RE: Splitting Composite Video Source

[otm]

Hello Everyone,

Really not sure where to put this, so if it's wrong, sorry.

I want to split a composite video source (Yellow phono plug) along with a mono audio source (white phono plug) to two seperate places for simultaneous viewing (one on a TV, another on a PC capture card).

The first thought that came to mind was the need to split the signal, but also to amplify.

I had a look on google, and to my suprise, nothing was really avaliable, they either had to many outputs, or cost a small fortune for what they were. Realistically £15 is the most that I would want to spend on this small device.

A thought crossed my mind though, instead of using a composite specific splitter, I could use a VGA splitter (used to take one monitor feed in, and split it to two seperate monitors) and simply wire the video source to pin 1, and the audio source to pin 2, and the ground to pin 3 thus splitting the signal, whilst amplifying at the same time.

Is there any reason why this is a stupid idea, if so, I could make one, but wouldn't know where to start... What chips would do the job? (I understand Maxim possibly do these things, but haven't got a clue other than that..).


Cheers,

Owen.

 

[Nigel Goodwin]

The only reason you need a splitter is because of the 75 ohm loads on the video inputs (you don't need a splitter for the audio), if you can remove the 75 ohm load from one of the inputs, you can simply parallel them - the load is often switchable on composite monitors for this exact reason.

Also, without altering anything, you 'may' be able to loop out of the TV to the capture card - particularly if the TV has two SCART sockets, in on one, and out from the other.

 

[crutschow]

The only reason you need a splitter is because of the 75 ohm loads on the video inputs (you don't need a splitter for the audio), if you can remove the 75 ohm load from one of the inputs, you can simply parallel them - the load is often switchable on composite monitors for this exact reason.

If you do that you need to daisy chain the video from the device from which your removed the 75 ohm load to the device that still has the load. If you split it at the source you will get reflections (ghosting) at the unterminated device.

The proper way to run the cable is go first to the unterminated load. From there use a T fitting at the unterminated connector for the cable to go to the terminated device. That way there is just a short unterminated tap to the first load, which should give no significant reflections.

 

[crutschow]

A thought crossed my mind though, instead of using a composite specific splitter, I could use a VGA splitter (used to take one monitor feed in, and split it to two seperate monitors) and simply wire the video source to pin 1, and the audio source to pin 2, and the ground to pin 3 thus splitting the signal, whilst amplifying at the same time.

That may work. One possible problem is that the VGA signal level is 0.7V max. and the composite is 1V max. As long as the VGA splitter can handle 1V you should be ok.

But you don't have to run the audio through the splitter (it may not even work well if you do that). Just use an RCA T to split the audio.

 

[otm]

Hi guys,

Thanks for the replies.

I think I should have added, the cable length between the TV and the Capture card will be about 10m, that might affect things..

So correct me if I have this wrong..

Theres no need to use any form of distribution amp for the mono sound source, (Here's a good question, why?).

And I should place a T-connector as close to one of the terminated input sources as possible (In this case a denon 1910 amp) to reduce the effect of reflections (That reminds me of an experiment I've done recently).

This hould allow me to use the PC capture card to record the video source at all points, whilst allowing the TV to be watched intermittantly, without affecting the Capture cards recording which would still be happening in the background.

Would putting in a T-split not cause the current of the video siganl to halve at both the Amp and Capture Card causing image issues?

Plus, I never though about the max signal of VGA, cheers for pointing that out.

Cheers,

Owen.

 

[crutschow]

I think I should have added, the cable length between the TV and the Capture card will be about 10m, that might affect things..

Theres no need to use any form of distribution amp for the mono sound source, (Here's a good question, why?).

And I should place a T-connector as close to one of the terminated input sources as possible (In this case a denon 1910 amp) to reduce the effect of reflections (That reminds me of an experiment I've done recently).

Would putting in a T-split not cause the current of the video signal to halve at both the Amp and Capture Card causing image issues?

Plus, I never though about the max signal of VGA, cheers for pointing that out.

10m of cable should not be a problem.

Sound inputs have a high impedance so there is no significant reduction of signal amplitude for inputs in parallel. And the sound frequencies are too low to have to worry about reflections for any practical length of an audio cable in the home.

For video the T would be at the un-terminated load (only if you can remove the termination resistor).

The T-split will not reduce the signal if there is no termination resistor at the load with the T.

 

[otm]

Thanks for that,

However, now you've got me stumped. The T would be at the unterminated load.. I get what your saying, but that doesn't allow me to do what I wanted to do surely?

The video source is one of thoes birbox cameras, so my parents want to watch the bird inside, at any point of the day, but I would also like to use the PC capture card to record video file (1 each day) of the whole thing, therefore, if my parents want to watch the camera on the TV, they don't need to stop the TV capture, plug the composite cable into the back of the TV...

