plugs I have.

UPDATE: The video output is 75 ohm.  I found a different supplier that listed the specs in their info.  If anyone cares the audio is 2.0v RMS.
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