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Aspect Ratio

Discussion in 'HDR-FOX T2 Freeview Recorder' started by Black Hole, Aug 19, 2012.

  1. Black Hole

    Black Hole May contain traces of nut

    An observation for anybody struggling with 4:3 images stretched to fill 16:9.

    The "WIDE" button on the HD/HDR-FOX handset cycles between the following modes (this presumes a 16:9 TV, and Menu >> Settings >> Preferences >> Video >> Screen Ratio is set to 16:9):

    Pillarbox displays 4:3 unstretched, with black bars down the sides.

    Zoom fills the width of the screen by enlarging the image, but loses a strip top and bottom off the edges of the screen (circles remain circular).

    Auto expands the image horizontally so that the whole screen is filled without any loss, but circles are no longer circular (everybody gets fat).

    Personally I use Pillarbox; when the source material is tagged as 16:9 the setting is ignored and the image displays normally, but when it is tagged as 4:3 it is displayed with black side bars automatically and circles remain circles.

    The problem is that when 4:3 material is being broadcast, adverts are 16:9 and the aspect ratio switches (and as an aside might be responsible for some of the HDMI glitches some people experience). Sometimes, very occasionally, the programme resumes but the aspect tags don't switch back - and then the original 4:3 source gets displayed stretched (like Auto). In this case it's the broadcaster's fault*.

    When the Humax video preference is set for a 4:3 display, there are a different set of options on the Wide button to suit the variations for fitting a 16:9 or 14:9 source onto the screen. I won't bother going into detail about those because they are not applicable to most people and will become decreasingly so with fewer and fewer 4:3 sets around. However, if you have a 16:9 display and are trying to show a movie file with a 2.35:1 aspect ratio, you can achieve it almost exactly by setting 4:3 in the video preferences and selecting "Letterbox 16:9" on the "WIDE" button.

    Of course, all this presumes you do not also have the TV settings playing with the aspect ratio!

    * Footnote: I'm not sure about this. I had a 4:3 programme get stuck on 16:9 after the ads, but then it magically resumed 4:3 when I switched into iPlayer and back.
  2. Black Hole

    Black Hole May contain traces of nut

    I conducted a set of experiments (see HERE - click) and have concluded that the broadcast 16:9 "standard" is to presume a 4% overscan at the display (in other words approx 2% of the picture is off the edge of the display all round, so the source 1280x720 or 1920x1080 picture is actually enlarged by the receiver). The TVs I experimented with all showed this characteristic when displaying from their internal tuners with aspect ratio set to 16:9, with no option to cancel it.

    This seems very peculiar to me. Analogue TVs (with CRT displays) used overscan to hide any irregularity in the electron beam raster generated by analogue methods and/or imperfections in the broadcast image, also some of the 625 lines of PAL broadcast were used for syncing and later for Teletext data, so they had to be kept off-screen. The broadcasters knew only the central 90% (or whatever) of the image was going to be visible to the viewer so anything important was kept away from the edges, and the video around the edges was supplied as a "guard" region - may be visible, may not. The overscan was trimmed at the receiver by some knobs either on the back panel or only accessible inside the case.

    There is no need of this in digital TV. HiDef broadcasts are 1280x720 or 1920x1080 pixels (16:9), and the TV receiver would be perfectly capable of displaying the full 1280x720 (or 1920x1080) right up to the edge of the screen without any guard area (it is taking the broadcast in the form of digital data which describes how to reconstruct the image on precisely that pixel matrix). And yet, observations of the full broadcast image show a degree of variability around the edges which strongly suggests the broadcasters expect to have a guard area to play with.

    The problem with this is that, having decoded a picture to (say) 1920x1080, the TV (presuming it has a full-HD display) has to apply a transformation to enlarge the image to 1998x1128 and then display the central 1920x1080 portion of it. That means the final display is less HiDef than when it left the production suite. The only way this makes sense to me is if the broadcast standard is actually 1998x1128 (or the equivalent for StDef).

    Of the TVs I played with, it was only possible to select a screen ratio of "1:1" or "Just Fit" or "Screen Fit" for external inputs, ie HDMI. When using this with the HD/HDR-FOX as the tuner, the edge effects become visible (particularly on something like BBC Breakfast where there are plenty of different video sources and on-screen graphics). However, this (it seems to me) is the only way to display a full HiDef broadcast unblemished by unnecessary interpolation. Setting the TV's aspect ratio to "16:9" returns to overscan, even for HDMI input.

    This is important for video sourced from a PC - the graphics are rendered to the individual pixel, and fine detail (small text for example) will not tolerate being interpolated onto a different pixel matrix (the desktop will appear blurred). If using a TV as a computer monitor (and let's face it: a small TV is bigger and cheaper than a dedicated monitor these days) it will be vital to ensure the PC's graphics are set to the same resolution as the TV, and the TV is set to 1:1 pixel mapping.

    The reported 720x576 for StDef isn't even 16:9 (1024x576 would be 16:9), so that presumably means the pixels are not square in the first place (a transmitted flag tells the receiver whether the data constructs a 16:9 image or a 4:3 one) - but it hardly matters for StDef!

    Updated in the light of subsequent posts
    Last edited: Jun 14, 2017
  3. prpr

    prpr Well-Known Member

    No it isn't. I don't know where you invented that from.
    720 samples x 576 lines in this country. Some crappy channels transmit 544 instead of 720 I believe.
  4. Black Hole

    Black Hole May contain traces of nut

    Beg your pardon, you're right, thanks (but a simple correction without the abuse would have been sufficient). 1280x720 is another HiDef resolution (as you would have realised if you thought about your posts as long as I think about mine).

    Regardless, it does not alter the thesis. Post 2 updated.
  5. Nontech

    Nontech Member

    Black Hole, Thank you for a very informative study.
    But just to get my thinking right, if I want to view a film shot in 4:3 do I:-
    Go to Menu/Settings/Pref/Vidio/Set Ratio as 4:3
    Then select "Wide" button on remote & Select "Pillar Box" ( forgetting that adverts, which I skip, will be shown in 16:9)
    Thanks in advance for reply.
  6. Brian

    Brian Administrator Staff Member

    I find the best settings to be

    MENU > Settings > Preferences > Video > Screen Ratio > 16:9
    MENU > Settings > Preferences > Video > Display Format > Pillarbox

    If you are watching a 4:3 programme, you will get the programme shown correctly with masks each side of the 4:3 frame, there is no need to set the ratio to 4:3.
  7. Black Hole

    Black Hole May contain traces of nut

    This setting tells the Humax what the aspect ratio of the TV screen is. A 4:3 source displayed on a 16:9 screen but with the Humax told it is outputting to 4:3 will be stretched horizontally to fill the 16:9. There is no pillar box setting at 4:3, only letterbox.