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 Post subject: VGA to composite adapter
PostPosted: Sep Mon 30, 2019 2:03 am 
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I have a question about VGA to composite adapters.

I recently modded a Sony B/W TV to accept composite video and I had used this VGA to composite converter

https://www.amazon.com/Sewell-Direct-SW ... 871&sr=8-5

with a composite to X/Y/Z converter for use with an X/Y display.

My question is this.

Are all VGA to composite adapters going to produce the same quality of composite signal or will a better quality adapter produce a better quality composite signal? As in is the adapter I'm using producing the best standard composite signal or will a better adapter produce a composite signal that is closer to the best composite signal possible for the analog composite video standard?


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 Post subject: Re: VGA to composite adapter
PostPosted: Sep Mon 30, 2019 4:38 pm 
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There are differences between converters.

You should check the video on a modern CRT TV to confirm signal quality.

Also your XY(deflection?) Monitor if it is monochrome you may get dot crawl/noise with composite because of how color is encoded in analog. There are a ways around this: 1 use a filter to cut everything above 3.5MHz. 2 get a VGA to S-video converter an S-video cable, cut one end off the cable and fit RCA connectors to each of the 2 coax lines....one line will be composite monochrome (with more detail than the low pass filter) and the other will be color info...patch the monochrome into your XY converters composite input.


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 Post subject: Re: VGA to composite adapter
PostPosted: Sep Mon 30, 2019 5:50 pm 
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Keep in mind that going from VGA to composite will require the resolution to be scaled down to 480i. The quality of the resolution conversion process will vary a lot, but a more expensive adapter isn't necessarily going to be better. Some adapters allow you to adjust the scaling and color/brightness for best results.


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 Post subject: Re: VGA to composite adapter
PostPosted: Sep Mon 30, 2019 10:40 pm 
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I posted this before finding a video on youtube where a comment said a S video output would give better quality for a B/W TV using only the luma output and it didn't dawn on me until later that my laptop has an S video out jack on it.

So I'm going to first try that.

Then if it doesn't work I'll try another adapter.

Unfortunately I only have a smaller lower resolution LCD display with composite input to test the converter with so I couldn't properly test it.

That said the S video output of the laptop should give the best quality I would think.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Tue 01, 2019 6:25 pm 
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The S-video luminance channel from the s-video connector will work fine, it also has the Sync information, but you may have to add a termination resistor from signal pin to Sheild if the video looks over driven, I cant remember for sure, but you will know if it looks over driven

I would use a S-video cable and cut the other end off and add an RCA to the end of the wire,
you do not want to just stick wires or pins alone into the s-video socket because on some of the sockets there is a Switch that is pushed in when you insert the cable into the socket
not all have this switch but your computer may use that to enable the output ( that little black plastic square pin in between the pins )
once you do this you will have to go to Display properties on your computer and look for the s-video in the display properties and enable it, I take it you may have a windows xp or win 7 laptop ?

if you still have problems getting the signal out you could go back to the adaptor you bought and after you connect it you may also have to go to display properties to enable it
if you have win 7 or higher and your keyboard is a Windows keyboard then on the lower left of the keyboard look for the "windows " key, its usually in-between the CTRL and Alt keys
it just has the windows flag on it, this serves as a shortcut key
you push and HOLD the Windows key while pressing the "P" key, then on the right side of your laptop display you will see a selection pop up that will allow you to select the mode
modes you will see are :
"PC" screen only selection only allows the laptop screen to display
"Duplicate" selects both the laptop screen and the VGA output to display the Same thing
"Extend selection" is to extend the laptop screen to the VGA output screen ( usually it will be black )
"Second screen only" selection will only output the 2nd screen or VGA device you have connected

also some of those little cheap convertor adaptors do not supply an ID Bit or DDC channel to the Computer to tell the laptop that there really is something connected
and also almost forgot- if the adaptor you purchased does not contain the I.D bit and DDC ( Data display channel) then you may very well have to turn the Resolution in your display setting to 640 x 480

to do this close or minimize all pages and Right click on the center of your screen ( windows home screen ) and select " display Settings" then scroll down and you should see the settings for Resolution, then select 640 x 480 and you may in fact have to do this to get your S-video connector to work properly, it all depends on the computer and video cards installed

if all else fails then you could buy an Extron VSC-500 scan convertor, these were popular years ago and you can find them dirt cheap on ebay
these also have centering and size adjustments to compensate for Sync pulse timing issues that vary from computer to computer

hope all this helps

Terry


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Tue 01, 2019 9:38 pm 
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The cable I bought is an S video four pin to dual RCA plugs with the chroma on one plug and the luma on the other plug.

My laptop is a Vista laptop, but is currently running Windows 10 64 bit without a problem.

Not sure if that will cause any issues with the S video port.

I did test the TV using a piece of test equipment at work which has a composite video output (outputs text only and is B/W as it has an internal B/W CRT display) and when I added a 50 ohm load resistor I had to readjust the sync and the picture no longer looked right. The signal was high enough to where I could have the picture control turned all the way down and still get a usable display on the screen. The adapter I used at home didn't produce nowhere near that much signal and I doubt the computer will either.

