Tapping for audio and video signals...

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Externet
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Tapping for audio and video signals...

Post by Externet » Sat Dec 29, 2007 11:48 am

Hello all.
To use the audio output amplifier section of a -say stereo, or TV- , the convenient point to inject 'line' level audio is at the volume control potentiometer.

To tap same level audio -say to record from-; the same point is the convenient one.

Now, how to tap video signal? Is it the contrast potentiometer wiper on a TV the right point?
- As to use/adapt as 'video in' to monitor NTSC on a TV , or to get a video output from a TV that has no composite video 'RCA' connectors-

Thanks,
Miguel
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Post by jwax » Sat Dec 29, 2007 5:58 pm

Tough call, Miguel! No one spot will answer all the possibilities. Most likely you'll need a schematic and select a point from there.

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Post by dyarker » Sat Dec 29, 2007 7:23 pm

Is that a CRT TV?
Dale Y

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Externet
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Post by Externet » Sat Dec 29, 2007 7:43 pm

Yes, a plain 'generic' CRT with potentiometer controls. :sad:

Edited: , Said volume potentiometer; changed to potentiometer controls.
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Post by dyarker » Sat Dec 29, 2007 8:14 pm

Then good chance it's a hot chasis set. Any wires you add into, or out of, the box will need isolation. Audio isolation would be easy, baseband video isolation is harder. Or, permanently power the TV through a 1:1 115VAC isolation transformer. Then signal common inside the TV can be ground ref'ed outside the TV without sparks flying and breakers popping.

Cheers,
Dale Y

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Post by haklesup » Sat Dec 29, 2007 11:18 pm

Unlike Audio, the video signal is not divided by any pots. If there are any pots, they are used to make referecne voltages for op amp circuits.

THe best place to inject video on an NTSC TV is right at the output of the tuner. Typically, analog TV's have some sort of coax coming out of the tuner and into the video processing circuits. In many cases its stll an RCA jack amking it easy to interface.

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Post by Externet » Sun Dec 30, 2007 11:15 am

Thanks.
It is not a hot chassis, it is a portable 12V DC TV.
I thought the tuner output 'RCA' was 45MHz intermediate frequency, not NTSC yet in the circuitry. Perhaps am wrong.
It is to use a portable TV as an underwater video camera monitor.
Miguel
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Post by jwax » Sun Dec 30, 2007 11:42 am

I went down that route also, Miguel. I wanted a video monitor for a telescope camera, but found that although the camera resolution was excellent, a crappy TV/CRT monitor ruined a nice image. Go with a real video monitor- either a good security monitor, or an LCD that has video input. Several are available for auto (12 VDC) use.
I found an in-dash 7" TFT LCD video monitor on ebay for about $50.
Only wish it was a brighter screen.

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Post by haklesup » Sun Dec 30, 2007 12:53 pm

I thought the tuner output 'RCA' was 45MHz intermediate frequency, not NTSC yet in the circuitry. Perhaps am wrong.
You might be right. It probably depends on the tuner and vintage of the set. BTW, NTSC is the over the air RF signal, composite video is what you are after.

Wouldn't it be easier to get an RF modulator and input the video on channel 3 or 4 of the old set. Nothing to open and its an off the shelf part. You might already have one in your junk box (AKA video game adapter).

Here is a nice example with multiple switchable A/V inputs and RF output for $30.
http://www.shoppingwarehouse.net/prod-7 ... h%2BEngine


JWAX has a point, portable CRTs weren't known for their high resolution. I suspect it is hard to get a very high voltage starting with 12VDC for the tube.

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Post by dyarker » Sun Dec 30, 2007 7:21 pm

"BTW, NTSC is the over the air RF signal, composite video is what you are after."

I'm pretty sure "NTSC" is spec for number of lines, scan rate, sync pulse width, etc., and modulation types for video and sound. So, the RF would be modulated NTSC, and composite would be baseband NTSC video with sound separated out. I would still call it NTSC because as RF or as composite, it is different than PAL or SECAM for lines, scan rate, etc.etc.

(I almost didn't post this 'cause I don't want to sound like Chris. So, tried being very careful with wording instead.)

Cheers,
Dale Y

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Post by Robert Reed » Sun Dec 30, 2007 8:29 pm

"(I almost didn't post this 'cause I don't want to sound like Chris. So, tried being very careful with wording instead.)"

LOL when I read this Dale. If you gave it your best shot on your best day, you couldn't even reach Chris's bootstraps when it comes to BS and gibberish :grin:

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Re: Tapping for audio and video signals...

Post by gmclam » Fri Jan 04, 2008 2:33 pm

Externet wrote:...Now, how to tap video signal? Is it the contrast potentiometer wiper on a TV the right point?
- As to use/adapt as 'video in' to monitor NTSC on a TV , or to get a video output from a TV that has no composite video 'RCA' connectors-...
There is really 2 separate questions here. The first is where to tap the signal to OUTPUT the TUNER video. Tap the signal at the VIDEO DETECTOR. There are tons of issues though, depending on the TV. You need to consider the signal level (you want 1.0 vp-p but some of these points can be 2.0v or 4.0v p-p); whether the set is a "hot chassis"; and how much of a load that signal can drive. I doubt it will drive 75 ohms.

The second half of the question is where can you apply an external video signal; such as to convert a TV into a monitor. Some TVs are easily converted and some are not worth the trouble. In a good set you should be able to inject your signal at the point where the output of the video detector was feeding (above). You still have to consider voltage level, ground, and whether or not the TV you're modifying is a good candidate.

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