Dolby Audio Coders

Forward adaptive bit allocation

Fixed bit allocation(Dolby AC-1)

Backward adaptive bit allocation(Dolby AC-2)

Dolby AC-3

Linear predictive coding (LPC) signal encoder and decoder

• Linear predictive coding involves the source simply analyzing the audio waveform to determine a selection of the perceptual features it contains

Linear predictive coding With this type of coding the perceptual features

of an audio waveform are analysed first These are then quantized and sent and the

destination uses them, together with a sound synthesizer, to regenerate a sound that is perceptually comparable with the source audio signal

With this compression technique although the speech can often sound synthetic high levels of compressions can be achieved

In terms of speech, the three features which determine the perception of a signal by the ear are its:

Pitch: this is closely related to the frequency of the signal. This is important since ear is more sensitive to signals in the range 2-5kHz

Period: this is the duration of the signal Loudness: This is determined by the amount

of energy in the signal

Linear predictive coding The input speech waveform is first sampled and

quantized at a defined rate A block of digitized samples – known as segment

- is then analysed to determine the various perceptual parameters of the speech that it contains

The output of the encoder is a string of frames, one for each segment

Each frame contains fields for pitch and loudness – the period determined by the sampling rate being used – a notification of whether the signal is voiced (generated through the vocal cords) or unvoiced (vocal cords are opened)

And a new set of computed modal coefficients