Brushing Up on Upconversion Bruce Jacobs Twin Cities Public Television.

Brushing Up on Upconversion

Bruce JacobsTwin Cities Public Television

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At Stake

Millions of hours of SD contentNew consumer HD displaysAt best, 480i becomes 720pAt worst 360i becomes 1080p!

UC

720

360i

1920

1080p

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Brush Up

Upconverter Theory- Digitize- Deinterlace- Scale

Upconverter Practice- Software- Hardware

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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