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2:1 v mbps
Hi all.
Something I am thus far unable to workout.
Does Avid use terms 1:1, 2:1 etc as just a neat way of determining data rate??
For example, if I digitize footage from digibeta, which is about 90mbps and apparently equal to 2:1, at Avid 2:1, does quality remain the same. As in, Avid is accepting a data rate of 90mbps, and therefore as digibeta is already at this quality, there is no reduction??or, is it simply a resolution issue. As in, if i capture 2:1 from Digibeta (which is about 2:1 also) am I really ending up with footage that is 1/4 quality of uncompressed SD. (because 1/2 of 1/2 is 1/4)?
This has really been bugging me for a while so I would love to know if anyone can help.
RB
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4 Replies
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Any form of compression gets decompressed when going out analog or over an SDI spigot. The term codec itself comes from COmpession DECompresion. When coming off the deck, the digibeta will output a full uncompressed signal before Avid (or other system) compresses it again. The signal does not get any better quality since the quality was already defined by the first compression in the process (usually at time of acquisition). Consider HDCAM – 135MB in 3:1:1 with reduce horizontal sampling. It gets decompressed when going over the HD-SDI to be 1920 x 1080 uncompressed to be captured either uncompressed or at any DNxHD resolution.
Michael
anything 24fps
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Those compression ratios are all relative to baseband uncompressed video (approx 170mb/s).
While Digibeta achieves around a 2:1 compression it has to be decoded to baseband video for playback out standard interface lines (Analogue and SDI). So the 2:1 in Avid is based on that rate.
The only video format where the native compressed video is transported off the cassette is DV where the native compressed video is transferred via Firewire. However DV played over analogue or SDI video will be decompressed to baseband video first.
Various solid-state and file-based formats (XDCAM, P2 etc) where files can be copied directly from recording medium are of course a different matter.
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Guys,
Thanks for the responses.
So the most important thing for me that I’m really trying to clarify is this:Because the video coming out of the deck is decompressed to 170 mb/s, capturing at Avid 2:1 roughly returns the video rate to 90mb/s which is close to digibeta compression. Thus there is no significant change in quality.
Is this correct??
Thanks for the help.
RB
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The compression algorithms are different, so while they have similar bitrates, the compressed footage in Avid is not simply being reverted to how it existed on the tape.
However the quality of the 2:1 compression is such that compared to a 1:1 capture of the same material there is very little difference (certainly none that you’d identify visually, and I’d say none you’d identify at all without a direct comparison).
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