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Old 8 Aug 2003, 12:11 AM   #1
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Question compressed domain transcoding

Hi there,

Reading some posts about "capturing with high quality and recompress" or directly capturing at the "right" Bitrate, I was wondering what the difference was between a "Reencoding" and a "Compression" (like DVD2one). I heard about "compressed domain transcoding" ? What is this ? has anyone informations ?

Thanks
Greg
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Old 8 Aug 2003, 09:11 AM   #2
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Transcoding or more specifically Compressed-domain Transcoding means normally a re-encoding process that changes the video or audio features, such as resolution or bitrate, by changing parts of the a/v content, but not by reconstructing the content again (which is the case in encoding process). Compressed-domain transcoding also maintains the format of the file same as in the original file.
Here is some info you may find useful:

Video compression algorithms are being used to compress digital video for a wide variety of applications, including video delivery over the internet, advanced television broadcasting, as well as video storage and editing. The performance of modern compression algorithms such as MPEG is quite impressive -- raw video data rates often can be reduced by factors of 15-80 without considerable loss in reconstructed video quality. However, the use of these compression algorithms often make other processing tasks quite difficult. For example, many operations once considered simple, such as splicing and downscaling, are much more complicated when applied to compressed video streams.
The goal of transcoding is to process one standards-compliant video stream into another standards-compliant video stream that has properties better suited for a particular application. This is useful for a number of applications. For example, a video server transmitting video over the internet may be restricted by stringent bandwidth requirements. In this scenario, a high-quality compressed bitstream may need to be transcoded to a lower-rate compressed bitstream prior to transmission; this can be achieved by lowering the spatial or temporal resolution of the video or by requantizing the MPEG data. Another application may require MPEG video streams to be transcoded into streams that facilitate video editing functionalities such as splicing or fast-forward and reverse play; this may involve removing the temporal dependencies in the coded data stream. Finally, in a video communication system, the transmitted video stream may be subject to harsh channel conditions resulting in data loss; in this instance it may be useful to create a standards-compliant video stream that is more robust to channel errors.

MPEG codes video in a hierarchy of units called sequences, groups of pictures (GOPs), pictures, slices, and macroblocks. 16 by 16 blocks of pixels in the original video frames are coded as a macroblock. The macroblocks are scanned in a left-to-right, top-to-bottom fashion, and series of these macroblocks form a slice. All the slices in a frame comprise a picture, contiguous pictures form a GOP, and all the GOPs form the entire sequence. The MPEG syntax allows a GOP to contain any number of frames, but typical sizes range from 9 to 15 frames. Each GOP refreshes the temporal prediction by coding the first frame in intraframe mode. The remaining frames in the GOP can be coded with intraframe or interframe (predictive) coding techniques.

The MPEG algorithm allows each frame to be coded in one of three modes: intraframe (I), forward prediction (P), and bidirectional prediction (B). A typical IPB pattern in display order is:


I0 B1 B2 P3 B4 B5 P6 B7 B8 P9 B10 B11 I0 B1 B2 P3 B4 B5 P6 B7 B8 P9 B10 B11 I0

I frames are coded independently of other frames. P frames depend on a prediction based on the preceding I or P frame. B frames depend on a prediction based on the preceding and following I or P frames. Notice that each B frame depends on data from a future frame, i.e.~a future frame must be (de)coded prior to (de)coding the current B frame. For this reason, the coding order is distinguished from the display order. The coding order for the sequence shown above is:

I0 P3 B1 B2 P6 B4 B5 P9 B7 B8 I0 B10 B11 P3 B1 B2 P6 B4 B5 P9 B7 B8 I0 B10 B11

MPEG requires the coded video data to be placed in the data stream in coding order.

The goal of MPEG transcoding is to process one MPEG-compliant video stream into another MPEG-compliant video stream that has properties better suited for a particular application. Transcoding differs from the encoding and decoding processes in that both the input and output of the transcoder are MPEG video streams. A naive solution to the transcoding problem, shown in the top of Figure 1, involves the following steps: first, the MPEG-coded video stream is completely decompressed into its pixel-domain representation; this pixel-domain video is then processed with the appropriate operation; and finally the processed video is recompressed into a new MPEG video stream. Such solutions are computationally expensive and have large memory requirements. In addition, the quality of the coded video can deteriorate with each recoding cycle.
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Old 8 Aug 2003, 09:27 AM   #3
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"Here is some info you may find useful:"

Unless I'm mistaken, what followed was a direct quote from an outside source. If, in fact , that is the case, you should indicate in your post what the source is...
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Old 8 Aug 2003, 10:18 AM   #4
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The first paragraph isnt, the rest is, hence the 'info you may find useful'!!!
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Old 8 Aug 2003, 10:39 AM   #5
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So, what WAS the source?
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Old 8 Aug 2003, 10:42 AM   #6
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I cant remember, i did a search in yahoo and read through a load of stuff until i found something that i thought would be helpful!
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Old 8 Aug 2003, 10:51 AM   #7
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"I cant remember, i did a search in yahoo and read through a load of stuff until i found something that i thought would be helpful!"


Well intentioned, to be sure - but also disappointing, especially for anyone who might be interested in pursuing this information further...

(Perhaps I'm expecting too much of you, based on the statements you've made about yourself in other posts - in the "Off Topic" and other sub-forums here)
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Old 8 Aug 2003, 10:57 AM   #8
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waht do u mean by that? What expectations are there upon me? My intention is to help when possible and to seek help when needed. nothing more, nothing less.
Here is your url

http://www.hpl.hp.com/personal/Susie...7/hpidc97.html
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Old 8 Aug 2003, 11:00 AM   #9
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Default Shame on you Oriphus!

"The first paragraph isnt, the rest is, hence the 'info you may find useful'!!!"

The first paragraph is a verbatim copy from afterdawn's glossary at:

http://www.afterdawn.com/glossary/terms/transcoding.cfm



"waht do u mean by that? What expectations are there upon me?"

I would expect that someone who indicates him/herself to be a "seasoned veteran" (experienced) poster, would ALWAYS give credit to a source of information, rather than run the risk of creating the illusion (intentionally or otherwise) that the words included in his/her post were his/her ORIGINAL thoughts...

Last edited by setarip; 8 Aug 2003 at 11:05 AM
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Old 8 Aug 2003, 11:02 AM   #10
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Yeah, but i have permission to use any of the glosary terms from dRD, the guy who runs it, on any post i like. this is in return for writing user guides for afterdawn. their main user guide was written by me and adjusted/changed, reformatted and published by dRD.
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Old 8 Aug 2003, 10:03 PM   #11
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thznk for those very precise answers !
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Old 9 Aug 2003, 10:03 AM   #12
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I hope it helps you, there is quite a good read on a few f the searches i did on Google. Have a look and see what you come up with.
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