Why VM picture is cut on each side?

x70

Standard Member
Hi,

I have a SD box and i have noticed that on some channels (surely the basic ones like BBC, ITV, etc.) the images sent by VM are cut on each side; i would say roughly 10-20%, which IMO is a lot!
This is nothing to do with TV zoom settings (4:3, 16:9, e.g.).

Is it something to do with the settings of the box?
If not and that is the normal case, does that happen also with a V+ HD box, with SD and HD signals?

Thanx,
x70
 

AndyCob

Well-known Member
Overscan?

As a legacy of CRT most displays, even flat panels like LCD and plasma overscan the picture slightly (basicly zoom in slightly and cut the edges off the screen) unless told not to (irritatingly some don't let you turn this off and insist on overscanning even a native input thought that's rare thankfully). Of course the overscan on a plasma/LCD is done electronically rather than physical as with CRT or RPTV but it's there and is a seperate setting to the normal screen zoom modes like 4:3 etc. Turning this off can get you back your lost picture edges, though it can also expose various edge effects (flickering) and embeded signal elemets like identifiers (little while dots/bars along the top edge of the screen) that the broadcasters expect to go unseen due to overscan being used though this moslty a factor with SD and not HD.

If your dislpay has a setting for overscan try turning it off.
 

dante01

Distinguished Member
It isn't cut on either side, it is the native aspect ratio of what you are watching along with overscan:


It is usually the case that you have to also make adjustments to your TV's own screen settings in order to get the native aspect ratio being output by the STB.

Also note that some programmes are pillarboxed so that you still only get what appears to be 4:3 even when you specify widescreen. This intentionally prevent 4:3 content from being stretched and distorted. Pillarboxing is the display of an image within a wider image frame by adding lateral mattes (vertical bars at the sides); these mattes are part of the image.

Pillarboxed image:
325pxpillarboxed.jpg


Letterboxing is the practice of transferring film shot in a widescreen aspect ratio to standard-width video formats while preserving the film's original aspect ratio. The resulting videographic image has mattes (black bars) above and below it; these mattes are part of the image. The term refers to the shape of a letter box, a slot in a wall or door through which mail is delivered, being rectangular and wider than it is high.

Letterboxed image:
240pximagecropping235x1.jpg


If you force either a pillarboxed or letterboxed image to fill the TV screen, you either get overscan (not all the viewable image displayed) or distortion (the image is stretched laterally and/or vertically).


Additionally, VM output is subject to overscan. Overscan is extra image area around the four edges of a video image that is not normally seen by the viewer. It exists because the edges of the programmes being broadcast would display edge distortion and noise were the entire area displayed and older programmes would not display as originally intended because they were made during a period of time where overscan was the norm and only the central area was ever intended to be seen by the viewer. This is normal practice for TV broadcasts and not a mistake on the part of VM, the programmes or the individual channel broadcasters. When using a set that has the option to view content with native pixel mapping or 1:1 pixel mapping, this option shoud be switched off for TV broadcasts and normal settings should be used for the viewing area.


There are several contributing factors that determine what you see on the screen when dealing with the output of either a cable or satellite box. The aspect ratio can be affectged by one or all of these factors.

What has to be cleared up is the actually widescreen settings associated with the STBs. The widescreen setting has no influence on what the actually box is receiving from the network or the actual individual channels broadcasting on that network. The widescreen setting on these boxes is simply the box manipulating the image to stretch it into a 16:9 format aspect ratio. In the case of SKY, this is done by stretching the outside left and right edges of the image more than the central area. The Virgin boxes stretch the image consistently across the entirety of the image. These two methods appear to have given rise to the misconception that SKY's widescreen setting really does allow material otherwise broadcast and displayed as 4:3 to truly become 16:9. This is not the case and the image is still being distorted, but more noticeably towards its left and right edges and less so in the central region.

Native 4:3 aspect ratio on a 16:9 display:
native430n169.jpg

Native 4:3 aspect ratio stretched via SKY's widescreen setting on a 16:9 display:
skystretch.jpg

Native 4:3 aspect ratio stretched via VIRGIN's widescreen setting on a 16:9 display:
virginstretched.jpg


What is being broadcast?

Not all channels broadcast with a native 16:9 aspect ration and not all channels use 4:3. Some channels have a native aspect ratio of 16:9 widescreen while others still use 4:3. Each has to treat native 4:3 and 16:9 content differently.

A channel who's native aspect ratio is 4:3 has to letterbox native 16:9 material:
169letterboxed.jpg

On a 16:9 widescreen TV this would appear as follows:
letterbox43169.jpg

On a 16:9 widescreen TV with the Virgin STB set to Widescreen this would appear as:
letterboxstretch.jpg

Native 16:9 content broadcast via the 4:3 channels is not scaled proportionately by the Widescreen setting of the STB, it is simply stretched horizontally. The black bars to the top and bottom of the image are integral to the image being broadcast by the 4:3 channel and the STB has no way of differentiating the image area from them.

