Examples of audio coding
Table of Contents
 Transform coding with timedomain aliasing cancellation based on an alternate application of 512point modified discrete sine and cosine transforms (MDCT and MDST)
 frequency resolution 93.75 Hz
 The 512point transform is done every 256 points (overlap)
 if necessary, preechoes can be prevented by reducing the block size to 256 points
 Adjacent tramsform coefficients are then grouped into subband ranges which approximate the critical bands of human hearing.
 From the absolute values of the transform coefficients within such subbands a logspectral envelope of the current block is estimated and used for a dynamic bit allocation routine.
 The coder uses no perceptual model except the critical band division.
 AT&T`s Perceptual Audio Coder (PAC) extends the idea of perceptual coding to stereo pairs.
 Transform coding
 PAC uses both L/R (left(right) and M/S (sum/difference) stereo coding, switched both in frequency and time in a signal dependent fashion.
 In M/S stereo coding, the sum and difference signals (L+R and LR) are coded instead of left and right signals.
 Hybrid (subband/transform) coding
 Used in MiniDisc
 In ATRAC the signal is first split into three subbands
(05.5, 5.511.0, and 11.022.0 kHz)
 A modified DCT with 50 % overlap is then applied to all subbands combined with dynamic window switching.
 The AC3 multichannel coder is a transform coder with Dolby`s AC2 filterbank at its core.
 The frequency resolution is 93.75 Hz.
 Bit allocation can occur down to the individual transform coefficient level, with neighboring coefficients obtaining different allocations.
 AC3 employs a core backwardadaptive bit allocation routine which runs in both the encoder and the decoder
 bit allocation routine controlled by a logdomain spectral envelope
 based on a psychoacoustical model
 certain parameters are explicitly sent to the decoder by which the actual psychoacoustic model in the decoder can be readjusted by the encoder
 If the encoder uses an improved bit allocation routine it can compare its results with that of the core routine used by the decoder and send correction information to finetune the decoder`s bit allocation. The final bit allocations of the encoder and decoder are, of course, identical.
 Masking is exploited during periods of high bit demand to reduce the overall bit rate.
 It takes advantage of the fact that ear is not able to independently detect the direction of two high frequency signals which are close in frequency.
=> A number of individual channel transform coefficients can be combined into a common coefficient.

 Dolby AC3 is a 5.1 multichannel audio system that encodes the channel separately, i.e., without compatibility matrixing thus avoiding "unmasking" artifacts.