Previews, device control etc. with DV.

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"If it's all digital, how will I preview my clips on the monitor?" "And what about device control?" The devil is in the details. If you have more questions, send mail to the DV-L mail list, and an enthusiastic crowd of international experts will come to your rescue. Hopefully. Maybe. Most probably.

With all digital
Firewire board,
such as the Spark, you connect the DV camcorder or DVCR to the Firewire board. Then you plug an analog monitor into the camcorder.

Boards that have a hardware codec, such as the FAST DVMaster, accept an analog
monitor at their analog output.
With either solution, you can also watch your previews on the computer screen.

Any questions?

How are previews handled?

Some people like previews on the computer screen. Some like it on a separate monitor. In order to avoid alienating the wrong faction, the smart hardware or software developer implements both ways of previews: On screen and on a monitor.

All the boards inspected so far offer on-screen previews.

A board with analog I/O, such as the FAST DVMaster, allows you to hook an external monitor directly to the analog port on the card. For this, DV data must be run through an on-board DV hardware codec, which increases the cost of the board considerably.

A purely digital board, such as the DPS/Adaptec Spark or the Radius PhotoDV have no analog outputs. The Spark handles the external monitor in a very clever way: The external monitor connects to a DV camcorder or DVCR. The DV camcorder or DVCR connect to the Spark via Firewire. Previews are sent from the Spark to the camcorder as DV data and instantly converted to analog video by the hardware codec present on the DV camcorder or DVCR. This may sound complicated, but it's actually quite simple: Previews appear instantly on the monitor hooked up to the DV camcorder or DVCR.

With any solution, implementation details are handled by the software driver accompanying the products. These drivers can and will change.

What about device control?

Device control allows an editing application to operate the camcorders or VCRs connected to the capture board. By doing this, the capture process can be automated. Whole projects can be edited in low resolution and the recaptured and processed on full auto. This is called "batch capture." Up until now, the connection between the computer and the VCRs was a complicated setup, usually involving serial lines, adapters and often conflicting protocols. One of these protocols is LANC, also known as CTL-L. A LANC port is available on DV machines like the Sony VX-1000. Conventional device control can therefore be used.

The preferred way with DV is to dispense with the external cabling and headaches altogether and use device control implemented via Firewire. DV over Firewire carries video, audio and device control signals, all through the same medium. No extra cables. All that's needed is the proper drivers. Their implementation may be different across the boards. One fly in the ointment: For a long time, there was no real device control standard specified in DV, so manufacturers could (and did) pretty much roll their own protocols. A device driver writer must therefore detect the device the driver is talking to and then adjust the commands accordingly. This can be a headache. Expect first revisions of these drivers to be a bit rocky. For instance, the first FAST DVMaster drivers didn't get along too well with the DHR-1000 DVCR by Sony. Also, early versions of some boards may lack tight integration of device control with Premiere and other editing applications. With major architectural changes imminent from ActiveMovie 2 (or, as Microsoft calls it now, DirectShow) and Quicktime 3, developers probably will wait for the dust to settle.

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Written mostly by Bertel Schmitt. Maintained by Alexei Gerulaitis.

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