Onboard graphics connected monitors don't sleep while discrete video connected monitors do sleep

[luger]

I've got 5 monitors connected to a Dell XPS 8700 with connections as follows:

1. Intel onboard graphics - HDMI
2. Intel onboard graphics - DisplayPort
   
3. Nvidia discrete video - HDMI
4. Nvidia discrete video - DisplayPort
   
5. Nvidia discrete video - DVI

Using Windows 7 power options, I can set the "Turn off the display" option, and when the timeout is reached, the computer no longer sends a signal to the 3 monitors connected to the Nvidia card and they enter sleep mode. The 2 monitors connected to onboard graphics go to a black screen, but they do not enter sleep mode, so they must still be receiving a signal from the computer.

The onboard graphics is Intel HD Graphics 4600.

All the monitors do function properly when I put the computer to sleep - all monitors will then enter sleep mode.

Any ideas on why this might be happening?

 

[luger]

Some digging around in the Intel graphics control panel shows that under Power Management Support, standby mode and suspend mode are "Not Supported". Strange as these are Dell and Asus monitors made in the last 1-2 years.

 

[luger]

Resolved.

Intel HD Graphics Control Panel > Audio Settings > Audio Without Video setting. This was ON. Turning it to OFF allows the monitors to enter sleep via Windows 7 Power Options display turnoff setting.