[etaf]

If i have a HD TV say 1080p
and a PC with a resolution of 1080 pixels high
connect with a VGA cable to the PC input on the TV
PC plays a HD BlueRay DVD
the TV screen displays - HD is this correct correct

whats the advantage (ignore sound) of connecting via a HDMI cable

[Chris Muriel]

In some cases (or setups) HDMI may incorporate the audio too.
The VGA input on some TVs supports a limited number of resolutions; that can normally be unearthed by studying the detailed specifications of a particular display model.

[etaf]

so assuming the input resolution does support 1080 or even 720 height
then that would be no different to HDMI
on a 1080 tv
oe 720 tv

[Chris Muriel]

OK, moving from the simple answer to the technical that I avoided earlier....
In theory the HDMI route should be better because the video from the PC can be sent in digital form to the LCD TV whereas with VGA it has to go through a triple video DAC (more properly known as a DENC, or Digital ENCoder) to be sent as (analogue) VGA, then digitised by the LCD TV via a video ADC (AKA decoder) to the panel electronics which uses digital input.
Therefore using the VGA route involves 2 extra stages of data conversion which could produce its own artifacts, distortion and non linearities; in practice you probably won't be able to tell the difference.
Comparative results depend on the system - so you could suck it and see.

[etaf]

Brilliant thanks for that, i did understand the technical bits ...and should have known myself really (ex electronic Engineer, 1970s) ..... had not thought it through....

[Chris Muriel]

Ok, glad it sunk in
I have the advantage that I am still practising electronics for a living (paid to be a geek). Unfortunately, however, too much of my time these days is spent doing IT work as I am in charge of training our distributors' applications engineers worldwide (I work for an IC firm).