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2.2.8.
MPEG Audio Layer 2 coding
Application: mono or stereo music, high quality.
As shown on Table 3, this coding algorithm features a maximum flexibility, with many variations for bit
rate, mono or stereo channel mode, sampling rates...
The two channel modes exist in three variations:
Stereo: coding of each channel stays independent
Dual mono: coding is similar to the previous case, but this choice applies to channel with no
acoustic relationship, e.g. two languages for commentaries
Joint stereo: applies to stereo programme, but here the encoder exploits the interchannel
correlation for improved coding. To be used only for a stereo programme
The 16 and 24 kHz sampling rates feature a moderate bandwidth (respectively 7 kHz and 10 kHz) and are
rather useful for commentaries.
Latency is rather high with these sampling rates
MPEG L2 is available for ISDN, wired or mobile IP networks.
ISDN mode specific aspects: J52
For ISDN transmission, MPEG L2 is proposed with two variations:
"MPEG L2 J52" variation, using the ITU-T J52 protocol for the link negotiation and inverse
multiplexing (B channel aggregation for connections requiring more than 64 kbit/s). Moreover,
optional protection against transmission errors (FEC) is also available. For more details refer to
2.1.2, ISDN interface (page 11).
MPEG L2" variation, without the J52 protocol, using a system for inverse multiplexing two B
channels which is proprietary but compatible with several codecs on the market.
2.2.9.
MPEG AAC algorithms
Application: mono or stereo music, low capacity transmission channels.
These algorithms feature a very high compression ratio, for a given audio quality, compared to Layer 2.
They can operate at a sampling rate of 32 or 48 kHz, and several bit rates: 16, 20, 24, 32, 40, 48, 56, 64,
96, 128, 192, 256 kbit/s. Three coding variations are available:
MPEG
AAC-LC
("Low Complexity"): lower compression than other variations, but lower latency.
MPEG
HE-AAC
("High Efficiency" AAC): higher compression, and the bit rate is limited to
128 kbit/s for this variation.
MPEG
HE-AAC v2
("High Efficiency" AAC version 2): compared to the above, this coding further
enhances the performance for a stereo program (not available for mono). The bit rate is limited
to 128 kbit/s for this variation.
AAC codecs are available for ISDN, and wired or mobile IP networks.