Adjusting telly picture size - TV settings

amjl2000

Member
I read a while ago that ideally with en LCD telly the best picture will be gained if you have adjusted the size of the picture on the screen so that the number of pixels disaplayed perfectly equals the number received. The received signal is bigger than it needs to be (overscan - and as has always been) presumably to allow for carp tellys that aren't set up right. But where is the sweet spot in reality?

As an example, take the BBC News time, ticker and other on-screen gubbins you see usually during breakfast. Should there be picture below the bottom ticker strip, or should that demark where I should be adjusting my telly to? Same with the 'Live' label they put at the top.
 

Black Hole

May contain traces of nut
This is irrelevant for a digital signal. The TV will map the received picture (decoded from the compressed data stream) directly onto the pixel map for the screen*. I don't think I have ever seen a digital TV with picture width/height adjustments for the digital tuner, only the analogue tuner and analogue inputs.

For reference, on my cheapie kitchen TV (16:9, picture by HDMI from a HD-FOX) the Breakfast clock strip (and banner, when it comes up) is floating above the bottom of the displayed picture by about a third of the height of the strip.

If by some means you were able to adjust the picture width/height away from a 1:1 pixel mapping, it would look horrible (unless the TV has some very sophisticated resampling capability). The width of an analogue picture source is not so critical, because the analogue signal can be sampled at the desired rate, but as the picture is transmitted as lines, again the line sampling has to match the pixel grid (although if the picture is from a true analogue source rather than a digitally generated analogue source, the smooth variation of content line-by-line will mean that interpolation is quite successful).

* This post later proven incorrect. See post 10.
 
OP
A

amjl2000

Member
I'll double check, but I know I can adjust the X-Y of the picture, I believe I can also zoom in-out a couple of notches - hence the question. You'd expect a poor picture if the pixels weren't mapped properly, but then again, would you really notice with a freeview signal?
Sounds like my telly is about the same as yours... so there we go.
 

Black Hole

May contain traces of nut
Could it be that the LCD on your TV is not full res, so the TV is already resampling the picture?
 

Black Hole

May contain traces of nut
Further to this, my Qumi projector displaying a live broadcast via HDMI from a HD-FOX seems to show more of the scan than any of my other screens (the image is definitely resampled, because the Qumi produces the image using a Texas Instruments DLP chip with a diagonal pixel matrix - it does not match full 1920x1080 HiDef, but it's pretty damn good).

The Breakfast clock strip floats one full strip height above the bottom edge of the picture and to the right edge (left edge also when the strip is shown in full). But the curious thing is: although the studio image fills the frame, frequently other picture sources are not full width and/or full height even though the clock strip remains where it is and continues to display to the "real" edge of the frame.

Clearly if the final display overscans (puts the edges of the frame beyond the edge of the physical display), these little things will not be noticed - and it is probably intended that they are not noticed. I don't understand why video sources in a modern broadcast system are not all 16:9 and fill the frame regardless, and why with digital delivery and display overscan is needed.

Does this mean that a "1920x1080 pixel" TV screen is actually less than that so there is an ignored overscan area, or is the image being resampled to enlarge it slightly?? It does rather call into question my assertions in post 2.
 

Mike2

Forum Supporter
I read a while ago that ideally with en LCD telly the best picture will be gained if you have adjusted the size of the picture on the screen so that the number of pixels disaplayed perfectly equals the number received. The received signal is bigger than it needs to be (overscan - and as has always been) presumably to allow for carp tellys that aren't set up right. But where is the sweet spot in reality?

As an example, take the BBC News time, ticker and other on-screen gubbins you see usually during breakfast. Should there be picture below the bottom ticker strip, or should that demark where I should be adjusting my telly to? Same with the 'Live' label they put at the top.



I am convinced that BBC One SD and BBC One HD send a different view. Am I imagining this?
 

Black Hole

May contain traces of nut
Nope, using the Breakfast stripes as a reference and displayed on my Qumi setup (because I seem to be able to see the whole frame) I can see no difference between BBC ONE HD and BBC ONE.
 

