# Video codec information: delta frames
Set to Yes if the compressor produces delta frames.  Delta frames record only differences from the previous frame, so all frames from the last key frame need to be decompressed first to view a delta frame.  Some compressors set this value to No even though they support delta frames, such as the MS MPEG-4 series of video compressors.  These codecs internally perform more advanced key frame analysis.

# Video codec information: FOURCC code
All video compression drivers respond to one or more unique fourcc (four character code) IDs that uniquely identify the video compression algorithm.  For instance, any video stream tagged with ‘IV50’ is compressed with the Indeo 5 algorithm.  This information is useful for identifying driver conflicts, manually installing a codec driver, or forcing a video stream to another compatible driver.

# Video codec information: Driver name
This is the name of the DLL that contains the video codec.  Usually a video codec can be installed by adding a vidc.fourcc = driver name line to system.ini under the [drivers] or [drivers32] sections.  This field also shows you if a driver is handling more than one format.

# Video codec information: Format restrictions
When you select a video compression algorithm, VirtualDub will probe the driver and attempt to figure out which formats it can and cannot accept.  For instance, codecs which block the image into 8x8 tiles may refuse to compress if the width and height are not multiples of 8.  Due to limited error reporting capability in the Video for Windows architecture, the driver cannot tell you what part of the image format is wrong.  Use the information from this box to determine an output size that the compressor will accept.

# Quality setting
The quality factor for video compression has several meanings.  Ultimately, the compressor can interpret this value any way it wants – even ignore it – but there are two general uses for this value:

If you do not have the “data rate” box checked, this slider usually sets a threshold for compression.  The video compressor will then use just enough space for each frame to hit this quality level.  More complex scenes use more space, and the size of the output file cannot be determined until processing has finished.  A higher quality level corresponds to a better quality frame.  This mode (thresholding) is sometimes faster than data rate control.

If you have the “data rate” box checked, this slider determines how hard the compressor works to hit that data rate.  Lowering the slider speeds up compression, but can result in video streams over bitrate and/or low quality output.  With data rate control, this value should almost always be set to 100%.

# Target data rate
Some compressors support data rate control as an alternative to thresholding.  Rather than using a varying amount of space to hit a fixed quality, the compressor varies quality to maintain a constant data rate.  This has a number of advantages, including a predictable file size, much better quality during “quiet” scenes, and dependable CD-ROM playback due to the constant data flow.

To use this feature, type in the desired data rate in kilobytes per second (K/s or Kb/s).  For instance, a 1X CD-ROM rate would be no higher than 150 Kb/s.  The video compressor will then do its best to make sure that the video stream does not exceed 150 Kb/s on the average.  The video stream may end up slightly larger or smaller than expected depending on the compressor’s ability.

# Key frame interval
Enable this value to guarantee key frames at specific intervals throughout the file.  When seeking in a file, players must play to the desired point from the previous key frame, so larger key frame intervals increase seek time.  Key frames also tend to be larger and higher quality than compressed ones.  In general, use a value between a quarter of a second to a 3 seconds for a good balance between seek time, image quality, and file size.

If this feature is not enabled, the only guaranteed key frame is the first frame in the stream.

# Sampling rate
Select a new sampling rate for the output audio here, or choose No change to leave the audio sampling rate alone.  Raising the sampling rate doesn’t help quality, but increases the audio track size; lowering the sampling rate diminishes quality but also lowers the track size.  However, you may need to force a conversion here if an audio compressor cannot accept the current audio format.  If you don’t want to change the rate, be sure to choose No change rather than clicking the appropriate sampling rate because the sampling rate might be slightly off, as in 22047 Hz, causing the stream to be converted to 22050 Hz.

Depending on the audio input, sample conversion can introduce noise into the output.  If the audio contains a lot of high-pitched sound, such as a high female voice or percussion, you may want to click the High Quality box to enable filtering on the output.  For even better quality, extract the audio track and perform the conversion using a good sound editor.

CD quality audio has a sampling rate of 44100 Hz.

# Sampling rate: Integral conversion
Check this box to force the new sampling rate to be an integral multiple or division of the old rate.  For instance, if you have an audio track with a slightly nonstandard sampling rate, such as 22047 Hz, choosing 11025 Hz with integral conversion enabled converts the track to 11023.5 Hz.  Similarly, the 44100 Hz setting would cause a conversion to 44094 Hz instead of 44100.

# Sampling rate: High quality
This option enables a triangular Tent filter on the resampled output.  When upsampling to a higher rate, the filter reduces quantization noise and hiss.  When downsampling to a lower rate, the filter attenuates high frequencies that are normally aliased to lower frequencies.  Without the filter, noisy high-frequency sounds like the hiss of a trailing s can drop in pitch and drown out the audio.

# Precision
Allows you to change the precision of audio samples.  If the volume is sufficiently well adjusted, lowering the audio sample precision from 16-bit to 8-bit halves the audio track size with little difference in quality.  You may have to expand an 8-bit track to 16-bit for audio compression; however, it is better to start with 16-bit audio to begin with.  Never reduce a 16-bit track to 8-bit before audio compression, if possible.

CD audio is 16-bit.

# Channels
Allows you to mix a stereo track into a single mono track, halving the track size, or duplicate a mono track to form a stereo track, doubling the size.  Conversion to mono is performed by averaging the two samples together.  Stereo tracks take twice the space, so if the audio is highly central, such as voice, then consider mixing to mono for the final output.

CD audio is stereo.

# Bandwidth required
Tells you how much space is required for each second of uncompressed (PCM) audio at the selected values.  For instance, dropping a CD-quality track to 22KHz, 8-bit mono reduces the bandwidth requirements from 176K/s to 22K/s, saving a lot of space.  Reducing the sampling rate to 11KHz again would only save another 11K/s of bandwidth, and make a much smaller difference in file size.