This is a "generic" answer to your enquiry. Your video adaptor
may have different characteristics.
I suspect that your video drivers are setting the output to
provide a specification that ALL display devices can accept.
By default, your laptop has a 16:9 display - and probably has a
maximum refresh rate of 60 Hz. However, when you connect a 4:3
auxiliary monitor, plug & play will detects the monitor and
configure the video output to a setting that both monitors can
accept. Furthermore, you will not be able to set the refresh
rate to 75 Hz, if either monitor is limited to 60 Hz.
Without testing, I can't be certain what would happen, if you
connected an auxiliary monitor with a 16:9 display. However, I
suspect both devices would display the 16:9 aspect ratio. The
same principle would probably apply, if you connect a 16:9 HDTV.
If not, you would probably be able to configure the output
manually (via Control Panel / Video Settings).
I have the reverse situation with my HP laptop, which has a 4:3
screen. If I connect it to my HDTV, the TV display will also
display the output in 4:3 aspect ratio, with black bands at
either side. However, if I turn off the laptop display and use
the TV as the default monitor, the aspect ratio increases to
16:9.
To obtain a more specific answer, it would be helpful to have
details of your video adaptor.
If your computer uses an Intel Graphics Media Accelerator Driver
for Mobile, the latest versions of this driver allow you to
create pre-set video configurations (or "schemes") to meet your
personal requirements. For instance, I have created the
following "schemes" on my laptop:
1. Laptop only. 4:3 ratio. 32 bit. 1024 x 768. 60Hz.
Auxiliary output disabled
2. Laptop and TV. 4:3 ratio. 32 bit. 1024 x 768. 60Hz.
Auxiliary output enabled
3. TV only. 16:9 ratio. 32 bit. 1360 x 768. 60Hz. Laptop
display disabled
