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Topic Title: High quality, 720x480, video capture
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Created On: 01/14/2006 05:00 PM
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 01/14/2006 05:00 PM
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Have you ever noticed how mini-DV movies have terrible smear of action? One suggestion for the cause of this is that the camera has long exposure time. However if you hook up a video out signal directly to you TV and watch live action there is not nearly as much smear as when you record to mini-DV tape and play it back. The real reason for the smearing is that mini-DV is compressed about 5:1 compared to standard NTSC video. The compression used for mini-DV is excellent for stiil or slow moving shots but sports/action are hopelessly smeared by the compression process. This leads to the question of how to capture video in an uncompressed format.

The following is a how-to for building cheap system that will do lossless capture of YUV2 video at 720x480 pixels and 30FPS (full quality standard NTSC). This is basically a data throughput problem starting at the camera and ending at the hard drive.

First we can estimate the data rate. For YUV2 video there are 2 bytes of data for each pixel. So the video data rate is calculated by the number of bytes per pixel, times the number of pixels per frame, times number of frames per second.

In this case:
2 x 720 x 480 x 30 = 20.736 MB/s

The data rate for cd quality video is:
2channels x 44100samples/s 2 bytes/sample = 0.1764 MB/s

So for full quality NTSC video capture you need about 21 MB/s sustained data throughput.

Theroretically USB and Firewire could provide the bandwidth, however, actual data rate for USB and Firewire is often below 20Mb/s so there is a huge potential for dropping frames with either. In addition to this the uncompressed digital stream is not available with mini-DV camcorders. So for the reasosn just stated, no Fire-Wire or USB 2.0 links can be in the signal chain.

One option is to use video capture at the computer. I am using a JVC camcorder with S-VHS output ($229). The capture solution I tried was to use an ATI TV Wonder tuner card and capture from the video input. This setup did not allow the full speed capture for me. The next step was to try a ATI all-in-wonder 9800 AGP card. The 9800 was able to capture direct from video without dropping frames. This system was using Windows ME. Windows ME only supports FAT32 which has a file size limit of 4GB. That limits clips to under 3.5 minutes.

Installing windows XP solved the file size limit problem but created a new one. The hard drive would not keep up with the video capture data and Pinnacle Studio refused to capture at best quality because the hard drive no longer tested fast enough. It showed a data rate of only 19Mb. I first tried putting the OS on a 40Gig HD on as a new C: drive and then put the 80Gb drive on the secondary IDE as a single drive. This is the reccomended set up for a capture hard drive. Unfortunately this was slower than when I used the 80Gig drive alone for both system and capture. The reason why is not apparent. Since the drive was proven capable of the data rate I decided thatr I needed a different data path.

After much searching the best solution appeared to be buying an inexpensive raid controller and hard drives. This is RAID 0 which means data is "striped" into both drives. Drive 1 takes the first chunk and while it writes the data the controller sends the next chunk to drive 2. The card and drives I bought was $25 + $115 +$115, and is able to get better than twice the data rate of the 80GB drive. The new drives are IDE WD 200GB units with the 8MB cache. Pinnacle Studio test results give 47MB/s data rate and the drive space allows 5 hours of uncompressed video capture.

The system is a Sempron 2200+ on a SiS 741GX motherboard with 512MB DDR3200. Better performance was given by overclocking from 1500 Mhz to 1800 Mhz.

Many people have told me that I need an Athlon 64 to do this kind of work but so far the low budget Sempron system which I have does it.... for about $1000 total.
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