User manual

128 MPEG glossary
www.magix.com
MPEG glossary
Motion estimation
Motion estimation is a further element for reducing data used in
MPEG encoding.
Motion estimation also occurs in the B and P frames. The image
difference that still exist after prediction (view page 133) are
examined. Complex algorithms are used to search for an original
occurrence of the macro block in the reference frame of each macro
block of the P or B frame (these are units of 2x2 blocks specially
combined for this purpose), which have been moved either by
movement or by camera pan. They can then be left out in the P and
B frame. Only the information by how far and to where the macro
block has been moved is saved instead. This vector is called the
motion detector.
In the General encoder settings (view page 122), you can specify the
quality of the final MPEG video. This factor also influences the time
required for encoding. The longer it takes, the better the quality.
Bit rate
MPEG is a format used for storage and transferring. With older
formats (e.g. AVI) you could predict that 20 seconds of movie would
result in 20 MB of data. The file size is this a direct measurement of
quality.
This is different for MPEG: The amount of data available can be used
differently for different display modes. 20 MB can be 4 seconds of
DVD Video or 5 minutes Internet streaming in thumbnail format. The
quality of an MPEG video is measured by the width of the created
data stream, the bit rate. This is the amount of the transmitted data
per time unit; it is stated in kBit/s or bit per second.
Bits, not bytes are used, since the data word width has to address
the transmission restrictions.
The file size can be calculated from the average bit rate, if its length is
known: