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Nick Feamster. Development of a Transcoding Algorithm from MPEG to H.263. Motivation:. Transcode. MPEG - High bitrate - DVD, Digital Television - 1.5 Mbps, 30 fps. Development of a Transcoding Algorithm from MPEG to H.263. H.263 - Low bitrate - Wireless, etc. - 500 kbps, 10fps.

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Nick feamster

Nick Feamster

Development of a Transcoding Algorithm from MPEG to H.263


Development of a transcoding algorithm from mpeg to h 263

Motivation:

Transcode

MPEG

- High bitrate

- DVD, Digital

Television

- 1.5 Mbps, 30 fps

Development of a Transcoding Algorithm from MPEG to H.263

H.263

- Low bitrate

- Wireless, etc.

- 500 kbps, 10fps


Transcoding issues
Transcoding Issues

H.263

- I, P Frames

- Unrestricted Motion Vectors

on boundaries

- Fewer I macroblocks per frame

MPEG

- I, P, B Frames

- Restricted Motion Vectors

on boundaries

- More I macroblocks per frame

vs.

January 26, 1999


Computation vs quality
Computation vs. Quality

Goal: A fast transcoding algorithm which preserves

picture quality.

Method: Make use of available decoded information

to avoid unnecessary computation.

January 26, 1999


Mpeg vs h 263
MPEG vs. H.263

January 26, 1999


Mpeg vs h 2631
MPEG vs. H.263

Conclusion: 1.5 Mbps MPEG Source minimizes

loss due to recoding.

January 26, 1999


Initial method

H.263

Motion

Estimation

MPEG

Decode

Drop B Frames

H.263

Encode

Initial Method

• 9-10 Seconds per frame

• Too much computation for real time transcoding

January 26, 1999


Alternative reuse information
Alternative: Reuse Information

H.263

Motion

Estimation

MPEG

Decode

Drop B Frames

H.263

Encode

Motion Vectors

What is the best way to do this?

January 26, 1999


Issue mpeg i frames

B

I

I

B

B

B

P

P

B

B

MPEG:

P

P

I

P

H.263:

Issue: MPEG I Frames

Problem: Each MPEG GOP has an I frame with

no motion vectors.

Solution: Copy motion vectors from preceding

MPEG P frame.

MV’s from

P frame

Yes

Drop B frames

I frame?

MPEG

Decode

H.263

Encode

MV’s from

current frame

No

January 26, 1999


Issue small mv errors
Issue: Small MV Errors

Problem: Quantization losses result in

motion estimation discrepancies

Solution: Half pel search to refine

motion vectors

January 26, 1999


Issue mpeg intra macroblocks
Issue: MPEG Intra Macroblocks

  • Often more useful to code an H.263 macroblock as an intra block, even if the block is coded inter in MPEG

Solution: Use the minimum error threshold specified

by the H.263 encoder to correct modes.

January 26, 1999


Resulting method
Resulting Method

MV’s from

P frame

Yes

I frame?

MPEG

Decode

Drop B frames

Mode

Correct

Half Pel

Correct

H.263

Encode

No

MV’s from

current frame

January 26, 1999


Other considerations
Other Considerations

  • Boundary Error

    • 0.5 dB loss, worst case (sequence dependent)

    • Much of this loss can be recovered with mode correction and half pel search

    • At a lower spatial resolution, boundary motion vectors take on increasing importance.

January 26, 1999



Other considerations1
Other Considerations

  • MPEG I Frames

    • Tried weighted average of neighboring P frame MV’s based on overlap, not as effective as simple replacement.

    • Replacement works well on sequences with correlated motion vectors

    • High quality P frame from MPEG I frame ® Less degradation of subsequent P frame quality

January 26, 1999


Results
Results

Sequence Method 1 (sec) Method 2 (sec)

Carousel (30 IP) 323 26

Bus (30 IP) 254 26

Girl (18 IP) 106 15

Bus (10 IP) 80 9

Carousel (10 IP) 90 8

Football (10 IP) 92 8

~ 10x Efficiency Improvement

January 26, 1999


Results1
Results

Carousel

Bus

Girl

January 26, 1999


Conclusions
Conclusions

  • Reusing motion vector information from decoded MPEG stream saves 85-90% of computation time with an average of 1.4 dB loss.

  • Much of the loss can be attributed to motion vector errors resulting from requantization of the original frame pixel values.

Frame A’

MV(A)

MPEG

MPEG

H.263

H.263

MV(A’)

Method 1

Method 2

January 26, 1999


Future work
Future Work

  • Algorithmic research (motion vector search for I frames, etc.)

  • Real-time implementation

  • Allow MPEG interlace as transcoder input

January 26, 1999


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