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Old May 31st, 2007, 05:32 AM   #1
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My HD uses less space than my SD?

I have a canon HV20 and a Sony Vx1000. The Sony uses more hard drive space. Is it because of the one chip vs three chip? Just curious.
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Old May 31st, 2007, 06:05 AM   #2
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Should be exactly the same ... HDV is compressed with less keyframes so it takes up the same amount of space as traditional SD...
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Old May 31st, 2007, 07:47 AM   #3
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Regular DV and HDV both have the same data rate of 25MB/sec so theoretically they should be about the same. The number of chips in the camera has nothing to do with the format of the data.

Just taking a quick look at a couple projects on my Mac I see 2:30 HDV clip that's 478.4 MB and a 2:30 DV clip that's 540.8 MB. So it does appear that HDV is a little more compact. I believe this has been discussed before and it has something to do with the fact that HDV compression works with groups of frames while DV compression is applied to individual frames. Perhaps someone else can explain the technical details.
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Old May 31st, 2007, 08:26 AM   #4
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The difference might be that audio is compressed with HDV, but not with DV.
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Old May 31st, 2007, 10:05 AM   #5
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First, I keep hearing 19 point something being thrown around with HDV rather than 25. I've been curious when that was brought up.

Second, my guess is that even though it may have the same "bandwidth" on the tape, when it is encoded, the resulting file size will vary depending on what is going on in the actual scene. A scene with a lot of movement, has a lot more information to construct frame for frame.

Can anyone tell me if I'm close.
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Old June 1st, 2007, 07:30 AM   #6
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The file size cannot vary from its bandwidth. Bandwidth times duration equals file size. The bandwidth is for the encoded file, so there is no extra step.

It is the audio that causes the apparent difference between DV and HDV, when both are described as 25Mb/s streams -- that describes the video only.

GB
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Old June 1st, 2007, 02:44 PM   #7
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I believe R Geoff Baker is correct: the audio is to blame. DV and HDV 1080i are both recorded at 25 megabits per second (that's Mbps, not MBps), but only the video. DV audio is, as I understand it, PCM. It's uncompressed--at least I think so, engineers feel free to smack me if I'm wrong--so at 16 bits per sample, and forty-eight thousand samples per second, that's 768 kilobits per second, whereas HDV's audio is MPEG 1 Layer 2 recording the same bit and sample rate at 384 kilobits per second.

And if you're talking about 720p HDV, the video is 19.7 Mbps, but the HV20 only does 1080i as far as I know, so that's another story.

All this and more can be found at Adam Wilt's site: http://www.adamwilt.com/

And the HDV version 1.0 main spec can be seen here: http://www.hdv-info.org/
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