Implementation of PET

We have applied PET to the transmission of video over the INTERNET. In a particular sample stream, a side-by-side comparison of MPEG stream encoding with and without PET demonstrates the dramatic improvement of picture quality due to PET, using only 24% redundancy on top of the standard MPEG data. For this particular group of pictures and frame sizes, this represents a more than five fold reduction in transmission rate over the JPEG mode to achieve comparable quality.

We note that a priority hierarchy is inherent in MPEG compressed video since it uses three different frame formats:

I (intra frames), coded as a still image, provide random access but only with moderate compression
P (predicted frames), predicted from the most recently decoded I or P frame
B (bidirectional frames), provide the highest amount of compression but requires the closest two I or P frames for prediction

A sequence of these frames forms the group of pictures within an MPEG stream:

So, from the above, we can see that a lost I frame affects at least a whole group of pictures (GOP) and B frames require an additional P frame to be correctly decoded.

For our particular example stream, we assigned priorities in a way that header information can be recovered from any 10% of the encoded packets, I frames from any 60%, P frames from any 75% and B frames from any 90%. Computing the redundancy distribution:

A total overhead of 23% enables the I frame to sustain losses of 40%. Respectively P frames sustain losses up to 25%, B frames up to 10% and important header information up to 90%.

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