PS3 wont go 1080i/720p

Connect your PS3 to your tv using a normal AV cable. Check some Display settings like 720p or 1080p. I've your TV is 720p, 1080p probably wont work.
I'm not sure of this all, but its worth a shot.

hi
There maybe some problem with your TVs resolution. ps3 outputs full hd high resolution video(1080p) through its HDMI. May be your TV is not supporting that resolution. Connect ps3's component video out to your TV's component video input and then try. if it ask for any resolution selection. select 720p or 1080i, above that resolution your TV will not support. Your tv is hd ready one, not full hd. ps3 's HDMI out is full hd (1080p).
or try this
Connect your ps3 to any full hd tv and then goto video setup of ps3. change resolution to 1080i (down scaling ). then connect to your TV via HDMI. surly u will get the video.
ok

When you switched cables, you mean you changed the cord running from the PS3 to the TV? HD content requires an HDMI connection from the PS3 to the TV. It is possible that when you changed cables, you replaced the PS3's HDMI cable with Component Cables which can only deliver in Standard Definition and not in High Definition (720p, 1080i and 1080p).
HDMI:
http://www.playstation3hdmicable.com/images/hdmi2.jpg
Component: http://www.componentcable.net/images/Component%20Cable%20PS3.jpg

hi
there maybe some problem with ur tvs resolution. ps3 outputs full hd high resolution video(1080p) through its hdmi. may be your tv is not supporting that resolution. connect ps3's component video out to ur tv and then try. if it ask for any resolution selection. select 720p or 1080i, above that resolution ur tv will not support. ur tv is hd ready one, not full hd. ps3 's hdmi out is full hd (1080p).
or try this
connect ur ps3 to any full hd tv and then goto video setup of ur ps3. change resolution to 1080i (down scaling ). then connect to ur tv via hdmi. surly u will get the video.
ok

You have make sure the game can run in 1080p first by looking at the back of the box. If it can, and you're using hdmi etc then simply do the following:

1. Go into display settings.
2. Select HDMI, then "custom"
3. Uncheck 720p and 1080i, leave 1080p selected.
4. Start up the game and double check it is on 1080p.

Otherwise, you're stuck because the majority of games are in 720p to avoid lag within the game.

This usually happens when you first buy your ps3 and try to use the HDMI connection immediately. The problem is that the ps3's default output resolution is set to 480i out of the box, and when you try to use an HDMI connection first, HDMI can't do that resolution. HDMI only does 480p, 720p 1080i and 1080p.
See :)

This also happens if you move your ps3 a lot. As it is, HDTV's are either 1080p (full HD) or 720p (including 1080i). If, for example, your tv is 1080p, when you set your display settings in the ps3 menu, it will set your XMB, or ps3 main menu to the max resolution of your tv, 1080p in this case. The resolution will automatically adjust when you start a game, and that game has a native resolution output. That's why, most of the time, games will play in 720p, because that's there native, or original, resolution. This is also the reason why sometimes people have no display on there TV, or a black screen, and when they play a game, that will actually display. That's because your tv can ACCEPT that resolution of that game.

• Have your ps3 in standby mode (red light).
• Now, press and hold the power button in the front down. Your ps3 will beep twice. Let it go almost exactly after you heard the second beep.
• Wait 10-15 seconds. A display will show up, and just answer yes to all!

This is natural, the PS3 does not stretch the signal like the Xbox does so games play at there native resolution (720p) there is nothing wrong with your TV or your PS3 and you would not see any difference in the games graphics if it was in 1080i. I'm playing my PS3 on a 46" 1080p TV and my PS3 games look great in 720p.

Hi

Try This links and follow the instructions! CLICK HERE

Thanks!!

Change the resolution on the ps3 to accept 480p, 720p, 1080i, 1080p.

SOURCE: http://boardsus.playstation.com/playstation/board/message?board.id=confrontation&thread.id=735278&view=by_date_ascending&page=2

If your playing with lag on your new HD LCD your at a competitive disadvantage! Everything you see happens seconds later than your opponent who has his/her settings right.
The fix:
1. Connect your PS3 using HDMI Input 2.
2. SELECT or RENAME the HDMI Input to PC. Already pointed out, but this does not make the majority of tv's run in PC mode.
3. On the PS3 start-up menu go to Settings---> Display ---->
Choose HDMI ----> DO NOT choose AUTOMATIC ----> SELECT CUSTOM ----->
4. Now, Select all the Resolution options EXCEPT 720p. This will force your games into 1080p mode. What it will also do is drop any game that doesn't have 1080i/p support into 480. Games like Burnout Paradise, Vegas 2, Heavenly Sword, NHL 08 and more will be dropped into SD (480) rather than HD. Your picture will be terrible. 720p is standard gaming resolution currently, 1080i/p is not.
5. Save and complete the display setup.
6. Turn off the PS3. Turn off the TV.
7. Turn on the TV. Turn on the PS3.
8. Complete! (Lag eliminated). Please spread this around!

*The key to eliminating ALL LAG is modern LCD's need to be in PC MODE (NOT GAME MODE) On many tv's, PC mode requires you to use a VGA or DVI connector. You'll either need to buy an HDMI adapter or an HDMI to VGA / DVI cable if that's the case.
and the Source MUST BE IN 1080p so that the TV can use it's NATIVE RESOLUTION.* Please, check your tv's native resolution. If you have a plasma that only does 720p, your native resolution is probably 720p. You do not need to follow these steps, it will only make things worse. If your tv's native resolution is 720p, obviously you want to keep your display settings at 720p on your PS3. If your native resolution is 1080i, you shouldn't go selecting 1080p. If your native resolution is 1080p on your TV, you need to be aware that removing 720p as an option will force many games into 480.

Keep these things in mind and remember that the steps above are far from a cure all solution. .

Lag gone! Worked for me! (SAMSUNG LNA52850)
Source : http://answers.yahoo.com/question/index?qid=20081225141902AApX7cJ