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| Appendix B
MPEG-4 High Efficiency AAC (HE-AAC, previously known as “aacPlus™”)
HE-AAC is an extension of AAC with the addition of Spectral Band Replication, a technique (developed by Coding
Technologies) of synthesizing high frequency audio content based on the lower frequency data and side-chain
information. SBR dramatically increases the efficiency of coding when using low bitrates. HE-AAC v1 is AAC plus
SBR, while HE-AAC v2 adds parametric stereo to further increase the efficiency of coding stereo signals. For more
on the SBR technology, see the papers on the Telos web site about “aacPlus,” which was the name of this codec
before acceptance by MPEG.
MPEG-4 Enhanced Low Delay AAC
High audio quality, low coding delay, and very low data rates: AAC Enhanced Low Delay (AAC-ELD) is the perfect
choice for any delay sensitive application that demands full audio bandwidth at data rates down to 24kbps.
AAC-ELD combines the strengths of its two main components, MPEG-4 AAC Low Delay and Spectral Band
Replication (SBR). Whereas MPEG-4 AAC-LD features low encoding/decoding latency, SBR provides high quality
audio at very low bit rates. SBR is also used in MPEG-4 HE-AAC, one of today’s most efficient audio codecs.
AAC-ELD became an MPEG standard in January 2008.
ISO/MPEG Layer 2
MPEG Layer 2 was an extremely popular early perceptual coding method, primarily because it’s easier and less
expensive to implement – particularly at the encoder – and practical devices using it were available earlier than
Layer 3. It’s a preferred choice for applications where very large data capacity is available, such as satellite links,
high capacity Primary ISDN or T1 circuits, and hard disk storage systems using Ethernet for signal distribution.
We include it in the Z/IP ONE to offer compatibility with the widest variety of codecs, and for use at high bit rates.
Layer 2 J-Stereo
The Layer 2 joint stereo mode uses an “intensity coding” method. This method has high coding power and is quite
effective, but hurts stereo separation on some program material. Audio above 3 kHz or so is combined to mono and
panned to one of seven positions across the stereo stage.
G.722
This technology pre-dates perceptual coding. It is much simpler than the transform methods, but suffers from
poorer audio performance. It has the benefit of low cost and the unique advantage of low delay. It has been around
as an international standard the longest, and is probably the most widely used system. In our view, this technology
is acceptable for mono voice where high fidelity is not necessary. It is good also for cueing and intercom channels.
We have included G.722 in the Z/IP ONE because:
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It had been the most popular coding method early on, so there are many of these codecs in use. Because it
is a standard, codecs from various manufacturers have a good probability of being able to interwork with
one another.
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G.722 has the lowest delay of all popular coding methods.
