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Digital Video for Multimedia: Considerations for Capture, Use and Delivery
Section 2: Digital video: issues and choices
Considerations and Guidelines for Digital Video Capture
What do you need?
- A capture card
- A computer, with fast processor and lots of RAM
- Lots of hard disc space
- Sound card
- Tape drive for backup
- Editing software for video and sound (see further on in this section)
- Time - quite a bit
- Recording equipment for video sources if not already obtained
Choosing the system
If your budget allows, choose the best option in each category. At least purchase the highest quality capture card that you can afford assuming that you do not have to obtain the analogue video material first. The latter will carve a large hole in any budget.
As a general rule, quality, speed and cost are directly proportional to one another. If the project demands top quality digital video, you will need a top quality capture card.
If your computer has a PCI bus (v2 or later) investigate the new cards that are currently being released for this bus. Current models include the FAST AV Master and the Miro DC. These cards are able to perform a lower compression for the initial capture (typical ratios of 6:1 for full frame video) due to the throughput of the PCI bus. This provides the potential for much higher quality which is especially important if the final output is to tape.
- If possible, evaluate the capture card in your own computer. We have experienced a number of significant problems with a capture card purchased to run on a PCI (v2) bus Pentium.
- Check that the capture card is supplied with drivers for your particular operating system.
- Read the specifications carefully, that £10 Widgets Inc. WunderCap may say 30fps and 320x240 capture -- but not both at the same time!
The more sophisticated cards are also capable of capturing both fields of an analogue signal. Each frame comprises two half frames of fields which are interleaved on screen. If tape is the destination for the finished video this function is important.
Some video capture cards (e.g. FAST AV Master) also incorporate sound capture and this can alleviate the 'lipsync' problems referred to in the previous section on sound.
Having difficulties with your capture driver? Try another capture program. We did with our capture card:
The proprietary capture software would connect to the card's driver only very erratically
Microsoft's VidCap would connect most of the time, unless we had used the proprietary software first -- then VidCap wouldn't connect at all.
Adobe Premiere would connect every time
PC and RAM: Buy the fastest computer you can afford and fill it with as much RAM as possible! Our initial video work was carried out using a 486 DX2/66 MHz with 32MB RAM. A two minute video sequence would sometimes take 4-5 hours to compress in software. This machine was upgraded to a Pentium 133 MHz with 32MB EDO (Exteneded Data Out) RAM. Compression is approximately 8-10 times faster. This class of computer is a good starting point. You can, of course, use a lower specification PC but you will have to wait a long time for the results. Most capture software allows video to be digitised directly to RAM which is much faster than capturing to hard disc which may cause frames to be dropped. Large amounts of memory are therefore useful even with slow systems.
This means that short sequences (eg 10seconds of 320 x 240 pixels x 24 bit colour at 25fps with 24MB free RAM) can be captured in this way without dropping frames. These sequences can then be saved to disc and stitched together later. Note that free RAM does not mean swapfile which refers to a space used on hard discs as a substitute for RAM.
Graphics display card: A fast graphics display card is important. Desirable features include video acceleration, 24 bit colour display to enable editing, compression, etc., to be undertaken without palette problems.
Add as much as possible. The price of hard discs has recently fallen dramatically. SCSI devices have several advantages over IDE/EIDE devices, including the ability to daisy-chain them (up to seven devices per controller card). They are also generally faster, available in higher capacities and, if housed externally, can be swapped between computers. This is useful if the final video needs to be taken to a bureau for writing to CD-ROM. Fast-wide SCSI controllers and high rotational speed hard discs on a PCI bus currently have the highest throughput and are certainly worth considering.
The hard discs should preferably be of the AV (Audio visual) type. These hide the thermal re-calibration that all hard discs perform periodically which could lead to dropped frames.
The introduction of new types of removable media discs (eg Iomega Jaz and Syquest SyJet) with capacities of 1GB plus are likely to prove very useful for digital video activities. They have reasonable transfer rates (approximately 4MB per second for the SCSI devices) and media costs are also reasonable ( approximately £65 per GB).
A sound card will, of course, be needed to capture the sound component of any video sequence. Most sound cards and, increasingly, those built onto motherboards, will probably suffice in the majority of cases. However, if sound quality is a significant component, you may need to budget for a higher quality card. It is also worth considering a 'proper' amplifier and speakers to use for assessing sound quality. Speakers supplied with computer are not always adequate. If this is not possible, purchase a pair of good quality headphones. Although previously mentioned, this last point is worth repeating.
Tape Drive for Back up
Raw video files are large and valuable. It will probably be much easier to retrieve a file from tape than locate the original material for recapture. Obtain a fast, high capacity tape drive for backups -- and use it!
As with the capture card, the quality of digitised video is directly proportional to the quality of any camera or recording equipment. File sizes will often be smaller if high quality equipment is used. Sophisticated recording equipment also produces less noise. Noise will be interpreted by the codec as content and be included in the file. If noise is a problem, investigate the use of software clean-up filters for digital video which are beginning to appear on the market.
- Allow a maintenance budget for equipment. Dust, dirt, head wear, etc. will all tend to generate noisy video.
- If the capture sequence is difficult and requires a number of trial runs, work from a duplicate tape and save the master for the final digitisation. Tape wear is another source of noise and something to avoid incorporating on a master tape.
- Use fresh tape for the original recording and wind through to reduce the chance of tape drop out.
- If capturing a lot of sequences it is probably worth investing in computer controllable playback equipment. Make sure that suitable drivers are available for your capture software.
- Consider whether timecode accuracy is needed. If so, this will be a requirement for any recording and playback equipment purchased. Alternatively timecode can be generated after recording by a bureau.
Virtual Environments Visualisation