RF output DOES NOT equal Composite!!!!

From: Dave Woodman <dave_at_naffnet.org.uk>
Date: Sun Jun 16 12:44:54 2002

John Honniball wrote:

> Dave Woodman wrote:
> >
> > Except, of course, in those large parts of the world that use PAL, and other
> > parts that use SECAM, and....
>
> Thanks for pointing that out!

You're welcome!

>
>
> Just to add a bit of technical info, in the PAL-I system, as used
> in the UK, the composite video is inverted before being amplitude
> modulated onto the carrier. That means that the sync pules are
> peaks of high-amplitude radio signal and bright areas of the
> image are low-amplitude signals. The reason for this is to
> reduce the average RF power required at the transmitter. Quite
> a clever idea, really.

There is also another benefit:- any impulse noise that interferes with
the video results in a white pin-point on the picture, as oppose to a black
blob (especially when the HT is poorly regulated...).

>
>
> Our sound sub-carrier is at 6MHz from the main vision carrier,
> FM modulated. The PAL colour subcarrier is at about 4.43Mhz.

Let's not forget the 'PAL' bit:- the chrominance information is
presented with its phase inverted on alternate lines. This is then used
to correct any phase errors in the signal (the reason that NTSC is
often called Never Twice Same Colour is because, uncorrected,
this distorts the colour information).

The net result is that colours are stable, but cannot ever be
completely saturated, a small price to pay for stable colours and not
needing a hardware patch - the 'hue' control.

The mechanism that was used in early TV sets to achieve the
correction was rather interesting:- the set contained a glass block,
with an ultrasonic transducer at each end (one sending, the other?
well, no prizes for guessing!). The delay in the block was one
transmitted line so the output could be directly compared with the
following line. Ah, the wonders of old technology...

Cheers,

        Dave.
Received on Sun Jun 16 2002 - 12:44:54 BST