how do I attenuate a composite output?

[pjturner]

I'm a relative electronic newbie; I've been trying to sort the video output on my HV and have decided that it must be giving out too strong a signal as suggested in the manual; the answer is supposedly to attenuate this signal however the only attenuators I can find are for RF aeriel co-ax cables - does anyone know the answer to this one and also what level of attenuation is normally needed?

Many thanks for any expert assistance

Phil Turner, Congleton Cheshire

[katman]

I'm a relative electronic newbie; I've been trying to sort the video output on my HV and have decided that it must be giving out too strong a signal as suggested in the manual

Hi Phil,

what makes you think the output is too strong ?
what are the symptoms ?

Most domestic video equipment is pretty tollerant of incorrect levels and will often display a stable but dark image at 50% of the correct level (double terminated video signal) or a bright washed out image at 200% of the correct level (unterminated video signal).

Can you give a bit more detail on what you are trying to connect to Homevision eg, what type of display (or modulator if you are trying to distribute the output), and what video sources you are feeding into it (if any).

Do you only have a problem when trying to overlay video ?
Do you have a problem displaying on a solid background ?
Do you have a problem trying to send video straight through?

If you can supply a bit more information I might be able to suggest a possible fix.

Regards

Keith
www.kat5.tv

[pjturner]

Hi Keith - thanks VERY much for taking an interest and my huge appologies for taking so long to return to the site to find your reply. I will answer some of your questions

what makes you think the output is too strong ? I'm SURE I read somewhere that this could be the case but can I find confirmation of this anywhere now?? ...NO!
what are the symptoms ? a fairly scrambled picture (I will try to e-mail you a photo)

Can you give a bit more detail on what you are trying to connect to Homevision - eventually TIVO but currently using dig terrestrial box output from scart to composite to test - this produces perfect picture thru AV3 front socket on my 6 yr old LG TV but scrambles when passed thru HV, however the "overlay onto solid background" option works ok and shows my HV video screen menus

and what video sources you are feeding into it (if any). LG TV as above

Do you only have a problem when trying to overlay video ? yes
Do you have a problem displaying on a solid background ? no
Do you have a problem trying to send video straight through? straight thru HV - yes

It sounds like the problem described in manual where it suggests you adjust RV1 video signal resistor except my HV is newer model (C3.1A is on IC label) which doesn't have these adjustable resistors

Thanks again for anything you can suggest, Phil Turner.

[katman]

I will try to e-mail you a photo

Received the photo OK thanks

Can you give a bit more detail on what you are trying to connect to Homevision - eventually TIVO but currently using dig terrestrial box output from scart to composite to test - this produces perfect picture thru AV3 front socket on my 6 yr old LG TV but scrambles when passed thru HV, however the "overlay onto solid background" option works ok and shows my HV video screen menus

It may be that your DTT box doesnt like the input to HomeVision.

I will try and explain....

a video signal MUST be terminated by a 75 ohm resistor to achieve the correct signal levels. All video equipment also contains a DC blocking capacitor on the input to prevent strange things happening if there is a slight DC offset on the signal being applied.

The termination resistor can be fitted in one of two locations, either BEFORE the capacitor like this...
(hope the ASCII art works OK if not this will look stupid!!!)

Input-------------||--------
|
75R Video circuitry
|
Ground---------------------

or AFTER the capacitor like this....

Input-------||---------------
|
75R Video circuitry
|
Ground---------------------

The main difference is that putting the resistor BEFORE the capacitor provides a DC path to ground for the input signal whereas putting it after does not.

Some source equipment will not work correctly if it doesnt see a DC path to ground. In the early days of KAT5 I had about 6 cases of this so I have moved the termination to be before the capacitor.

Do you only have a problem when trying to overlay video ? yes
Do you have a problem displaying on a solid background ? no
Do you have a problem trying to send video straight through? straight thru HV - yes

Which indicated that problems ONLY occur with a video input irrespective of whether OSD is enabled or not. The only peice of source equipment I had that exhibited a dislike of no DC path was also a DTT box, a Pace Ondigital box. It now works fine with KAT5 units.

There are two things you can try.

1. Temporarily try a different piece of video kit as the source eg VCR or your Tivo.

2. Try connecting a resistor across the input. A 1K resistor will provide a DC path which may stabilise the picture with no loss of signal. If that doesnt work try progressively lower values but you shouldnt need to go below about 220R.

Hope that helps

Keith

[katman]

(hope the ASCII art works OK if not this will look stupid!!!)

ASCII art didnt work and it does look stupid .... DOH !!!

[pjturner]

you're quite right, Keith - it was a funny output from my DTR box - I was sure I'd previously had the same trouble with direct output from TIVO but I obviously hadn't as it seems to work perfectly - I've had to hack apart a SCART cable to allow me to send the composite signal through Homevision but keep the pin 8 signal that tells my telly to switch to AV2. Just have to do some more exciting things with my HV custom screens now!
MANY thanks for your help, Phil.