Television

*** also see Conventional Analog Television - An Introduction  and  Worldwide TV Standards - A Web Guide

The word, Television, is from old French linguistics, where tele = far  and  visio = to see.  Television is the the transmission of visual images of moving and stationary objects, with accompanying sound.  Television is also the name of the device used to receive the signals and convert them to a sequence of images.  The standards were developed by many committees such as the ANSI, FCC, CCIR (Consultative Committee for International Radio) for D-1 digital component recording format (Ampex created D2).   NTSC (standardized color TV), ATSC (Digital TV), SMPTE (timing standards), etc.

Here we will cover the back-end of a TV, which is the part we look at - the screen, and the technologies used to place the images on the screen.  We will not cover the electronic circuitry inside the TV.

Introduction

Movies were developed long before Television.  The process of filming movies is called "cinematography".  Movies are displayed as a series of images (frames), which are placed onto a filmstrip at 24 cells, or frames per sec.  The projector in the theater actually breifly stops the film in a holding gate and flashes light through each cell 3 times, so the actual frame rate is 72 fps.

Television -  30 frames per sec (actually 29.97), 60 fields per sec, 525 horizontal scan lines (486 visible)

So far, even after 60 years since early television - most Televisions are still analog.  Basic monochrome (Black & White) TV was established in the 1930's.

Color was added in the early '50s. This is when American television picked up its name, NTSC. The initials stand for National Television Standards Committee. This is the committee that established the standards for our television system.  

1954 Admiral Color TV

The next standard will be HDTV (High Definition TV) - the sets are already being sold, and some programs are broadcast in this format today. 

To truly understand PC video you need to understand a bit about basic Television.  Just like PC video - TV video is nothing more than images - a series of images that are displayed rapidly, one after the other.  Your eyes and mind can only detect single images if they stay on the screen for a bit.  But with video, as soon as one image shows, another takes it's place, and your mind blends them together as one continuous moving picture.

TV and movies are what we all know about in terms of video.  Movies are specialized for the big screen, so we won''t discuss them.  Television is based on NTSC standards, called "Broadcast" standards.  First let's look at how a TV works.  We don't care about the electronics - but the way the screen shows the images is very important !!

 

When you look a a television screen, as stated - you are viewing a bunch of pictures.  But these are not flashed on the screen in the way a film projector does.  Instead, the images are "painted" on the screen, as a series of horizontal lines.  The lines go left-to-right, from top to bottom - just as in reading a book.

For simplicity, the example below shows a screen with only 8 scan lines, which is two fields - 4 lines each.  The image on the right shows the beam with "flyback" scanning, also called "vertical retrace".  The beam is shut off during retrace   :

                   

NOTE:  .  Actually, there are no "black bars" - these are only there to show you the pattern of scanning the lines across the faceplate of the CRT.  In actuality, the phosphor glows for a very short time after the beam passes across it - but the scanning is fast, so by the time the beam reaches that area again, it is still glowing slightly.  The previous scanned field is merely "painted over" again . . . updated continuously.  Also, the beam must "retrace" (travel back up to the top) after each field is scanned - this is invisible to you, since during the retrace, the beam is turned off.

 

Links

 

http://www.tvtechnology.com/ 

http://www.fcc.gov/mb/  FCC Media Bureau

http://www.smpte.org/  Society of Motion Picture and Television Engineers (but must pay for standards papers)

http://www.ee.washington.edu/conselec/CE/kuhn/ntsc/95x4.htm (but has wrong info on vertical retrace lines)

http://www.uwasa.fi/~f76998/video/conversion/ 

HDTV - http://www.ee.washington.edu/conselec/CE/kuhn/hdtv/95x5.htm 

Links to standards bodies  www.tvhandbook.com 

 

Misinformation

Be forewarned - the web, and people are full of misinformation about the technical aspects of Television !!  The info has proliferated as people create their own sites based on other sites.

This has been the most frustrating research, as the facts have been double and triple checked.  In some cases, no factual conclusion could be reached, because the sources quoted so many varying facts.  Hence the very unusual step of warning you about the great deal of bogus information out there !!!  

 

To come up with this section - the first step was to define television scanning and resolution.  It soon became apparent that the multitudes of websites, and newsgroup postings on this topic - are all across the board with their facts and numbers.  

 

Normally there is no need to consult newsgroups and personal websites - since the facts are accurately contained within the standards papers and on the Manufacturer websites.  However, there is a dearth of television standards on the web.  NTSC apparently has no website.  In addition, www.itu.org does not list a recommendation for analog TV standards and their digital television  BT.656-4 is very brief.  

 

Hundreds of hours have gone into these pages - wading through all the various numbers quoted for scan lines, retrace lines, resolution, etc.  In order to NOT re-proliferate misinformation, you will see notes made where several numbers have been found and no apparent standard exists.  For example, the number of lines cropped has been quoted by various sources as 0, 4, 5, and 6.   Instead of choosing one - all of them are listed.  In other cases where one or two sites posted one number, and 10 posted another - the vast majority rules.  In addition, a lot of information, fortunately - was available in an old standard book, "Basic Television", by Bernard Grob.