It gives errors when I use AT&T HDMI

If you did connect the computer to the tv with a HDMI cable, you have to look there. HDMI also protects giving through copy protected content. Please read everything about HDMI and digital rights.

just plug in the HDMI cable from what ever equipment your using,then switch it on, put the other end into the back of the tv, take out any scart cables you have plugged in, just for now.
then find your AV source button on your remote, press it until it says HDMI.
the chances are,if it flickers or wont show, then you'll need to hook up the external equipment by scart cable & go into the picture settings & change it to a lower rez. (The tv wont be the problem)
i can't help you any more because you haven't said what it is your trying to do or hook up.....SKY BOX, DVD PLAYER, GAMES CONSOLE...e.t.c.
Or what make of TV it is.....

Change the settings on your HDMI output to 720 progressive for resolution. The tv likely cannot handle the 1080 resolution, or the tv can only use interlaced (1080i or 720i) format.

So I had this same problem. I found that when I hooked to a 1080p TV my PS3 was working, however when I hooked to my 1080i it said Not Supported Mode. So I then hooked to the 1080p and told my PS3 to limit it's self to 1080i mode and hooked it to my Samsung TV and it worked just fine. Give that a try and it should solve the problem. Hope this helps.

Simply attach a scart lead from the rear of the DVD to the AV input in the rear of the TV. if vboth the TV & the DVD have HDMI connections then use a HDMI cable connector instead as this will give a better picture

If your Sony TV is hd ready you have a number of choices of connections you can use. On the back of the DCT 3400 you have regular AUDIO and VIDEO connections, the separate Y, Pb, & Pr connections (both of these use the regular RCA style connectors), RF in (which uses the standard cable connection), and the HDMI connector, which uses a special flat ribbon style connector. Your Sony should have all of these types of connectors on the back of the TV. The separate Y, Pb, & Pr, or HDMI connectors will give you the best picture.

if it worked fine at some point then it should work fine now... i use hdmi to hdmi from my laptop to tv all the time dvi would only cause quality loss.

the first thing i would try would be to change your resolution on your laptop to a couple different settings. a lot of times, if its not set to one thats compatible with your tv, you won't see anything.

if that doesn't work, go to personalization and open the tab that says "connect an external display" and toy around with it. but yeah it doesn't sound like you have any hardware problems there.

hope this helps

I had this issue once before, recheck all the conenctions on the tv, and make sure that the HDMI plug in for the TV * if its the older model * make sure it is placed to HDMI not TV.
Please let me know how it goes,
Thanks
Justin
Please rate me

Purchased a newer HDMI DVD player,(Sony)as well as found that I do need the video cables.Also found that I need to change the input to component on the DVD player by using a regular coxial to at least get a display from the DVD player had I kept the older one.

if I understand your question properly.. its because your direct tv output doesnt broadcast in high def so its giving you the standard tv definition inputs which are RGB or Component.so you will lose picture quality cos its running at fewer lines or pixels. hope this helps