1-10
INTRODUCTION ORBAN
MODEL
1010
distortion, or like warbling, comb filtering, or gurgling—an “underwa-
ter” sound.
Coding Efficiency
Different sounds will vary greatly in the efficiency with which a perceptual coding
system can encode them. Therefore, for a constant transmission bitrate, the mask-
to-noise ratio will constantly change. Pure sounds having an extended harmonic
structure (such as a pitch pipe) are particularly difficult to encode because each har-
monic must be encoded, the harmonics occupy many different frequency bands, and
the overall spectrum has many “holes” that are not well-masked, so that added
noise can be easily heard. The output of a multiband audio processor that uses clip-
ping is another sound that is difficult to encode, because the clipper creates added
distortion spectrum that does not mask quantization noise well, yet may cause the
encoder to waste bits when trying to encode the distortion.
Sophisticated encoders use a short “bit reservoir” to save up unused bits so they can
be applied to difficult-to-encode sounds. However, the length of the bit reservoir
will directly affect the coding delay, so dynamic allocation of bits occurs only over
rather short time windows (in the order of tens of milliseconds). Another feature of
sophisticated encoders is “redundancy reduction,” which encodes frequently ap-
pearing data with shorter digital words and infrequently appearing data with
longer words.
Encoding Stereo
Usually, there is some correlation between the left and right channels of a stereo
signal. At lower bitrates, one way to achieve higher quality is to exploit this correla-
tion when coding stereo information:
Depending on program content, the encoder dynamically switches between discrete
left/right coding and sum-and-difference coding. The difference signal often re-
quires fewer bits than the sum signal to encode with high audible quality, thereby
saving bits in the overall coding of the stereo signal.
There is no benefit to joint stereo coding when the two channels contain
independent information because there is no correlation between the
channels.
Opticodec-PC Codecs
Opticodec-PC offers two coding algorithms from the several standardized by
ISO/MPEG (Moving Pictures Experts Group): the AAC and aacPlus® v2 algorithms.
AAC is intended for very high quality coding with compression up to 12:1. The AAC
codec is about 30% more efficient than MPEG1 Layer 3 and about twice as efficient
as MPEG1 Layer 2. The AAC codec can achieve “transparency” (that is, listeners can-
not audibly distinguish the codec’s output from its input in a statistically significant
way) at a stereo bitrate of 128 kb/sec, while the Layer 2 codec requires about 256
kb/sec for the same quality. The Layer 3 codec cannot achieve transparency at any
bitrate, although its performance at 192 kbps and higher is still very good.
AAC stands for Advanced Audio Coding. Intended to replace Layer 3, AAC was de-
veloped by the MPEG group that includes Dolby, Fraunhofer (FhG), AT&T, Sony, and
Nokia—companies that have also been involved in the development of audio codecs
such as MP3 and AC3 (also known as Dolby Digital™).
