I used to own a 25 inch CRT television. I’ve replaced it with a 20 inch flat panel LCD television.
Both used the standard 4:3 television aspect ratio. However on the new TV, it seems that I see more at the top and bottom of the screen.
On the old CRT set, graphics that were at the top of the screen, usually those fixed scoreboards that appear on sports broadcasts, would sometimes be cut off a bit on the top or bottom.
Now they never are.
On a related note, this set has the option where I can turn the picture upside down or make it a mirror image?
Why would I ever want to use these features? Aside from annoying people.
The TV standards were developed with the idea that some of the pixels off to the sides weren’t going to be visible. A little extra slop to ensure that you didn’t have borders of black stuff around the edges of the pictures. (Actually, some of that black stuff wouldn’t be black. Modern sets tend to blank most of it out.)
Keep in mind that having the image appear in the right position and all was a tricky business by 1940-50s tech standards. Remember people always adjusting the veritical hold? So some leeway was needed to take into account that not everybody’s set would be adjusted right, especially as they aged in those days of vaccuum tubes and paper capacitors.
As Time Marched On, displaying the image right has gotten a lot easier. But still, for CRTs, is a bit of a challenge to keep things right (without having the owner adjust things). So they are designed to exclude a few pixels around the edges to avoid showing the black part, again aging is a factor.
But for LCDs, the image will always be where it’s supposed to be (excluding something horrible going wrong), so no need to allow for that.
I like to fix up garage sale TVs, so I know a lot about the various adjustments inside the TV that can fix up a weird image. But usually, once the image is messed up, something is starting to fail so it is a temp. fix. And since there are Lethal Voltages inside, it’s not something for the average DIYer to try. But a lot goes into making a CRT image look right. I will never have to do those kinds of things on an LCD to adjust the image shape or position.
I got rid of the old CRT because at times the image would shrink down to a single line and then slowly expand to full screen. I felt that this was the sign of an impending death.
ftg’s explanation is essentially correct, although his use of the term “pixel,” which wasn’t coined until the late 20th century, is slightly anachronistic.
The technical term is “overscan,” and it isn’t just a “few pixels.” The “safe area” for broadcast television (the area in which critical information must be placed to ensure that it will be seen by all viewers) is about 10% of screen width in from the edge of the screen on all four sides. I don’t know if this is actually codified in SMPTE (or other) standards, or if it is just a conventional practice (Q.E.D. will be along in a minute and tell us), but pro studio cameras often had “safe area” lines drawn on the viewfinder monitors to aid the operators.
IIRC, another reason for the variation in viewing area was the differences in the cabinetry the various manufacturers of early TV sets, which masked different amounts of those big, heavy picture tubes.
Going above and beyond Safe Action Area’s 90% of the image is Safe Title Area. Safe Title is the inner 80% of the image, and it’s assumed that all functional TVs will be able to display anything in the Safe Title Area.
As for standards - not quite a standard, but a SMPTE Recommended Practice:
SMPTE RP 218: Specifications for Safe Action and Safe Title Areas for Television Systems
This practice describes a method for locating the safe action and safe title areas for television systems. This document is intended for application in program production where the image aspect ratio of the acquired essence is the same as that of the display.
If you really, really want to know more, SMPTE will sell you a copy of the whole thing for $26.
As for flipping, inverting or mirroring the image, think along the lines of hiding the TV somewhere in some sort of built-in or fancy cabinetry and watching it via mirror.
Ooh, forgot to address the inverting, etc. question.
I make up my own (S)VCDs. I’m careful, other’s aren’t. (It has to do with original codecs and conversion software.) Some people end up with discs that have the image upside down, etc. The manufacturer might be trying to reduce complaints like “My niece Dolly’s wedding video is upside down? What’s wrong with your machine? I want a refund…”
I’ve heard stories of people getting DVDs that display the image upside down, but I have no idea what’s going on there.
Another possibility about flipping images, etc: In some applications (many arcade games, for instance) you have the CRT sitting with its surface horizontal, and a 45 degree mirror above it. If, for some reason, you wanted to set your TV up in such a way, you’d need to flip the image on the screen to correct for it.
What is the advantage of hiding the image tube (or LCD) in a cabinet and viewing the image in a mirror? Was it a function of how heavy the damned things were and that it was simpler to engineer them so the tube was upright? I think this was common on the earliest (1920s-era) sets, but I’m not sure.
And even if it made sense with early CRTs, what kind of sense does it make with LCDs?
It saves the cabinet designer from having to move the display itself and dealing with the power and signal cables. Cables that don’t move don’t tend to come loose. Also, a piece of mirror is thinner and weighs less than any display, no matter how thin.
I believe the first commercially made image orthicon (picture tube) sets in the U.S. came out in 1938. A 1939 model. The television sets of the 1920s had mechanical scanning discs instead of picture tubes.
Image orthicon tubes were camera tubes. Picture tubes are all CRTs.
But Walloon is right that electronic picture tubes were not commercially available until well after the 1920s (although 1920s-style death rays were available then). There were essentially no consumer receivers for the early electro-mechanical TV systems, because none of the systems ever became well established enough to support any significant level of manufacturing. They were mostly hobbyist sets or custom jobs.
Widespread marketing of TVs to consumers didn’t begin in the U.S. until after WWII, and TV didn’t really take off until the late 1940s.
The pictures at Walloon’s link answer the question about why the tubes were placed vertically and a mirror was used. The necks were so long that if placed horizontally, the cabinets would have protruded more than 3 feet into the room. The upright orientation allowed for a much more compact case and a better viewing angle.