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Title: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: David Allred on July 06, 2016, 11:09:34 am
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. 
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: John Roberts {JR} on July 06, 2016, 11:36:03 am
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
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: Tom Bourke on July 06, 2016, 11:43:16 am
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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: David Allred on July 06, 2016, 01:22:55 pm
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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: John Roberts {JR} on July 06, 2016, 01:30:49 pm
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
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: David Allred on July 06, 2016, 01:53:56 pm
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
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: John Roberts {JR} on July 06, 2016, 03:36:17 pm
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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: David Allred on July 06, 2016, 04:24:37 pm

 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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: Lee Douglas on July 06, 2016, 04:31:54 pm
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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: David Allred on July 07, 2016, 07:31:35 am
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.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: Lee Douglas on July 07, 2016, 08:16:23 am
This particular TRRS is nearly 3/16" longer than other TRRS plugs I have.

Deleted: I missed this part when I replied.
Title: Re: Resister req'd across video (rca) connector to mimic connection to tv?
Post by: Craig Hauber on July 10, 2016, 12:59:25 am
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.

what would be wrong with using a little 3" LCD camera monitor with it all the time? 
Small enough you can tape the 2 devices together and the little monitors I have you can turn the contrast and brightness down so low the screen is black at the lowest setting.