Jim Yanik wrote:
AFAIK,the TV systems are STILL incompatible;
Europe uses different broadcast modulation schemes and different frequency
assignments.

I assume in this case you are talking about digital TV. It all depends upon
how you look at it. I don't know about the pre-war 405 line English system,
which finally was stopped in the 1980s. However the 525 line US system and
and 625 line English/French systems were basicly the same, a "flying spot"
of light, zero volts being white and about one volt being black. The scanning
speed was the same, the US system had less lines because it scanned 60 times
a second, the English/French 50.

A DC syncrchronization aka "sync" pluse was included to keep everything
together so if signal got scrambled, the TV would bring it back together
quickly.

Those rates were chosen because the studio lights were arc lights and flashed
on and off at the power line rate, so the TV cameras had to be syncronized
to them or you would get moving black stripes across the screen.

The RCA system for compatible color TV (compatible with black and white),
used 1/4 of the color information based on the fact that your eye only sees
about that much. The color information was encoded on a phase modulated
3.57mHz subcarrier, which at the time was beyond the picture information, but
still within the transmitted signal.

The original RCA system, alternated the phase of the carrier every line,
so that it would fix itself if there was a transmssion or syncrhonization
problem. To save money, the National Television Standards Commitee (NTSC)
which chose the standard, dropped the alternating phase.

When the BBC adopted their 625 line system to replace the 405, they used a
modification of the original RCA system with a 50 Hz field rate (25Hz frame
rate) which gave them 625 lines. Because there was more modulation, 3.57mHz
was still inside the picture, so they moved the color subcarrier up to 4.43
mHz. As an "in your face" they called the system PAL, Phase Alternating Line,
to differentiate it from the NTSC choice.

The French used a different color encoding system called SECAM, which was also
based on the RCA system (1/4 color, 4.43mHz color carrier) but designed
to be totally incompatible so that you could not watch French TV in England
and vice versa.

NTSC stands for National Television Standrds Comittee, PAL for Phase Alternating
Line, and SECAM is a French acronym for what could be loosely translated as
system of transmitting color TV.

Although the frame rates were different, and the color carriers at different
frequencies, the information was basicly the same, and pretty much encoded
the same way. So it was pretty easy, but expensive to build multisystem
TVs.

Except for the people in the channel Islands, or on the coasts of England
or France, there was no reception of signals anyway, so no one would buy
them anyway.

As the 1960's progressed and TV spread throught the world, variations of
NTSC, PAL and SECAM were adopted either because the standards fit the
former colonial powers that ran the countries or they did not fit the
country next door. So the UK used PAL, the French SECAM, Germany PAL (but
modified so that the tuners would not work with UK signals), East Germany
used SECAM (but modified to use the cheaper west German tuners) and so on.

So there were many ways of encoding the video, but it all came down to a
number between 0 and 1 for brightness and 1/4 color information.

In the early 1980's satellite TV became a problem. Multisystem TV sets
existed, once you put a signal up, there was no way to stop someone from
receiving it if they could see your signal. In the US, the requirment for
a Federal license for a satellite dish was dropped, and in many places there
never was one.

HBO was the leader of the movement to prevent people watching these signals
and pushed for a way of encrypting satellite video. What they did was to
embrace the original MPEG-1 video standard, which was then encrypted using
the US DES (Data Encryption Standard). DES was chosen because it was illegal
to export DES chips from the US, which made it illegal to export HBO
receivers.

The MPEG-1 standard was simply a digital compression based by taking the two
relevant bits of information, brightness and color and combining them and
using various mathematical compression algorythms. In the end though what
went in was very much the same INFORMATION in an analog TV signal because
that's what they had coming in and that's what they wanted coming out.

The MPEG-1 standard included various other things, such as the ability to
have more than one video program, more than one audio channel per program,
and several different digial audio compression choices from none to
what later became MP3 (shortend form of it's full name).

Over the years there have been improvements to the MPEG-1 standard, to become
the MPEG-2 (aka MP2) which is used in DVDs. DVD's for those that don't know
are MPEG-2 video streams represented in flat files, with some extra indexing
information.

In some places there was a short flirtation with encoding MPEG-1 signals
on CDs (video CDs). Commodore made a version of the Amiga called the PC-TV,
using the Philips system and I think there was a competing Sony one.

VCDs took off eventually because video tapes and players and later DVDs
were taxed over 200% in some countries, but computers with CD drives
were not. :-)

There are many compression techniques in use, but the ones used for TV
transmission still work very much the same way, with the light level and color
information being the same as it was in the RCA system.

The data transmitted is still almost universally MPEG transmission streams,
with different compression and encoding methods. Because some countries
still have TV sets that flash at 60 times a second and others at 50, the
frame rates of 25 and 30 have been kept, but are really meaningless. There
really are three rates in use, 24 (film), 25 (used for film and video) and
30 (video). TV set's just play them and whatever decoder box you use or disk
player just converts them to the national standard that is expected of them.

What is loosely called MPEG-4 standards have no frame rate per se, a frame
changes only when the information on the screen changes. So a live action
sporting event may have the full 25 or 30 frames per second, but a photo of
two people watching a sunset in silence may only have 10 or 12.

As for over the air, there are three currently used systems of digital
TV. It's up to the country to decide which standard is used in their country
and I'm sure politics matters. The most common is the DVB-T (digital video
broadcast terrestrial), which has been in use in the EU for a long time now.
It's relatively simple, cheap to produce and unencumbered by expensive
patents.

The US uses a system called ATSC (American Television Standards Committee),
which is different than the DVB-T, although it does basicly the same thing.
Compared to the DVB-T system, which is much older, it uses more sophistocated
chips, with more expensive patent licenses.

DVB-T and ATSC tuners are incompatible. My guess is that was done so that
US manufacturers could get a financial incentive for choosing that system,
in terms of licensing fees, instead of fighting cheap knock-offs from China.

There are companies that manufacture dual DVB-T/ATSC tuner chipsets, they
are targetd to laptops but will eventually find their way into pocket TVs
for travelers.

The third system which I mentioned is Japanese in origin and is incompatble
with the other two. I know nothing about it, except that a few south asian
countries have chosen it.

So if you are still reading, the answer is basicaly that while the INFORMATION
has not changed since the early 1950's, the way of encoding, compressing and
transmitting it has changed, but that does not make it inaccessable.

While you could buy a multisystem analog TV or VCR to cross borders as it were
you can still do so digitally. Since the videos transmitted are basicaly the
same (MPEG transport streams) world wide, it's just a matter of a tuner chip
if you go (signally) from country to country, and if you receive your signals
in another method (over the internet, from a recording, etc), then they are
pretty much the same.

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Those who cannot remember the past are condemned to misquote it.