This is what made me thought of a splittter becuase you need to provide two 75 ohm signals (to one source or the other, or even both at the same time, at any point in time) from one 75 ohm source.

Have I picked the wrong end of the stick again??

Cheers,

Owen.

 

[ericgibbs]

hi otm.
My CCTV's and TV's are directly linked with SCART box splitters [no internal resistors] , using 10mtr cables. no obvious problems with the picture or sound.

BTW: I'm just up the road from you in Pompey

 

[otm]

Hi Eric,

Wow it's a small world!!! I must say though, I spend most of my time at Loughborough University now, back for the holidays at the moment though!

Your advice is something simple that can be tried out at next to no cost, However, I wouldn't mind waiting to see what crutschow's reply is.

I do find it odd though that no simple device like this is available, theres load of SCART, VGA, HDMI stuff around that doesn't cost much, but next to no Composite video ones, and the ones there are, are too complicated.

Cheers,

Owen.

 

[ericgibbs]

hi Owen,
Are you the Owen we worked together on that QBasic parallel port weather station.???

 

[otm]

Hi Eric,

I am that very same person....

You also helped me with a side project I undertook with RF transmisson.

What sort of scart splitter are you using? and a proper stupid technical question, why do you think it doesn't have an effect?

Cheers,

Owen.

 

[crutschow]

However, now you've got me stumped. The T would be at the unterminated load.. I get what your saying, but that doesn't allow me to do what I wanted to do surely?

The idea is to have a short tap connection to the unterminated load, thus you put the T there. The other two T connections go to the source and the terminated load which can be any length cable.

 

[otm]

Crutschow,

I'm really sorry but am having a bit of a mental block visualising that in my head.

Let's try and get the terminology down...

When you say unterminated load, you mean the device that will be off most of the time ie. the TV receiver system.

what confuses me is when the capture card, and TV are both on, then there is no unterminated load.

Do you see my confusuion,

To be honest, I'm at fault for not being able to get a mental picture...

Cheers,

Owen.

 

[crutschow]

OK, the terminology is the problem.

Unterminated means that the internal termination resistor (typically 75 ohms) is physically disconnected. You have to go inside the device and physically disconnect the resistor. Whether the device is on or off has no effect on this termination.

 

[ericgibbs]

hi Owen.
If I follow your application correctly, all you need is a unit of the types shown on this link.
You can buy them with phono AV connectors and SCART connectors on the same box, use a short SCART cable from the TV link them thru the box.

Your Search Results | CPC

**broken link removed**

 

[otm]

Hi guys,

I think Eric is more on the right lines...

I've attached a picture with the system No. 1 being as it currently stands, and System No. 2 what I require.

I again may have picked up the wrong end of the stick, But Crutschow, Your system acts more a selector switch between the two possible outputs, when I require both outputs to be active at the same point. From what I understand, for what your suggesting to work, My parents would need to unplug the TV capture card to watch the birdbox on the TV, and to record using the capture card, they would need to disconnect the TV, and plug the cable into the Capture card.

this means you don't record for the 5 or so mins you watch the TV, which might be the 5 -10 mins that something really big happens.

I hope this clears things up.......

Looking at it Eric's idea seems better, but I would prefer a system that's more suitable if you know what I mean.


Like I said, I might have the wrong end of the stick, and Crutschow your System might do exactly what I want, but I don't know it yet....

Cheers,

Owen.

 

[Nigel Goodwin]

Isn't there an AV output on the back of the Denon?, feed the capture card via that.

 

[otm]

Nigel,

I looked into that at the beginning, I can only use th composite Monitor output when the Amp is on, and you are watching the incoming from one of the composite inputs. As soon as you change to one of the HDMI inputs say, the output stops.

I will look into it further, seeing as the Amp is fairly programable.

Cheers,

Owen.

 

[crutschow]

I again may have picked up the wrong end of the stick, But Crutschow, Your system acts more a selector switch between the two possible outputs, when I require both outputs to be active at the same point. From what I understand, for what your suggesting to work, My parents would need to unplug the TV capture card to watch the birdbox on the TV, and to record using the capture card, they would need to disconnect the TV, and plug the cable into the Capture card.

No. If you remove the termination resistor from one of the receivers, then you can connect them both at the same time. That's the purpose of removing the resistor.

You say outputs but don't you mean inputs?

 

[otm]

AAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!

I can just imagine you tearing your hair out when you read my last comment!!

Ok now I get it. However, isn't that slightly impractical... (This is where you just loose it!)

This is where you tell me it's a really simple set up and I realise how stupid I've been.....

Thanks for your help so far..

Owen.