Mike2

Forum Supporter
Nope, using the Breakfast stripes as a reference and displayed on my Qumi setup (because I seem to be able to see the whole frame) I can see no difference between BBC ONE HD and BBC ONE.

I believe you.

I just tried the same on my TV Freeview tuner. Conveniently there are signed programs on at the moment on BBC2, with the picture in a frame. The frame is set higher on BBC2 SD, so less of the picture is visible. That is what I recall on the Breakfast show on BBC1. I agree, the T2 shows no difference.

Just tried Foxsat HDR and there is no difference between the SD and HD broadcasts, both show more picture as per the TV's HD Freeview broadcast and the T2.

Now tried the TV's built in Freesat tuner. Same as the Freeview tuner. The bottom of the screen looks the same, but the picture is slightly stretched vertically in SD (or compressed in HD?) so that the top of the screen is missing in SD. Strange!

In all cases I have set screen size to 16:9.
 

Black Hole

May contain traces of nut
I've been having a play, and refuted some of my previous assertions. On my Samsung 32EH5000 (receiving 1080i by HDMI from the Humax), the screen resolution is (presumably) 1920x1080 and yet with the picture mode set to 16:9, the image overscans by about 24 pixels top and bottom and 39 pixels left and right (that's roughly 2% all round). If I switch to "screen fit", I see the whole thing, and supplying a digital test source (ie computer graphics) the resolution is much better than the standard 16:9 (there is no interpolation going on).

This means that the frame size shifts I see on the Qumi are normally hidden in the overscan areas on a TV with the default settings. I have no idea why the broadcast industry needs an overscan in this digital age.

Thus, in respect of the OP: if the TV in question has a 1:1 option, that will provide the best pixel mapping for HDMI or DTV display. Obviously a 1080 picture will display best on a 1080 display, and a 720 picture will display best on a 720 display, but 720 on 1080 only has to be up-scaled 2:3 so that is reasonably easy.
 

Black Hole

May contain traces of nut
More info: with my Samsung set to "Screen Fit", even the Breakfast stripes didn't make it all the way to the edge! The only full frame picture I found was when the travel news slide came up on the local bulletin.
 

Mike2

Forum Supporter
More info: with my Samsung set to "Screen Fit", even the Breakfast stripes didn't make it all the way to the edge! The only full frame picture I found was when the travel news slide came up on the local bulletin.

On my UE40ES8000, screen-fit isn't available with the inbuilt tuner. Also, playing a widescreen 2.39:1 film direct via DLNA causes it to be stretched to 16:9, ie, unwatchable. I don't have this problem with a WD TV Live and a Sony BD player used to play from the same DLNA source. The Fox T2, however, messes up in exactly the same way as the TV, ie, it stretches cinema widescreen vertically to 16:9 format.

Very annoying!
 

Black Hole

May contain traces of nut
Things Every... (click) section 16.

I will have to check out the native tuner. With the HDR-FOX as the tuner, a few spot checks have shown that BBC channels are all over the place re screen width, and C5 is full screen!
 

Black Hole

May contain traces of nut
LG 32LD690: "Just Scan" option disabled for tuner input.

Samsung 32EH5000: "Screen Fit" option disabled for tuner input.

Qumi (no tuner): I found a video overscan option which was turned off, turning it on expanded the displayed picture to the full width (and cropped the edges).

Conclusion: overscan is presumably a "standard" for the display of broadcast video, and expected. BBC1 Breakfast actually seems to work with that expectation, only (in the main) supplying a small amount of picture that will exceed the edge of the display. The overscan is presumably an artificial enlargement of the image by the receiver, so that only the central portion of the full 1920x1080 or 1280x720 frame is being mapped onto the TV's 1920x1080 display. This would not be acceptable on a computer monitor, and if you want broadcast HiDef in full HiDef - best source it from the Humax and make sure the TV is 1:1!

Further observations: C5 generally fills to the right, but I have observed a slight indent on the left on occasion. Breakfast also does not fill as far to the left as it does on the right on occasion. The West local bulletin provides full-width slides, but the studio content is slightly indented on the left.
 
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