dual monitor: 1 card (dual head) vs 2 cards (agp+pci)

[hahher]

hey i wanted to know what the differences were. i mentioned this in another thread.

you can do clone, true dual (separate resolutions/refresh) through 2 cards right? can you also do span? plus you get dual dvi and can upgrade the agp card without losing dual dvi.

is suround gaming the only difference?

 

[Viper96720]

If the single card has dual ramdacs can do seperate resolution refresh with one card.

 

[Peter]

The main difference is, with two cards, yes you do get two rendering engines. Dualhead cards do have independent output pipes for independent display, clone mode, span, or whatever. What's supported depends on the operating system and the available drivers.

The big caveat however is, PCI is slow, and using a graphics card there eats a lot of system bandwidth exactly because of the PCI bus being slow. You'll lose a lot of I/O performance. This might be showing as loss of HDD performance, and various other symptoms down to e.g. disturbances and crackling from your sound solution.

 

[MercenaryForHire]

Originally posted by: Peter
The main difference is, with two cards, yes you do get two rendering engines. Dualhead cards do have independent output pipes for independent display, clone mode, span, or whatever. What's supported depends on the operating system and the available drivers.

The big caveat however is, PCI is slow, and using a graphics card there eats a lot of system bandwidth exactly because of the PCI bus being slow. You'll lose a lot of I/O performance. This might be showing as loss of HDD performance, and various other symptoms down to e.g. disturbances and crackling from your sound solution.

Not to mention this little bitch called "Driver Issues" - one card with two outputs is a lot easier for the OS to handle and talk to than two cards.

- M4H

 

[hahher]

do all current generation nvidia (5200 - 5950) and ati (9000 - 9800) cards have dual ramdac?

if a card has 2 outputs but 1 ramdac, that means it can only do clone right?

 

[Peter]

Each and any analog output (TV, CRT) needs to have its own RAMDAC. If a dualhead card had only one RAMDAC, then one of the monitors would have to be a digitally connected TFT. No matter whether the contents are cloned or separate.
This is frequently seen e.g. in notebooks, where head #1 is the internal TFT, and head #2 uses the sole RAMDAC to provide a VGA monitor connector.
On the other hand, cards with dual RAMDACS do not necessarily have to be dual head capable. This is frequently seen in older cards that do have VGA and TV outputs, but can only do clone mode.

All ATi desktop graphics chips in the Radeon series have dual RAMDACs. With the exception of the original Radeon (aka 7200), they're all dual head capable too, including the Radeon VE aka 7000. However not each and every card actually implements dual outputs. The rock bottom cheapos don't (but for $3 more it's in), and many of the All-in-Wonder cards don't simply because there is no room for the 2nd plug.

As for NVidia, the generic answer is Yes, w/ basically the same exceptions.