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Brushing Up on Upconversion
Bruce JacobsTwin Cities Public Television
2
At Stake
Millions of hours of SD contentNew consumer HD displaysAt best, 480i becomes 720pAt worst 360i becomes 1080p!
UC
720
360i
1920
1080p
3
Brush Up
Upconverter Theory- Digitize- Deinterlace- Scale
Upconverter Practice- Software- Hardware
4
ReferencesLarry Thorpe: "The HDTV Camcorder and the March to
Marketplace Reality" SMPTE Journal, March 1998
Jed Deame: "Motion Compensated De-Interlacing: The Key to the Digital Video Transition"Presented at the SMPTE 141st Technical Conference, November 19-22, 1999
Charles Poynton: "Digital Video and HDTV: Algorithms and Interfaces"Morgan Kaufmann, 2003
Digitize & Scale
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Spatial Frequency
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Spatial Frequency
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Spatial Frequency
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Spatial Frequency
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Tip #1
To successfully reconstruct a digitized signal, we must sample at more than twice the highest input frequency – the "Nyquist Rate".
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Four Sample Examples
At Nyquist Ratef(s) = 2 * f(max)
Below Nyquist Ratef(s) = 1.3 * f(max)
Double Nyquist Ratef(s) = 4 * f(max)
Just Above Nyquist Ratef(s) = 2.3 * f(max)(Rec 601 = 13.5/5.75=2.3)
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At Nyquist Rate
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Pan Right ⅛λ
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Pan Right ¼λ
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Tip #2
In a camera, direct pixel mapping isn't good enough, despite the "PC" experience.
- See Tip #1
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Below Nyquist Rate
Alias = 1/3 f !
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Pan Right ⅛λ
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Pan Right ¼λ
Alias Moved ¾λ Right!
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Tip #3
Aliasing is bad enough in audio where it is "stationary". In video, aliasing also moves, the wrong distance, often in the wrong direction, resulting in obvious and annoying artifacts.
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Pre-Filter
f(s)Aliasing
Lost Resolution
½ f(s)
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Tip #4
Avoid aliasing by using a steep LP filter before sampling.
- Steep filters are impossible with optics- Camera designers say some aliasing is better than degraded resolution
Avoid resolution loss by sampling at far more than double highest desired frequency.
- Requires extra pixels and higher bit rate
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Oversampling
f(s1)
½ f(s1) ½ f(s2)
Pre-Filter Post-Filter & Sub-Sample
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Tip #5
Use Oversampling 1. Increase sample frequency2. Increase pre-filter pass-band3. Add steep digital LP post-filter4. Subsample to original rate
- Legacy technique in SD CCD cameras- Improves resolution- Reduces aliasing- No increase in bit rate- Does increase camera noise
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Double Nyquist Rate
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Pan Right ⅛λ
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Pan Right ¼λ
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Just Above Nyquist Rate
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Pan Right ⅛λ
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Pan Right ¼λ
Alias Moved ≈2λ Right!
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Tip #6
Even with adequate sampling, mapping the output directly to perfect digital display does not produce perfect results!
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Reconstruction Filter
f(s)Aliasing
Lost Resolution
½ f(s)
Pre-Filter
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Tip #7
To reconstruct the signal, a low pass filter is needed, to remove the sample frequency image.
- Too steep results in ringing- An analog filter for analog output- Can be digital, if a higher frequency is used!
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Reconstruction LPF
Filter Magic
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Tip #8
Using "reconstruction filters" in both dimensions will upconvert the image!
- Doing this well (using many "taps") requires perhaps thousands of multiplications for every pixel, in every color channel, in every frame, all in real time!
Deinterlace
The interlace problem:- Simply using fields might leave only 180 V lines!
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Deinterlace
1.1
60 fields per second from source
even
odd
1.2
2.1
2.2
3.1
3.2
4.1
4.2
5.1
5.2
? ? ? ? ? ? ? ? ??
60 Frames per second needed for upconversion
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3/2 Pulldown
1
24 Fps
1.1
60 fps "3""3" "2""2"
even
odd
1.2
1.1
2.2
2.1
3.2
3.1
3.2
4.1
4.2
32 4
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Tip #9
If the 60i content came from 24p, weave matching fields back into frames, and ignore the duplicative fields.
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Pulldown Removal
1
24 Fps
1.1
60 fps
even
odd
1.2
1.1
2.2
2.1
3.2
3.1
3.2
4.1
4.2
32 4
1 1 2 2 3 3 3 4 41
60 Fps
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Tip #10
If the two fields are well correlated when there is no motion, weave them back into a frame and repeat it.
=
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Tip #11
If the two fields are completely uncorrelated, bob between fields with half the vertical resolution for each.
≠
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Tip #12
Adapt by using weave in regions of correlation, and bob between fields where not correlated.
≠
=
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Tip #13
Search for motion vectors, and compensate where possible.
Practice
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DemonstrationSoftware
- Final Cut Pro- Apple Compressor
Embedded Hardware- AJA Kona- Panasonic AJ-HD1400 DVCProHD VTR- Sony HDW-M2000 HDCam VTR
Stand-Alone Hardware- Leitch X75- Miranda XVP-811i- Snell and Wilcox Quasar PH.C- Teranex Xantus