A channel who's native aspect ratio is 4:3 who broadcast 4:3 content to a 4:3 set looks like this:
43set.jpg

The same broadcast appears as follows on a widescreen set:
native430n169.jpg

and as follows if you use the Virgin box's widescreen setting:
virginstretched.jpg


Channels that broadcast with native 16:9 still have to deal with content that has an aspect ratio of 4:3. In the case of HD channels, 4:3 content has to be pillarboxed. Black bars are placed either side of the 4:3 image area to make the broadcast up to 16:9:
native430n169.jpg

Such broadcasts cannot be stretched by the STB's widescreen setting because the black bars are integral to the image being broadcast and the STB cannot therefore differentiate the image area from the total area including the pillarboxing bars.

Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube (CRT).

Although it appears as though you are losing some of the image, there os no requirement to rurn off overwerscan. The content was produced to take into consideration the overscan and you were never intended to see the lost parts of the image around its edges. You may even experience some degradation f the image around the edges if you turn overscan off and display the picture as its native pixel mapping. All YVs have overscan switched on by default and all TV broadcasters flag their content as being applicable for overscan.

There is no hard technical specification for overscan amounts for the low definition formatts. Some say 5%, some say 10%, and the figure can be doubled for title safe, which needs more margin compared to action safe. The overscan amounts are specified for the high definition formats. The European Union has safe area recommendations regarding Television Production for 16:9 Widescreen. The official BBC suggestions say 3.5% / 5% per side for overscan, but up to 17% has been known.

Overscan is not just a legacy of the CRT era, but something that is still apparent and taken into consideration by the TV content producers and the broadcasters. TV production houses and studios still film more than what is intended to be seen on the screen and broadcasters still use these unseen areas to stream data and other otherwise unsightly anomalies. Turn overscan off and you get to see these anomalies. Older material may also suffer more toward its edges after over zealous compression and the overscan helps hide the unsightly effects of this.
 
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x70

Standard Member
first of all many thanks for such a comprehensive feedback!

Maybe i should have given more info on my setup, as i'm still unlcear on what is the issue in my case:

- I do have an old CRT 16:9 TV connected to the VM SD.
- i do not have distorted images
- the VM box settings (TV Display format) is TV 4:3
- being a 16:9 TV screen, every 4:3 image is bound to show black bands on each side (unless i start changing aspect radio from my TV, eg zooming, in which case at some point the black bands disappear but i obviously lose parts of the picture) and that is fine with me
- however sometimes the image is clearly incomplete, for example: part of the channel logo is cut, film titles are cut when close to the edges, etc.
- the same program is not cut when i watch it on my laptop using usb freeview receiver instead
- i'm actually unsure on which channels show this issue, though before i mentioned BBC, ITV, etc. Today i have noticed on living tv and yesterday on film4

I am sure the explanation is already between the lines of your feedback, but i'm still unsure what is the issue in my case. Maybe you have more details now to point to the exact cause.

Thx again,
x70
 

tankstage

Active Member
If your TV is 16:9, why do you send it a 4:3 signal from the V box ?
The V box is cropping the 16:9 signal to fit your requested 4:3 output.
This will only affect channels that are transmitted in 16:9 and not channels that are only 4:3.
When you watch on your laptop is that shown in 16:9 format ?

If you want to see the full picture, but in 4:3 you will have to "letterbox" the output from the V box.
 
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bsuttie

Active Member
Yes, if you have a 16:9 screen you should set your box to 16:9 (Widescreen). At the moment you have it set to 4:3 centre cutout which is what you are describing.

regards

Brian
 

AndyCob

Well-known Member
If your CRT is 16:9 and you are connected by scart then you should have your VM box on 16:9 not 4:3, hopefully if your TV has an auto setting for it's image zoom you should then automatically get 16:9 as 16:9 and 4:3 as 4:3, does the VM box have a scart control setting like Sky? if so you need that on to.

Also as you have now said your TV is a CRT overscan is to be expected. The amount of which will almost certainly be the result of the setting of a couple of variable resistors somewhere on the circuit board for each input.

It's entirely possible your TV is poorly calibrated, and the picture wasn't centered and sized properly for the AV scart input (have you ever owned an analogue VGA PC monitor, remeber fiddling with the dials for picture vertical and horizontal position and size), the same settings need to be made on a CRT TV but they are usually made once at point of manufacture and never meant to be user adjusted and they frequently seem to differ between the internal tuner and say a scart input, yet another reason for overscan as that tends to absorb any differences.

It's the same issue that used to casue the classic I put my sky/vm box on and the picture moves an inch to the left/right etc, the calibration for the av input is off compared to that for the tuner. Unfortunately that means your stuffed unless you have a circuit diamgram and/or are at least a reasonable ammateur electrician to find and adjust them, it's not worth the cost of a professional to look at it and that assumes I'm right about the probelm:rolleyes:. I actually did this a few times with some of my past old CRT tvs, and once broke the resistor:facepalm:, right pain finding another and soldering it in. Generally only something to be done if you don't mind risking knackering the set.
 
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