Aspect Ratio
and
Active Format Description
Gerry Field
American Public Television
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WGBH Public Television Quality Workshop
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Aspect Ratio

Television Aspect Ratio Defined:
Picture frame: Ratio of image width to image height.
Not directly tied to resolution


Historically, TV content and frame had been 4:3
Content is both 4:3 and 16:9.
 16:9, 4:3, 15:9, 14:9, 1.85:1, 2.39:1, mixed aspect ratio



TV frame is now 16:9 in > 50% households
Millions of 4:3 receivers remain in use
Channels are both 16:9 and 4:3
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It’s Simple
Source
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Program
Channel
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Viewer
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4:3 Image - 4:3 Frame
16:9 Image - 16:9 Frame
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4:3 Image - 16:9 Frame
16:9 Image - 4:3 Frame
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What Happens Downstream?
4:3 Channel
Letterboxed 16:9
Typical for Multicast or Cable
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16:9 Home Display
Adds Pillars
Results in “Breadbox”
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What Happens Downstream?
Center Cut Alternative
16:9 Frame
Center Cut 4:3
4:3 Channel
Center Cut 4:3
One Alternative
Typical for Multicast or Cable
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WGBH Public Television Quality Workshop
16:9 Home Display
Adds Pillars
No “Breadbox”
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However…
It Doesn’t Work For All Content
16:9 Frame
Center Cut 4:3
4:3 Channel
Center Cut 4:3
16:9 Home Display
Center Cut 4:3
NOT Center Cut Safe
NOT Center Cut Safe
NOT Center Cut Safe
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Worst Case
This Never Happens, Right?…
Letterboxed to 4:3
“Postage Stamp”
To a 16:9 Home Display
4:3 Frame
Letterboxed Content
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Upconverted to 16:9
Typical Primary Channel
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Downconverted for 4:3
Using Letterbox
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Anamorphic
Only Use as a Distribution Technique
Not as a Broadcast Format
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Most common is SD widescreen on D-Beta
Must be properly “stretched” prior to emission
If not, viewer sees “squashed” picture
Common practice is to upconvert to HD at a bitrate cost
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Speaking of SD & Widescreen
16:9 is not limited to HD
16:9 SD is included in ATSC A/53 standard.
and is regularly used in domestic and overseas markets
16:9 SD can be distributed if MPEG2 encoder flag is set to 16:9.
SD 16:9 can co-exist with SD 4:3
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Distributing SD Widescreen
Current workflow would need to be modified
Testing and vendor support required
Business requirements need to match
16:9 HD or 16:9 SD
Sencore IRD
MRD3187
4:3 SD Center Cut
Source SD 16:9
File or Anamorphic Tape
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4:3 SD, 16:9 Letterbox
Active Format Description AFD
A four bit code to define the active and protected image area.
Intended to guide professional video equipment, consumer
receivers and set-top boxes to optimize the displayed image.
Very few ATSC receivers and consumer decoders act on AFD.
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Active Format Description AFD
May also be used for format conversion signaling in the
post and pre-broadcast chain.
References:
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- SMPTE ST 2016-1:2009
- PBS TOS-1, 4.4
- PBS TOS-21, 4.3.7
- PBS Connect > Engineering/Compute >
Technical Reference Library
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AFD Codes

SMPTE ST 2016-1:2009 defines 17 AFD Codes.

Four are in common use:
1000 – Full Frame
 1001 – Full 4:3 / Pillarbox
 1010 – Protected 16:9
 1111 – Center Cuts

(14:9, 15:9, 2.35 etc. treated as 16:9)
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AFD = ‘1000’ (Full Frame)
In a 4:3 coded frame
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In a 16:9 coded frame
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AFD = ‘1001’ (Full 4:3 / Pillarbox)
In a 4:3 coded frame
In a 16:9 coded frame
In a 4:3 coded frame, AFD code ‘1001’
represents the same image display as
AFD code ‘1000’.
AFD code ‘1000’ is the preferred coding for a
full frame 4:3 image
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AFD = ‘1010’ (Protected 16:9)
In a 4:3 coded frame
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In a 16:9 coded frame
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AFD = ‘1111’ (Center Cuts)
In a 4:3 coded frame
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In a 16:9 coded frame
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Consumer Issues
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Consumer Issues
o
Broadcasters cannot rely on consumers to
maintain aspect.
o
AFD is not widely implemented on consumer
devices.
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So What do we do?
Educate.
 Implement AFD.
 Discuss with Cable and Satellite providers to
Implement AFD on their SD Down convert.
 Produce when appropriate 16X9 Center cut safe
material.
 If necessary up convert Letterbox to 16X9 Full
frame.
 Consider SD Widescreen distribution & channels

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Aspect Ratio
and
Active Format Description
Gerry Field
American Public Television
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WGBH Public Television Quality Workshop
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