Video Output Assignments Map

This page is a comprehensive description of what each specific preference in VirtualVTR and VirtualVTR Pro will do. It does not offer overall guidance on particular setups.

Introduction
The modern architecture in QuickTime requires video hardware devices to register their available outputs, and also any sub-modes in those outputs, which determine the types of video data which are compatible with those outputs. VirtualVTR in OSX offers a very flexible mechanism for routing any type of video media through any available video output device and output mode. This allows users to setup sophisticated routing and force alien format movies through the most appropriate output mode of any particular video card they have.

Media Types and Routings
Each QuickTime movie contains Video frames with a specific codec format, or media type. Common examples are DV, MotionJPEG and various flavors of uncompressed video. Each of these codecs store video data in a different format, and VirtualVTR needs to make sure that you feed particular data down a pipe which is expecting that type of data. For example, a common codec is the DV PAL codec. This is most often played back via a Firewire DV Bridge, and so in this case, the media Type 'dvcp' is routed to a Pal output device 'Firewire : Apple Firewire PAL'. DV Bridges typically only accept DV PAL or DV NTSC data, and no other type of data, so this is fairly easy to understand. In a different setup, you might wish to feed your DV Pal media through a DeckLink card, or an AJA IO. These devices offer a variety of 'output modes' or 'ports' with with to connect to the output hardware. Each output mode expects data in a specific format, and in the case of DeckLink, they offer a special output mode which is optimised to receive DV PAL media. Aswell as offering the most efficient route from your movie to the video output, this media-specific output mode may be performing other DV-specific functions such as re-ordering field priority, or clamping black levels. There are different output routings for PAL and NTSC movies, although note that VVTR determines PAL or NTSC by counting video lines, so anything which does not have 576 lines, may automatically be designated as NTSC, even if its an odd-sized PAL movie.
VirtualVTR includes a list of the most common media Types by default, and you can alter the routings for these types using the pop up menus in the PAL and NTSC columns. If you have a movie which is not already setup here, you can add a new type by clicking in one of the Media Type fields set to 'none' - then pick the movie in question. VirtualVTR will read the Media Type from that movie file and add it to the list ready to be routed. Make sure you do not include the same media type more than once or VVTR may be confused about what to do with it. Also note that VirtualVTR shows the media types of movies in the bin, in between the filename and the timecode range. Selecting 'default output' for a specific routing lets the Mac OS decide what to do when opening a movie. In most cases, this will simply open it in a window on the mac screen. You can then optionally use the 'Full Screen' and 'Resize to Fit' prefs in VirtualVTR to fill a mac monitor. In most professional situations it is preferable to use appropriate video output hardware instead of a mac window in order to genlock the video playback or to create proper PAL/NTSC Signals.

Options and QuickTime hints
There are also some specific options which can be applied to each routing. These include the ability to 'echo port' the video from the Video output back into the window on the Mac screen. Note that this option is not supported by all modes, so in some cases you can NOT have the video both on the mac screen and also the video output device. Other options include 'High Quality' and 'Single Field'. It is important to understand what these represent. VirtualVTR is NOT actually using these options in any way to alter the picture. What these options do is to send 'QuickTime Hints' to the video output mode you have chosen. If you choose 'Single Field' for a particular routing, this hint will be sent to the output mode you choose. The Output mode then decides how to interpret this hint. In some cases, the hint will be completely ignored and nothing changes, this is all defined by the specific output component's own behaviour. In the case of software decompression of DV Video for example, feeding an RGB Output mode (not using a dedicated DV output mode), the Apple DV Decompressor will respond to the Single Field and High Quality messages in basically the same way, turning dual field decompression on and off. This can be useful to eliminate annoying frame slippage, especially when the field dominance of DV movies has been reversed (this is a common problem with DV, due to a quirk in the official DV PAL technical specification).

Note that any changes made in the Output Assignments settings are typically not reflected until the movie is closed, and re-opened.

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