Data Compression 585systems accept PCM dual channel, digital audio (in the form of one or more AES pairs)
is windowed over small time periods and transformed into the frequency domain by
means of subband fi lters or via a transform fi lter bank. Masking effects are then computed
based on a psychoacoustic model of the ear. Note that blocks of sample values are used
in the calculation of masking. Because of the temporal, as well as frequency dependent,
effects of masking, it’s not necessary to compute masking on a sample-by-sample basis.
However, the time period over which the transform is performed and the masking effects
computed are often made variable so that quasi-steady-state signals are treated rather
differently to transients. If coders do not include this modifi cation, masking can be
predicted incorrectly, resulting in a rush of quantization noise just prior to a transient
sound. Subjectively this sounds like a type of pre-echo. Once the effects of masking are
known, the bit allocation routine apportions the available bit rate so that quantization
noise is acceptably low in each frequency region. Finally, ancillary data are sometimes
added and the bit stream is formatted and encoded.
19.4.1 Intensity Stereo Coding
Because of the ear’s insensitivity to phase response above about 2 kHz, further coding
gains can be achieved by sending by coding the derived signals (L R) and (L R )
rather than the original left and right channel signals. Once these signals have been
transformed into the frequency domain, only spectral amplitude data are coded in the HF
region; the phase component is simply ignored.
19.4.2 The Discrete Cosine Transform
The encoded data’s similarity to a Fourier transform representation has already been
noted. Indeed, in a process developed for a very similar application, Sony’s compression
scheme for MiniDisc actually uses a frequency domain representation utilizing a variation
of the discrete fourier transform (DFT) method known as the discrete cosine transform
(DCT). The DCT takes advantage of a distinguishing feature of the cosine function, which
is illustrated in Figure 19.2 , that the cosine curve is symmetrical about the time origin. In
fact, it’s true to say that any waveform that is symmetrical about an arbitrary “ origin ” is
made up of solely cosine functions. This is diffi cult to believe, but consider adding other
cosine functions to the curve illustrated in Figure 19.2. It doesn’t matter what size or
what period waves you add, the curve will always be symmetrical about the origin. Now,
it would obviously be a great help, when we come to perform a Fourier transform, if we