AUDIO CODING REFERENCE |
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This method was invented in the late 70s and adopted as a standard in 1984 by the CCITT, the Consultative Com-
mittee for International Telephony and Telegraphy (renamed as ITU-T in 1993). The technique used is sub-band
ADPCM (Adaptive Delta Pulse Code Modulation), which achieves data reduction by transmitting only the difference
between successive samples. G.722 does this in two audio frequency sub-bands: 50 Hz to 4 kHz and 4 to 7 kHz.
Deep Tech Note!
Only two bits are allocated per sample for audio frequencies above 4 kHz. This is sufficient for
conveying the sibilance invoice signals, but is not very good for intricate musical sounds. Also, the
“predictor model” used to determine the step size in the adaptive function is designed only for
speech. This is why music transmitted via G.722 has a distinctly “fuzzy” quality.
G.722 has a frequency response extending to 7.5 kHz with fairly poor fidelity. Unless there is no alternative, it
should not be used for music.
aptX and aptX Enhanced
Like G.722, aptX also dates from the earliest days of audio coding, and is also based on ADPCM. However, it is
designed to run at much higher bit rates, with more bands, and much more faithful audio replication. Due to using
ADPCM, rather than psychoacoustic principles, it also has a very low delay. aptX ‘cascades’ well, making it suitable
for audio that needs to be recompressed and retransmitted, perhaps several times.
aptX was developed in the 1980s, and became a commercial product in 1990. In 2000, the algorithm was updated
into what is now known as aptX Enhanced. Compared with the original aptX, the Enhanced version of the codec
features lower delay and better encoding accuracy across the audio frequency range.
