DV & Firewire


Editing Workflow

My hand does the work and I don't have to think; in fact, were I to think, it would stop the flow.

- Edna O'Brien

by Bertel Schmitt and Alexei Gerulaitis

When DV was first announced, many people thought that they would need specialised editing equipment and special editing software.  Actually, quite the opposite is true: the more things change, the more the stay the same.

Here's how it works, step by step:

bulletStep 1: Compression in camera.  As video is being shot, it is compressed and converted to digital form in the camcorder.  What used to be "video" now sits on a digital tape.  This digital tape can be played in a digital tape drive, such as the one in your camcorder, in a DVCR, or in a stand-alone unit such a DVDrive.
bulletStep 2: A transfer, please!  DV data, not video, is transferred electronically via the firewire to the computer's hard disk.  This not a capture process, it's a file copy process.  The copy process can be handled by a driver camouflaged as a regular capture driver, or by a stand-alone utility.  This is specific to the individual implementation of the driver for the various DV/Firewire boards, and it can also change as matters progress.  The hard drive has to be fast enough to cope with the 3.7 MByte/sec data rate (plus some overhead).  Theoretically, it is possible to stop the tape and restart if and when the hard drives can't cope with the data rate, but this is an involved process and may not be implemented in early versions of DV drivers.
bulletStep 3: You want it wrapped?  During the copy process, the DV data is "wrapped" into a file format commonly understood by computers, in this case either AVI for Video for Windows or Quicktime for the Mac.  Both file formats allow for "installable compressors," also known as codecs.  AVI, for instance, can work with a multitude of installable compressors, such as Indeo, Cinepack, VDO, et cetera to name just a few.  The compressed DV data is treated like data produced by just another installable compressor.  As a matter of fact, there is a DV compressor/decompressor (codec) installed on your system.  But during the file copy process, this codec is not needed.  We'll cover that later in more detail.
bulletStep 4: Look, Ma, no codec!  After the copy process has been finished, the DV data is sitting on your hard drive, wrapped into a file format any standard editing application can process.  Note: The actual DV data has not changed.  It hasn't been touched by a codec, it hasn't been recompressed, changed or altered.
bulletStep 5: DV Premieres. To edit your clips, you use any standard editing application that can work with industry standard file formats, such as the ubiquitous Adobe Premiere.
bulletStep 6: Ain't misbehavin'.  During editing with a program like Premiere, your DV AVI or Quicktime movie will behave just like any other video clips you used before.
bulletStep 7: So where does that codec come in?  When, and only where Premiere adds filters or transitions, Premiere needs the DV data in uncompressed form.  For this, Premiere will call the installed DV codec.  Premiere will hand it compressed frames retrieved from the AVI or QT file.  Premiere receives uncompressed RGB bitmaps back from the codec.  Premiere then blends, filters, combines, warps or alters these frames according to the specified transition.  When done, Premiere hands the finished RGB bitmap to the installed DV codec.  The codec compresses the bitmap to DV AVI or QT and hands it back to Premiere.  Premiere then stores it in the target file.  The installed DV codec can be implemented in hardware or in software.
bulletStep 8: Don't touch that.  Clips without filters or transitions are not being touched by the codec and are simply copied to the target file.  If you would have a project which consists only of hard cuts, the codec wouldn't be called for editing at all.
bulletStep 9: Here we go again.  After all edits have been finished, the resulting file must be copied from the computer to the DV device via Firewire.  During this copying process, the AVI or QT wrapper is removed, data specific to the receiving device is adjusted or restored.  This is usually done "on the fly" as data is sent to the DV device.  Some boards may need additional post processing.  The copy process usually is handled by a standalone utility, or by a Premiere "Print to DV" plugin.
bulletStep 10: (This is a 10-step program!)  You are done!  On your hard drive sits DV video, most of it as pristine as you've shot it.  No generation loss.  You've reached the holy grail of video editing - right on your computer.

Source: dvcentral.org 16 April 2003 © 1997-2003 by the authors all trademarks recognised

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