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Sony ENG / EFP Shoulder Mounts
Sony PDW-F800, PDW-700, PDW-850, PXW-X500 (XDCAM HD) and PMW-400, PMW-320 (XDCAM EX).


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Old March 1st, 2006, 05:30 PM   #1
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Question about VBR

In the highest mode it is 35Mbits/sec VBR. So how does it work? For VBR to work in theory you need at least 2 passes. So how can it do VBR on the fly and still get an average of 35Mbits/sec?

Also what is the limit for it? Does it peak to 50Mbits/sec? Or higher?

Thanks.
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Old March 3rd, 2006, 11:41 AM   #2
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Thats the advantage of being able to write a twice realtime speed. If I understand this right, the camera can take a number of frames into a memory buffer, analyse it for the bitrate needed, and then write it to the disc at twice realtime speed. Of course I'm probably wrong on that (and I usually am!)
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Old March 3rd, 2006, 11:53 AM   #3
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I would imagine VBR would introduce temporal noise, to a certain degree. It is interesting to note that JVC added motion blur to their progressive HDV cameras, which can be "viewed" as temporal noise added to smooth jitter when you pan the camera. It amazes me what manufacturers will put themselves through just to compensate for consumers who don't know how to treat a progressive format.
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Old March 9th, 2006, 06:54 AM   #4
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Presumably the VBR works in the same way as any MPEG2 single pass VBR.

Starting with an I frame the encoder analyses each frame on the fly to see where the changes are. In the case of HDV if there is a lot of changing detail, the codec runs out of bandwidth, and detail is lost as a consequence. Presumably with this camera if there is a large amount of changing detail in the frame, the encoder can pump more and more bits at it to minimize any loss of detail.
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Old March 9th, 2006, 08:27 AM   #5
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Trying to work out what the maximum bit rate is (as there is no indication whether 35Mb/s is the maximum or average bitrate)...

The disks are 25GB(?)

According to the brochure the proxies take up 1.5Mb/s for video and 0.5Mb/s for audio. The record time is around an hour, so that would mean that the proxies take up about a gigabyte, leaving 24GB. Presumably these disks are 25GB unformatted, so even if formatting took the capacity down to 23GB that leaves a maximum bitrate for 60 minutes of material of about 49Mb/s.

If this is true then that would be really nice!
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