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video level problem? SA8300HD, YAM RXV1200, Mits WS-55809

OK, maybe some of you can see what I am missing or know something that I don't that can shed some light on this one. We have a client that has had this Yamaha receiver and Mits HD ready set for a few years and just switched to Cox HD cable. The receiver has been working fine, switching both component and composite signals into the set. Hooking up the SA box through the Yam (component signal) gives a pix in which the whites are "blown out" or "crushed" like the video is too intense. Black levels look normal. The effect is like looking at video that is not properly terminated at 75 ohms. Other sources besides the SA box look fine using the same inputs on the Yam and Mits. Here is the catch. Hook the SA box up to the set directly and it looks fine. OK, so the Yam is the problem, right? Well, pulled it to the shop, assumed it might have a bad ground or something and checked, re-seated everthing in the vicinity of the video board. Threw every kind of source at it that I could, and the input and output looked identical, pix looks fine with every source 480i to 1080i on every one of 4 different technology displays.

Put it back in the system assuming that we had missed something or maybe a bad cable or something and the same thing happened. I have not looked at the output of the SA box yet, since we do not have Cox at the shop and can't get it. Guess I will be hauling a scope out to the site. Maybe someone will come up with some ideas before I go.

The Yamaha seems to be a very straightforward buffered switch with about

60Mhz bandwidth. I could not detect any degradation in signals passed through it and I confirmed it terminates at 75 ohms. Maybe the output of the box is really hot and the Yamaha is not tolerant and clipping the signal? Looks fine going directly into the set, however.

Any ideas?

Leonard

Reply to
Leonard Caillouet
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We came across some 'prosumer' gear at work that did not have correct output termination which caused some aggavation when trying to connect it in. In your case I'd take the source video into a BNC 'T' connector into the input of your scope and then connect the terminator on the other leg of the 'T'. If everyone is 'playing by the rules', the level should drop by 6 dB and be 700mV without sync, 1 V with sync.

Good luck sorting it out. GG

Reply to
G-squared

My first thought was unterminated video when I saw the symptom. We'll see what the level actually is, but the TV handles it fine. My second thought was that maybe the tri-level sync from the HD was getting distorted and the Mits was not handling it properly. We have seen this issue in other brands, but if there was a problem there it seems like the level would go the other way if the set were trying to clamp to the second half of the tri-level sync pulse. Maybe the SA box is not using the proper tri-level sync? Not possible at this late date that they could do something so silly, is it?

Leonard

Reply to
Leonard Caillouet

I agree with the previous poster. I bet the TV has a termination on it's input, which is why it handles the signal well. A lot of equipment has an option to terminate, or not terminate in order to bridge equipment.

Don

Reply to
Don Bowey

Could it be an output driver of the kind that require DC termination to define the working point in combination with a DC blocked input? I have seen some incompatible S-video connections and I heard of this problem with certain units. /Jan

Reply to
Jan B

I am the OP and the termination is appropriate at 75 ohms.

Leonard

Reply to
Leonard Caillouet

What do you mean by DC termination?

Leonard

Reply to
Leonard Caillouet

Hi, Receiver was doing fine and problem when Cox HD box is hooked up. Logic tells me, I'd try another cable box first. Tony

Reply to
Tony Hwang

That was the first thing we tried.

Leonard

Reply to
Leonard Caillouet

I mean a termination that works also at DC. Some drivers need the 75ohms to give the correct DC-load (and levels).

At least some front projectors aparently has a DC blocked termination (on S-Video inputs) and those units have a problem with each other. This can be measured with a normal ohm-meter. /Jan

Reply to
Jan B

All component, composite and S-video should be terminated at 75 ohms.

Some units have the option of switching the terminating resistors on or off to allow a loop out to another unit, where there should be proper termination. This may be what you are talking about. Nearly all inputs are coupled to the buffer stages through capacitors that block dc that can occur as a result of the negative going sync pulses. This is one of the advantages of tri-level sync used with HD...it has both positive and negative going pulses that average out. Still, I have not seen an input circuit in consumer electronics that was not capacitor coupled (after the 75 ohm terminating resistors).

Leonard

Reply to
Leonard Caillouet

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