OPTICODEC-PC INTRODUCTION
1-13
A low-bandwidth signal in the compressed bit stream provides “hints” to modulate
these created high frequencies so that they will match the original high frequencies
as closely as possible. Adding SBR to the basic AAC codec creates aacPlus, which of-
fers the best subjective quality currently available at bitrates below 128 kbps. At bi-
trates below 128 kbps, full subjective transparency cannot be achieved at the cur-
rent state of the art, yet the sound can still be very satisfying. (In the phraseology of
the ITU 1 to 5 subjective quality scale, this means that audible differences introduced
by the codec are judged by expert listeners to be “detectable, but not annoying.”)
Coding Technologies’ aacPlus v2, the latest in MPEG-4 Audio and previously known
as "Enhanced aacPlus," is aacPlus coupled with the new MPEG Parametric Stereo
technique created by Coding Technologies and Philips. Where SBR enables audio
codecs to deliver the same quality at half the bitrate, Parametric Stereo enhances
the codec efficiency a second time for low-bitrate stereo signals. Both SBR and Pa-
rametric Stereo are backward- and forward-compatible methods to enhance the ef-
ficiency of any audio codec. As a result, aacPlus v2 delivers streaming and
downloadable 5.1 multichannel audio at 128 Kbps, near CD-quality stereo at 32
Kbps, excellent quality stereo at 24 Kbps, and great quality for mixed content down
to 16 Kbps and below.
MPEG standardized Coding Technologies’ aacPlus as MPEG-4 HE AAC (MPEG ISO/IEC
14496-3:2001/AMD-1: Bandwidth Extension). With the addition of MPEG Parametric
Stereo (MPEG ISO/IEC 14496-3:2001/AMD-2: Parametric coding for high quality au-
dio), aacPlus v2 is the state-of-the-art in low bitrate open standards audio codecs.
The Coding Technologies codecs provide the absolute best possible sound per bit the
current state-of-the-art will allow, without the typical resonant, phasey, watery
character of other codecs.
Trading-Off Audio Bandwidth against Bitrate, Sample rate, and Channel Mode
High audio bandwidth does not guarantee good sound in codecs. In many cases,
especially at low bitrates, it is actually just the opposite. For example, FM radio is a
15 kHz medium, yet there are plenty of codecs claiming to have 20 kHz response
that sound much worse than FM radio.
The designers of the various codecs usually determine the optimum tradeoff be-
tween bitrate, sample rate, and channel mode (stereo or mono) by performing ex-
tensive listening tests. To maximize overall audio quality at lower bitrates, it is im-
portant to allocate the bits efficiently. This usually means allocating more bits to
those frequency ranges most important to music and speech.
Below a certain sample rate (which depends on the design of the individual codec),
codec designers have determined that limiting audio bandwidth to less than 20 kHz
achieves highest overall quality. For example, AAC requires 192 Kbps or more for 20
kHz+ response (Table 1-2 on page 1-12) and aacPlus requires 64 Kbps or more for 20
kHz+ response (Table 1-1 on page 1-11).
We recommend using aacPlus v2 for stereo streams below 48kbps. Be sure your tar-
get players support it; otherwise, the streams will play in mono.
