Information Technology Reference
In-Depth Information
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Requantization
Spatial Downscaling
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RT
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Bit-rate (kbps)
(a) Education video clip
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Requantization
Spatial Downscaling
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RTB
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Bit-rate (kbps)
(b) Onthestrip video clip
Figure 2.8
PSNR versus transcoded bit-rates using requantization and spatial downscaling
Specifically, the integrated transcoder comprises two parts. First, the original compressed
video is transcoded once offline using the largest quantizer value (e.g., 31 in MPEG-1) to find
the lowest bit-rate achievable by requantization. This is shown as the requantization threshold
bit-rate (RTB) in Figure 2.8.
Second, during online transcoding, this RTB parameter is used to determine the transcoding
technique to employ. In particular, if the desired output bit-rate is lower than RTB, spatial
downscaling is used, otherwise requantization is used to achieve better visual quality (i.e.,
PSNR).
Using this two-part design, we can eliminate the need to compute the PSNR versus bit-rate
curves, which is very computationally expensive, to determine the transcoding method to use
given a target bit-rate. Instead, we only need to perform transcoding once to obtain the RTB
for such purpose.
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