Image Processing Reference
First Choose the Codec if Necessary
Choose the codec you want to use if the choice hasn't already been forced on you by cir-
cumstances beyond your control. Decide how the encoder is going to process the content.
The trade-off is whether to encode quickly and sacrifice coding quality or to take some
extra time and produce a more compact file.
Set the Bit Rate for the Video
Set the limit in kilobits per second (Kbps) for the video track. In Popwire Compression
Master, when encoding with MPEG-4, the optimal range is from16 Kbps to 2000 Kbps.
Check the specifications for the codec you are using to find the optimal setting.
Table 38-1 shows some example bit rates/file sizes for encoding video. These are just
ballpark guidelines, and your service offering will dictate exactly what values you should
A small picture size implies a lower frame rate, typically 15 fps. The medium-sized
picture is about a quarter the area of a full-sized TV and can probably cope with a normal
frame rate. The full-size service is all the pixels and all the frames.
Rapidly moving content may increase the bit rate required or may reduce
the quality to remain within the same bit rate target.
Audio bit rates depend mainly on the number of channels. This is not as simple as it
sounds. A 6-channel surround-sound service can be delivered as 6 discretely coded audio
streams. In the new and emerging coding systems, it is coded with a mono mix with dif-
ferential signals added at lower bit rates for the additional streams. These need to be
decoded and cascaded to reveal the additional tracks.
Speech will code more economically than music.
The AAC codecs deliver very good audio at 64 Kbps and excellent audio at 128 Kbps.
Usable multi-channel audio is possible at bit rates in the 500 to 750 Kbps range and pre-
mium quality surround is possible at 1.5 Mbps or better. This will improve as codecs