# Warning: About to save an uncompressed file
By default, VirtualDub does not automatically recompress video data that it processes, resulting in raw video data being written to the AVI file.  This results in the absolute highest quality, because the processed output is written straight to disk, but also results in huge files.  In many cases, writing uncompressed data slows down processing because of the sheer amount of data being written to disk.

The simple fix is to enable video compression:
•	Under the Video menu, make sure Full Processing Mode is set.
•	Select Video, Compression… and choose a suitable video codec.

# Warning: Video compression not active
Direct Stream Copy mode tells VirtualDub to send data straight from input to output.  This is very fast and makes sure the output video looks exactly like the video, which is very useful for audio processing.  However, few of VirtualDub’s processing functions are available in this mode.

To enable video compression, the video mode must be set to either Normal Recompress or Full Processing Mode.

# Warning: Outdated compression codec
You have selected a video compression algorithm which is inferior to other available codecs.  By coincidence, the outdated codecs listed here have been included with Video for Windows ever since Windows 3.1, and are the only ones shipped with the original retail version of Windows 95, so you may want to ignore this warning and compress anyway.

Microsoft Video 1  This codec produces large files that appear very blocky, and more importantly, cannot produce a perfect frame no matter how much space you throw at it.  It had appeal during the days of the 80286 and 80386 CPUs because of its high speed, but finds little use today.

Intel Indeo R3.2  Intel’s first widely available codec.  It’s very blocky with highly saturated color gradients, and produces dithering in its output.  It has been shadowed by Intel’s own newer codecs; Indeo 4.1/4.5 is available on both Windows and Macintosh, and 5.0/5.1 for Windows.  The Indeo 5.1 codec decompresses slower but compresses several times faster than its predecessors, and with much improved quality.

SuperMac/Radius Cinepak  The long-standing favorite of Macintosh users before the appearance of Intel Indeo 4 for QuickTime and the recent reigning champion, the Sorenson codec.  This codec decompresses several times faster than Indeo 5 but is slower at compression and produces blockier output.  It attempts to intelligently mark scene changes with keyframes, but in general the output of this codec just sucks in both quality and size.

# Warning: Audio compression inactive
The audio mode is set to Direct Stream Copy mode, so the audio track in your input file will not be touched, and the current audio compressor will be ignored.  To use the audio compressor you have selected, the audio mode needs to be set to Full Processing Mode.

# Warning: Filters inactive
VirtualDub’s filters only activate in Full Processing Mode.  Most filters modify video data too extensively for any hope of preserving the original compressed video, and require recompression.  If you have not already set one, you will need to select a video compression codec as well with the Video, Compression… menu option.

# Warning: Video compression quality >90%
Most video codecs are designed to work at a moderate compression level, and work less efficiently as you increase the compression quality level; the step from 90-95% usually takes more space than the step from 80-85%.  Usually, reducing quality to 90% or so saves a lot of space without visible change in the output video.

# Warning: No keyframe interval set
Video players need to rewind to the last keyframe and ‘fast forward’ when you seek to the middle of a file.  If there are no keyframes in a file, the player has to fast forward from the beginning every time you seek in the video.  This can take more than five minutes in a large file.  For this reason, keyframes are almost mandatory in a video file.

To alter the keyframe interval, select Video, Compression… from the menu, make sure ‘keyframes’ is checked, and enter in a non-zero value.

# Warning: Output color depth is set to 16-bit
VirtualDub has been set to 16-bit output for either speed or space savings.  When video compression is enabled, this is usually undesirable – it won’t save any space, and usually adds banding to the file.  You can fix this by using the Video, Color Depth… option.

# Warning: Audio interleaving is off
Audio interleaving puts small chunks of audio in between video frames in the file, so the player can walk through the file in one pass and pick up both audio and video as it goes.  Without interleaving, the player must jump back and forth between audio and video portions of the file.  Enable audio interleaving through the Audio, Interleaving… menu option.

# Warning: Fast repack mode selected
Fast repack mode can significantly speed recompression time if the video codecs are able to negotiate a common YUV format.  However, some codecs do not handle YUV properly, occasionally resulting in upside-down video.  If this happens, switch to slow repack or full processing mode.

#Warning: Attempt to use a hacked compression driver
The “DivX” codec is a hacked version of the Microsoft MPEG-4 codec.  Because this is a modified binary, there is always the possibility that the changes to the codec may have destabilized the driver, causing VirtualDub to act erratically or crash.  If you decide to continue using this codec, you’re completely on your own with any problems you encounter.

#Warning: Using MPEG-4 V3 in an AVI file
The Microsoft MPEG-4 driver was a part of Microsoft’s NetShow package, which has now become Windows Media Tools and Player.  Beginning with build 4.00.3845 of Windows Media, this format cannot be used in AVI files; the drivers will refuse to compress and decompress the format.  If you generate an MPEG-4 V3 stream in an AVI file, Windows Media Player will not be able to find an appropriate decompressor for it and will refuse to play the file.  However, you can convert the resulting AVI file to a playable ASF using Microsoft’s vidtoasf tool.

If this is troubling to you, consider using MPEG-4 V2.  This format is close to or similar to V3 quality-wise, has better speed, particularly on non-Intel CPUs, and can freely be used in AVI files.  You may have to upgrade to the latest version of Windows Media Tools, however, to enable MPEG-4 V2 compression.

# Warning: Miro/Pinnacle capture drivers detected
Some miroVIDEO capture cards from Pinnacle Systems, such as the DRX/DC10, resort to an emulated overlay when used with a graphics card that cannot provide a true video overlay.  This “primary overlay” mode is still very fast but can result in a lot of flickering, garbage on screen, and very slow GUI window operation, even on fast Pentium II systems.  Disabling overlay, by switching to preview mode or turning off the capture view altogether solves the problem.  This doesn’t occur if the DCxx is using a true video overlay on the graphics card.

VirtualDub has built-in workarounds to alleviate this problem.  To enable the workarounds, select the Preferences… dialog, select the miroVIDEO driver, and click the Disable overlay for menus and dialogs option.  VirtualDub will then automatically disable and re-enable overlay as appropriate to speed up the user interface.

# Warning: Zoran drivers detected
Some TV tuner cards are modified Zoran H20 reference designs, and at least one device – the Iomega Buz – is based on the H22 design.  Drivers for these cards that were written by Zoran or were based off of such drivers cannot time frame grabbing more accurately than the nearest millisecond.  As a result, some of the popular framerates – such as 15.000 and 29.970 fps – cannot be hit with this driver, and attempting to do so can result in dropped frames and/or jerky capture files.

The only known workaround at this time is to adjust the capture framerate to the nearest millisecond.  There is a button in VirtualDub’s Settings… dialog to do this.  Adjusted framerates, such as 15.151 and 30.303 fps, are also available in the quick framerate menu.  The Average Frame Rate field in the side panel will tell you if the capture card is capturing off-rate.

#Warning: Brooktree Bt848/878 drivers detected
Depending on the modifications your capture card maker has made to the Brooktree reference drivers, you may have to manually disable overlay to capture above 240 scanlines, i.e. 640x480.  You may also get field swapping when capturing in the YUY2 (YUV 4:2:2) format.

On the good side, the reference drivers allow arbitrary image sizes (352x240) and some additional YUV formats (UYVY) that may not be exposed by your particular driver’s Video Format dialog box.  These extra features, if present, can be accessed through VirtualDub’s Set Custom Format… (Shift-F) function.