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Aspect ratios in recorded video and SageTV

This page talks about aspect ratios in respect of SageTV and NTSC SDTV.
For PAL SDTV, wherever a height of 480 is mentioned, assume it is 576!

Here is a brief description of various aspect ratios, with example images

4x3

16x9 letterboxed -- black bars are added to fill the 4x3 area

16x9 anamorphic -- the 16x9 image is enlarged vertically to fill the 4x3 area.

and should be stretched on playback to generate a true 16x9 image

What do TV's Do

Most OTA STDV is 4x3 or 4x3 letterboxed.

Digital SDTV (satellite or cable) and DVDs can sometimes be 16x9 anamorphic, This is indicated by a flag in the MPEG stream so that the player/STB can decide what to do with it.

If you tell your DVD player or STB that you have a 4x3 TV, then it will display the 16x9 video letterboxed with black bars

If you tell your DVD player or STB that you have a 16x9 TV, then it will send the true 16x9 anamorphic signal, indicating to the TV that it should be stretched to a 16x9 aspectratio. (This indication can be done using the Wide Screen Signalling signal in the video, or in Europe, setting a specific voltage on a pin in the SCART/PERITEL video connector).

Aspect Ratios in Recorded Files

Sage can record TV from a variety of sources.

OTA analogue TV, captured by an encoder card
Analogue output of a Digital TV's STB, captured and re-encoded by an encoder card
Digital TV MPEG stream directly from an digital TV tuner card.

In general the resolution of the captured files will be 720x480. This does not correspond to any real aspect ratio, but the Aspect Ratio flags in the MPEG stream indicate the playback aspect ratio to use.

The captured MPEG files from encoder cards will always set the 4x3 flag (even if they are 16x9 anamorphic) because most capture cards are not aware of WSS.

However The saved MPEG files from Digital TV tuner cards will have flags indicating the correct aspect ratio for the show.

In addition it can play back other media

DVDs
'square pixel' AVI videos
'non-square-pixel' AVI videos

DVDs, like Digital TV MPEG files will have flags indicating the correct aspect ratio for the show.

AVI's can also have these flags, but they are ignored by most players, so most AVI's have 'square pixels' -- ie the pixels match the aspect ratio: 640x480 or 848x480. Sometimes AVI's are created directly from the captured source/DVD, without resizing, in which case they will have non-square 720x480 pixels and will need to be resized to ythe correct aspect ratio on playback. This can only be done manually.

In summary, the video files we can have are:

4x3 labelled as 4x3 (correct)
16x9 letterboxed labelled as 4x3 (correct)
16x9 anamorphic labelled as 16x9 anamorphic (correct)
16x9 square pixel AVI (correct)
4x3 square pixed AVI (correct)
16x9 anamorphic labelled as 4x3 (INCORRECT)
16x9 non-square pixel AVI (INCORRECT)
4x3 non-square pixel AVI (INCORRECT)

Aspect ratios on Playback

Sage, or rather Windows, assumes that your display device has square pixels -- eg 640x480 for a 4x3 display device, or 848x480 for a 16x9 display device.

The decoder should[*] indicate on playback what aspect ratio to use for the file being played.

Therefore, if the file has the correctly labled AR, (or 'square pixels' in AVI files), then it will be displayed correctly in sage if the Sage aspect ratio is set to Source

4x3 shows with left/right black bars
anamorphic 16x9 shows filling screen

The exception is 16x9 letterboxed AVIs -- as Sage cannot 'see' the top/bottom black bars, it will display it as a 4x3 image, with both top/bottom and left/right black bars (using less than half of the display area).
To compensate for this, the 16x9 aspect ratio should be used to zoom into the letterboxed content, making it fill the screen. This can be done by setting a 133% vertical zoom.

For incorrectly labelled files, other custom aspect ratios must be used:

16x9 anamorphic -- should fill the screen so use 'Fill' AR (set to 100%x100%)
4x3 non-square pixel AVI' -- use 4x3 AR (set to 100%x100%)

[*] some MPEG2 decoders do not correctly do this, and so cannot autoswitch between 16x9 to 4x3 Aspect Ratios in DVDs when using 'Source' aspect ratio in Sage... this is a bug in the decoder

Non-square display pixels

If your Windows desktop resolution aspect ratio does not match the display device's aspect ratio, then the pixels are not-square (which is sometimes the case for special video modes). As windows will be trying to play things back at the correct aspect ratio you need to adjust the aspect ratio mode zoom % to compensate... In these scenarios Source aspect ratio does not work at all well, and it is best to use custom aspect ratios.

For example:

720x480 display resolution on a 4x3 display

When playing back a 4x3 video, in Source AR mode, Windows will use a 640x480 display area giving left/right black bars, when it should fill the screen. To compensate, use FILL, or adjust the horizontal zoom by 720/640*100=113% to fill the 4x3 area.

When playing back 16x9 video Windows will use a 720x405 display area, giving an image that is too high. (the height should be 360). To compensate, use 16x9 with a vertical zoom of 640/720*100=88%

720x480 display resolution on a 16x9 display

When playing 4x3 video, Windows will use a 640x480 display area giving an image that is too wide (the correct width should be 540). To compensate, use 4x3 with a horizontal zoom of 640/848*100=84%

When playing back 16x9 video Windows will use a 720x405 display area, giving top/bottom black bars when it should fill the screen, use FILL to correct

When playing back 16x9 anamorphic, windows will try to use 640x480, but you need to zoom to the full width and height, so use a horizontal zoom of 760/640*100=113% and a vertical zoom of 133%

There are 2 'Hidden' screens in Sage to help set Aspect Ratios -- These are unsupported. Use the Dynamic menus, or Studio to access them:

ARAdjustOffset -- to adjust the position of the current AR.
ARAdjustZoom -- to adjust the zoom of the current AR

Both screens allow the Apect Ratio settings to be modified by using the arrow keys -- like the overscan settings -- with a video playing in the background.

Example configuration

16x9 TV
Windows desktop resolution: 1280x720 (16x9 aspect)
Windows desktop fills 16x9 TV -- therefore square pixels

Source aspect ratio configured to 100%x100%
-- used for all correctly AR-labelled files and DVDs
4x3 aspect ratio configured to 100%x100%
-- used for non-square pixel/incorrectly labelled 4x3 videos
16x9 aspect ratio configured to 100%x133%
-- used for 16x9 letterboxed shows -- expands the letterboxed content to fill screen
Fill aspect ratio configured to 100%x100%
-- used for non-square pixel/incorrectly labelled 16x9 videos -- expands to fill screen.