View Full Version : TV 4:3 16:9 switch
11-17-2010, 11:40 PM
My tv takes composite input. It allows switching aspect ratio between 4:3 and 16:9. Selecting 16:9 results in wider letters and circles become ellipses. Does anyone know what the tv is doing? Is this something totally internal to the tv or could the tv driver be modified to get more lettes per line and more area for graphics?
11-17-2010, 11:56 PM
My TV has this capability. As far as I know, it simply adjusts the scan sweep to cover different regions of the display.
A Propeller could run a faster pixel clock when in "16:9" mode, and a slower one when in "4:3" mode. In a real sense, there is a change in detail per unit area of display going on. More pixels can be seen, though a composite signal may not be able to carry them to the display in a meaningful way.
S-video, or component video would be better suited for that.
So, if the TV is running 160 pixels in the 4:3 ratio display, and then was switched to 16:9, perhaps running 256 pixels would result in about the same size of pixel being seen, if one were to actually just measure the pixel on the screen.
11-18-2010, 12:03 AM
Would increasing the vertical expansion alleviate this at the expense of decreased vertical resolution?
11-18-2010, 04:05 AM
The signal carries the same data in either case. All that changes is the amount of screen area used to display that data.
Composite is low resolution, S-video is higher, component higher still.
Component signals are the best because they can just be driven to a higher resolution, because they have the bandwidth to carry it. A TV, with those signals, will be able to make good use of the aspect ratio change.
S-video to a degree does this, though it does have color resolution limits. Monochrome pixels can resolve to a high resolution however.
Composite signals can't even properly resolve on ordinary 4:3 screens. The 16:9 just stretches them some, and because of their resolution limits, driving faster pixel clocks really doesn't get one very much.
In the example I gave above, going from 160 to 256 pixels, the 256 pixels could be displayed either way, with it only being a matter of the actual pixel size changing with the aspect ratio change. On a 4:3 display, that's mostly a square pixel. At the 16:9, it's wider than it is high.
Double that resolution, and the 16:9 will display it over a larger screen area, which allows the viewer to see it with better clarity, however composite signals don't carry enough information for that to be practical. S-video connections are a improvement, component the best, able to easily display the increased number of pixels.
In any case, the amount of signal is the same, meaning there really isn't "extra" display space, just a difference in how it's presented to the viewer.
11-18-2010, 06:08 AM
The flat screen TV's use the horizontal and vertical sync signals as references to generate the clocks to convert the input signals into a screen image. That means the signal between horizontal syncs will be one line wide on the screen whether it is from a 16x9 source or a 4x3 source. Some sets do have a menu selection that allows you to choose if a signal is displayed as a 4x3 or 16x9 image.
11-18-2010, 04:58 PM
"Composite is low resolution, S-video is higher, component higher still"
This not stictly true in that the original resolution is the same but gets delivered with different quality results. Component has the least fudging to get to the screen and so looks the best, S-video tries to retain the full luminance detail and then paint on the reduced bandwidth chroma but the composite suffers the problems of trying to put together the luminance and reduced bandwidth chroma in a way that they do not interfere with each other too much (comb line filters, or more usually low pass filters on the luminance).
The aspect ratio is set by the capture device, if it starts out 4:3 then it will only look correct at 4:3. If it is stretched out wider then it will have to be stretched vertically too, and cropped off. The reverse is true for the 16:9 way around too
11-18-2010, 05:48 PM
Well, I was trying to address "extra screen space", with the fact that the signal is the same either way.
The "resolution" of the various signals is a function of their bandwidth, and the composite < S-video < Component is true enough.
When generating "pixels", there really isn't a capture device, which is why I phrased it the way I did. Some variation on the pixel clock can produce results on different aspect ratio displays, depending on the purpose and the signal used, in that whether or not more pixels equates to better use of the screen space, or more "resolution" seen, depends on the signal carrying that info to the display.
We are probably saying about the same thing, just approaching it from different angles.