Crystal Vision
Aspect Ratio Correction
5 Aspect Ratio Correction
The Q-Down 3G range includes the ability to deal with any HD to SD aspect ratio
conversion requirements. You can select a 16:9 Anamorphic output for 16:9 SD
systems and either a 16:9 to 4:3 Letterbox, 16:9 to 4:3 Letterbox compromise, or 16:9
to 4:3 Full Screen with centre cut for 4:3 SD systems. The Q-Down 3G range will also
allow you to customise the shape of your output picture. Each of the standard aspect
ratios can be individually adjusted away from the default values to create customised
versions by using four independent sets of size, position and crop controls.
The Q-Down 3G range ensures the picture is always the right shape by coping with
both analogue and digital SD blanking widths – particularly useful for feeds that are
up and down converted multiple times. When used with an Anamorphic conversion,
the ‘SD Analogue Line Length’ option puts the active 1920 pixels from a 1080i or
1080p picture into 702 pixels of SD (rather than 720 pixels), with the rest of the pixels
used to represent analogue blanking – which prevents the SD signal from losing the
sides of the picture. All other aspect ratios are adjusted by a similar amount.
The Q-Down 3G range can automatically choose the appropriate aspect ratio
according to the SMPTE 2016 AFD data embedded in the 3Gb/s or HD input video.
HD programmes are often made of a mixture of true High Definition sources and SD-
originated sources that have been up converted, and the SMPTE 2016 AFD code in
the signal gives information about which areas of the screen contain a picture and
which areas have black ‘padding’. The down converter needs to read this signal and
select the correct aspect ratio conversion to be used with the down conversion.
The Q-Down 3G range offers three options for the SD output: In
Auto 16:9 mode
the
SD outputs can be set to be always 16:9 and in this case it will perform the down
conversion without ever changing the aspect ratio; In
Auto 4:3 mode
the SD outputs
can be set to be always 4:3, in which case it will down convert and perform either a
Letterbox or centre cut, depending on the source material; In
Auto Adaptive mode,
if
the HD input is 16:9 full frame the picture is down converted with no aspect ratio
conversion and is output as 16:9 SD. If the SMPTE 2016 data indicates that the HD
input is a 16:9 Pillarbox, however, a centre cut is performed and the output is 4:3 SD.
Although it would normally be a requirement for an automatic system to label all the
3Gb/s or HD video images to show their format and control the down converter, with
the Q-Down 3G range it is only necessary to label those that are not full frame 16:9
images.
When the input is not labelled the down converter uses its default assumption that an
HD image is a full frame 16:9. It is therefore possible to have a powerful system with
the majority of signals not containing SMPTE 2016 information.
Fixed Aspect Ratios
There are four fixed aspect ratios which are - Anamorphic, 16:9 Letterbox, 14:9
Letterbox and Centre Cut. Anamorphic will map the input picture directly to the native
aspect ratio of the viewing display along with the resulting distortion associated with
this. Should it be necessary to view the output on a 4:3 display, setting the aspect
ratio selection to Letterbox will give the correct picture dimensions by adding black
bars to the top and bottom of the picture. Selecting 14:9 Letterbox will crop the picture
to 87.5%, and depending on the aspect ratio of the display will either add black bars
Q-Down-AG 3G/ATG 3G User Manual R2.1
14
11 July 2019
