DMDX Help.


Digital Video

    DMDX has the ability to display a streaming Digital Video multi-media file, beit AVI, MPEG, QuickTime or whatever else DirectShow supports (see TimeDX's Digital Video help on Codecs), or it can also just use just the Streaming Audio abilities (see below). In order for this to work DirectShow must be installed on the machine, usually with the DXMWeb.exe package. The machine to run it should also have a PII processor, while a P133 can do it it is usually dropping a lot frames and a P200MMX isn't much better, while TimeDX on a P200MMX can display Digital Video files with relative ease DMDX has a much harder time as it must do whole lot more than TimeDX is.
    As with all DMDX natural color related things, a 16 bpp screen mode is highly recommended. As with the
<animation> switch all frame buffers (including the visible primary buffer) are cleared at the end of an item so all Digital Video items should end with a blank frame, for example:
10 <dv> "movie" /;
    When DMDX processes an item with a Digital Video file in it it uses a completely different method to load images into the display memory. Instead of buffering any number of frames in the video card's memory ahead of time DMDX now only moves a frame into video memory immediately prior to it's being displayed due to DirectShow's nature. Additionally, digital video files have enormous initial latency (kind of like pre-roll time on tape machines) in the order of hundreds of milliseconds, DMDX automatically re-schedules the rest of the display queue once a digital video frame has actually started producing images to be displayed.
    Multiple digital video files can be played at once (assuming you have the CPU horse power) and the frames can be combined with other frames in normal DMDX ways. The DMDX RT clock can be turned on at a particular digital video file frame number using a new clockon switch,
<DVClockOn> and the duration of the DMDX frame can be set to expire when a particular digital video frame is displayed with the <DVFrameDuration> switch.

    When playing back a file with the
<DigitalVideo [N,N[,N,N]]> switch the first optional pair of Ns specify the top left corner location of playback, the second the bottom right, DMDX switches <X> and <Y> are overridden by these. In the same way that the <X> and <Y> switches can be real and express a fraction of the screen dimensions these values can be too. If neither pairs of coordinates are present the images are displayed centered on the screen in the size they are stored in the file in, if only the first pair is present the top left corners of the images are moved to that location, if both coordinates are specified the images are stretched to fit (overriding any <BMPMultipliers>). If all four values are zero the image is displayed full screen. If a -1 is used for a field the default value for that field is used, if -1 is used for either or both of the first coordinate pair the image is centered in that axis, if -1 is used for either or both of the second pair the original image size in that dimension is used. For example:
<dv 64,48>
    This would display the images near the top left corner of the screen in the original file size.
<dv 64,48,164,148>
    This would display the images near the top left corner of the screen as 100x100 pixel images.
<dv 0,0,0,0>
    This would display the images full screen.
<dv -1,48>
    This would display the images near the top center of the screen in the original file size.
<dv -1,48,-1,0.5>
    This would display the images near the top center of the screen in the original file size width but stretched to half the screen height.
<dv -1,-1,0.5,0.5>
    This would display the images in the center of the screen in one quarter of the screen area.


    When turning the clock on for responses at a given digital video frame with the <
DVClockOn> switch the normal DMDX clock on switch should not be used as well. This switch must occur in the frame that defines the digital video file name regardless of whether another DMDX frame will be displayed by the time that digital video frame is displayed. The way this functions is that as the code is displaying video frames it takes note of the number of that video frame in the video sequence, when it sees that the video frame number is greater than or equal to the clockon frame number (and it hasn't already turned the clock on) it turns the clock on. In order to determine which video frame contains the relevant stimuli TimeDX can be used to seek around the digital video file to find a particular video frame and it's number can be noted and then entered into the item file (sorry, cursors don't exist in any generic fashion for digital video files).

    If synchronization between a digital video sequence and the rest of DMDX is needed the <
DVFrameDuration> switch can be used. In the same fashion as the <DVCLockOn> switch, when the specified video frame has been displayed DMDX moves to the next element in the display queue and re-schedules everything accordingly.

    If display of the digital video needs to be abortable <
AbortItemKeyName> can be used, if the display needs to be contingent upon a subject's response (or some other condition I can't currently imagine) <AbortItemExpression> can be used (see jobstatus example).


Streaming Audio

    To facilitate the use of large audio instruction sets a subset of the Digital Video abilities have been implemented with the <StreamingAudio> keyword. The problem with large multi-minute (and therefore multi-megabyte) audio files is that they must be read into main memory before DMDX plays them if the <wav> keyword is used, this can take appreciable amounts of time on some systems and can also cause portions of memory to be swapped out to the paging file if really large audio files are used that are a significant percentage of or larger than the RAM installed in a machine, slowing things right down. Another problem is that the file must be a plain vanilla .WAV file, no compression. Using the <StreamingAudio> keyword the audio is streamed from disk (only the next fraction of a second is read at any one time) and DMDX also uses the codecs installed on the machine to decode the file.

    To use compressed audio files (say .mpg or .au files) a suitable codec must be found, see
TimeDX's Digital Video help on Codecs.

    The downsides are that any significant degree of precision or synchronization is lost as DirectShow is now doing all the work. The use of .WAV cursors is no longer possible, instead the millisecond accuracy of the <PlaySAFrom> and <PlaySATo> keywords is available (these are in fact synonyms for <PlayDVFrom> and <PlayDVTo>, the frame duration for a streaming audio file is one millisecond). <StreamingAudio> does not support any of the <Sound> (<wav>) keyword's <SetVisualProbe> functionality, the visual probe is always at the start of the audio file (the audio file always begins playing at the frame's scheduled time) and the duration of the frame is either the duration of the audio file or the explicit % frame duration. Nor does it support the <Volume>, <Pan>, <DVFrameDuration>, or <DVClockOn> keywords.
   
    It is currently not possible to have the same instance of a streaming audio file playing twice (the old <
wav> keyword limitation in a slightly different incarnation) as the <StreamingAudio> keyword really is designed for simple instructional purposes -- should you really need to do so then use slightly different parts of the file, say <PlaySAFrom 0> in one frame and <PlaySAFrom 1> in the other.
   




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