Variable Bit Rate

In any given audio section, certain fragments are more complex than others, e.g. a whole orchestra playing compared to a single instrument. As a result the number of bits needed to faithfully encode varies with the program material. In order to encode in the best possible way, it is advantageous to save bits from the simple sections and use them to encode complex ones. This is what variable bit rate encoding does.

Think of movie soundtracks, which can contain a very wide variety of sound complexity; dialogue, music, sound effects, background noise, sections of silence. These variations can occur in any combination at any point in time. In a typical movie soundtrack, for most of the time a bitrate of approximately 384 kbit/s is sufficient to encode the 5.1 channels fully 'transparently' (i.e, indistinguishable from the original). However, the peak bit rate needed for transparency can extend to over 600 kbit/s for some particularly complex sections, e.g. at times where there is music, sound effects, back- ground noise and dialogue all happening at the same time. What are the benefits over constant bit rate encoding'!

Operating at a certain (average) bitrate, a fixed or constant bit rate encoder provides variable quality. For fragments that are simple to encode, the constant bit rate encoder is applying a bitrate that is higher than is really needed. For complex sections the bit rate available is lower than that required, and artefacts may become noticeable. A constant bit rate encoder would have to operate at a bit rate high enough to encode the most complex sections transparently, i.e. around 600 kbit/s, but a variable bit rate encoder can operate at the aver- age bit rate, namely around 384 kbit/s, which represents a significant increase in encoding efficiency.

A variable bit rate encoder provides fixed quality. It always applies the number of bits necessary to encode without noticeable artefacts. It has been suggested that a constant bit rate encoder, constantly operating at the same peak bit rate as the variable bit rate encoder, will sound equally good or better. But the variable bit rate encoder needs the high peak bit rate only for a fraction of the total time. Applying a fixed bit rate is clearly excessive. Another variable bit rate bitstream with e.g. a second language version could easily be added instead

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