It does verify though that I can feed a pretty hefty signal to the TV before the signal overdrives the circuitry.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Tue 01, 2019 10:08 pm 
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Composite video is 75ohms termination, but I cant remember what and if any termination is required for S-video Luma Termination, maybe none at all, depends on the Gain and all of the Video amp you are connecting to, I guess just viewing image and adjusting picture and brightness controls would be fine
not sure if win 10 supports the S-video output, all you can do is try, can't hurt anything
good to see someone else being creative :)

Terry


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Tue 01, 2019 10:34 pm 
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Perhaps I should have tried a 75 ohm resistor instead. Could explain why the 50 ohm resistor caused the image to mess up somewhat.

Perhaps no termination was also why the signal was higher in output than what came from my adapter.

I can try a 75 ohm resistor and no capacitor and see if that works better.

Perhaps that would have given me a better image with the existing adapter, although just using the luma signal will improve the image as there's no color info.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Wed 02, 2019 1:21 am 
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The use of a 75 ohm resistor is independent of the use of a capacitor. The resistor would go directly across the jack the coax plugs into, not after any cap.

_________________
Tom


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Wed 02, 2019 4:50 am 
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I decided to try a 75 ohm resistor.

Not having one I used a 33 and 39 ohm in series for 72 ohms.

Needed that in order for the laptop to detect something was plugged into the S video jack.

Here's what the display looks like.

Attachment:
20191001_232701.jpg
20191001_232701.jpg [ 211.83 KiB | Viewed 773 times ]


I do have a topic about the TV where I detail the composite video mod and resistor replacement.

I put it in DIY as it was a modification, but perhaps I should have put it here so it would get more exposure from those who deal with TV.

viewtopic.php?f=12&t=365108


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Wed 02, 2019 10:06 am 
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Tube Radio wrote:
I decided to try a 75 ohm resistor.

Not having one I used a 33 and 39 ohm in series for 72 ohms.

Needed that in order for the laptop to detect something was plugged into the S video jack.

Here's what the display looks like.

Attachment:
20191001_232701.jpg


I do have a topic about the TV where I detail the composite video mod and resistor replacement.

I put it in DIY as it was a modification, but perhaps I should have put it here so it would get more exposure from those who deal with TV.

https://www.antiqueradios.com/forums/vi ... 2&t=365108


Looks great !
you could add an inexpensive Video D/A to that and run it to other sets that you have that have Composite Video in :)


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Wed 02, 2019 11:25 am 
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Perhaps, but this is the only one I have.

The eventual goal as I discussed in the topic about the TV is to at some point make it display an oscilloscope clock.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Fri 04, 2019 3:18 am 
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Decided to try this cable with my composite to X/Y/Z adapter I built and here's how it looks on my Tektronix 606A X/Y display.

There's some issues with grounding in the converter box which explains the barely visible lines and a slight lack of clarity as a result.

The camera doesn't do this justice.

Attachment:
Display 2.png
Display 2.png [ 721.65 KiB | Viewed 700 times ]


I also have an RCA AXR-095-L B/W TV that I may add a composite video input to if I can find the schematic.

That one would need an isolation transformer though as it uses no power transformer.

Why?

1. Because the TV is laying around and would otherwise be completely useless.
2. To see if I can do it on that TV and how well it works.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Fri 04, 2019 6:18 pm 
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If the tuner/RF system is working you could also save the effort of adding a video input and simply get an RF modulator (composite video/audio to RF converter). They can still be bought new but you can get them dirt cheap used in thrift stores. Most VCRs have built in RF modulators (just gotta set them to VCR mode and tune down to the AV input).


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Fri 04, 2019 6:44 pm 
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Perhaps, but I can get a better picture without going through the RF section.

Now if I were to feed the luma signal to the composite to RF converter perhaps I'd still get a quality picture, but then again conversion is very easy.

1. Find the correct point to insert the composite signal.
2. Disconnect the internal connection to the video IF.
3. Wire in an RCA jack.
4. Mount RCA jack if not using an existing jack on the TV.


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Tue 15, 2019 2:25 am 
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I'm trying to convert my Sony TV to composite input.

Could you help me out.

imgur[dot]com/a/ecYkQ2q

I believe that I need to inject composite into the base Q207. Is this correct?


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 Post subject: Re: VGA to composite adapter
PostPosted: Oct Wed 16, 2019 1:19 am 
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Here's a picture showing the part of the schematic and the change made.

You want to completely remove Q207 and the input is where the emitter of Q207 connected with the cable shield going to the nearest ground point.

If the sync, video and sweep circuits are on separate boards and receive power straight from the power supply, you can remove B+ from the tuners and IF board.

For the video input either use the external VHF antenna jack or the headphone jack. If the headphone jack is used, the speaker will have to be wired directly and all connections removed from the headphone jack. Use a good quality shielded wire to connect the jack to where the emitter of Q207 connected.

My suggestion is to also wire the input to the volume control to whichever jack was not used for the video input as that will allow you to also input an audio signal to the amp.

If using with a computer you need a good quality VGA to composite adapter. Look for one that can also output s-video. If using with a composite source no converter is needed.

You may or may not need a 75 ohm resistor between the video input and ground. Depends on how the video looks.

Attachment:
Composite input.png
Composite input.png [ 49.19 KiB | Viewed 505 times ]


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