Problems with hd via cat5e

Hi Dave

I think what is happening is that the turning on or off of a light is causing electro-magnetic interference (this is perfectly normal, it happens all the time). This is coupling in to your Cat 5 cables and causes the receiver of the signal to see a burst of bad data.

To explain: everything going over your Cat 5 is data, and this will use some protocol or other to package it up. Normally this stuff includes some extra information (often called a checksum, but it can be more complex than a classical simple adding up) which is used to validate that the information received is OK.

The way modern HD TV and digital TV works is to send lots of these packets of data. To cram in the vast amount of information in a TV picture, they mostly work by sending a FULL IMAGE once every so often (perhaps 1/2 to 1 second) and then the differences from that picture are sent thereafter. Differences are smaller than a whole image so you can cram more of these small difference packets down the wire.

This is fine and dandy BUT when a packet gets corrupted and can't be recovered, for example, by a big burst of interference, you have a problem. The TV can't decode the stuff any more, it needs to wait until the next full image comes down (this is called re-synchronising).

I see EXACTLY the same as you describe for free-to-air digital HD TV - when anything at all electrical is operated, the picture is lost and comes back about a second or 2 later. It drives me crazy but there is not much can be done about it.

The old analogue TV was *much* more tolerant of interference.

Because you are distributing using Cat5 there is perhaps a solution. You might not like it, and it might not even work. And it's expensive and horrible. Pull out the Cat 5 cable and replace it with a shielded twisted pair cable. This cable will be much more expensive, and the shield will need to be grounded at one end only. This is difficult, painful, expensive, and might not work. Another possibility is Cat6 cable which has more twists/inch and might give better common mode interference rejection. (If the common mod induced voltages are high enough, though, the front end of the receiver will still get clobbered and you are in the same amount of poo as using Cat5).

If you do find a solution (or anyone else knows of a solution that actually works) please post it for the benefit of the rest of us.

Typically this kind of thing is solved as part of a product design where tests are actually carried out, measurements made, and a suitable design is done. Sometimes this still happens but a lot of products are design out of cookbooks, with mediocre results. This might be what is happening here, also.

 

Excellent idea Don. I shoulda thunkda that one.

You can buy clamp on ferrites at places like Jaycar. (Don't bother with Dick Smith any more - they don't seem to sell electronic components any more.)

Jaycar items that might suit: U25, U16 or U15. Their cat numbers LF1290, LF1292, LF1294 respectively. About $10 (though one of those is a pack of 4 and the others are singly).

If this works, you probably need a clamp on ferrite at each TV - or pretty much at every place you make a connection to your Cat 5 wiring system - both feeding points and receiving points.