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Understanding LongGOP codecs


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#1 DS Williams

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Posted 11 February 2009 - 07:38 PM

I know what a group of pictures is, and I know how codecs like XDCAM EX and AVCHD go about compressing images, by recording and I frame and then b and p frames with changes information, but what I don't understand is how long GOP footage is shown on a monitor or television direct out from the camera.

If I hook up my EX-3 to a monitor via HD SDI, and go to the play back menu and watch recorded footage, does the footage have to be uncompressed out of it's 15 frame gop first? I assume the monitor requires all I frames
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#2 Tim Tyler

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Posted 12 February 2009 - 04:21 PM

I know how codecs like XDCAM EX and AVCHD go about compressing images...


A coDEC also DECodes. http://en.wikipedia.org/wiki/Codec

What you see on a monitor via SDI may be the image before it has been encoded too. But if you're playing back, then it has been decoded.
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#3 Phil Rhodes

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Posted 12 February 2009 - 05:31 PM

Further to the above, it's worth remembering that video over firewire, including HDV and DV, is unusual in that it is compressed. Practically all other point to point video networks are uncompressed, for whatever value of "uncompressed" applies to analog signals. SDI is comparable to computer-oriented protocols like DVI - it's a list of pixel values with comparatively little complexity.

Conversely, practically all video tape and solid state formats use compression, and it's the job of the tape deck to read the magnetic signals off tape (or a flash card reader to read the flash), and then to decode the image to a standard like SDI. Only in very specific circumstances do you get to see the compressed image data anywhere other than the internal electronics of a tape deck. One of those circumstances is DV video, and a wide-ranging standardisation effort had to be implemented to provide people with the convenience of handling compressed DV video over firewire. Computers had to be given an ability to decode the images compressed by the cameras, whereas with SDI, the computer doesn't need to care what tape format the images come from as it's already been completely decoded.

This has downsides, too. If you copy DV or HDV tapes from deck to deck, to clone a tape, you get exactly that - a clone - because the image data will not be decompressed (it might be decompressed by both decks just so you can view it on a monitor, but it is likely to be recorded to tape bit-for-bit as it came off the master). If, on the other hand, you connect two HDCAM decks together, the playback device will decode the image, send it over SDI, and then the record deck will recompress it again. There is a workaround for this known as SDTI, a variation which allows certain tape decks to exchange compressed data, using normal SDI hardware - usually, the tape decks must be of the same format (you can't record DVCPRO data to an HDCAM deck, for instance).

The Sony Xpri editor has (or had?) the ability to read HDCAM data over SDTI, and had software to decode the HDCAM format internally, but it's an unusual example and presumably only happened because Sony had internal access to information about how their own tape format works.

P
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