Telos ZephyrExpress user manual 722, Layer 2 Joint Stereo, Layer 2 Mono-128

Layer 2 Joint 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.

Layer 2 Mono-128

ZephyrExpress lets you combine ISDN channels and use the resulting 112/128kbps data stream for a single Layer 2 monaural channel. Since a bigger data path is available, this mode uses the least compression and can give you the highest mono quality for particularly critical applications.

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 ZephyrExpress because:

•It has been the most popular coding method until recently 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. (We’ve tested with many units and have found no problems so far.)

•G.722 has the lowest delay of all popular coding methods.

This method was invented in the late 70s and adopted as a standard in 1984 by the CCITT, the Consultive Committee for International Telephony and Telegraphy, a division of the United Nations. The technique used is Sub-Band ADPCM, Adaptive Delta Pulse Code

tModulation, which achieves data reduction by transmitting only the difference between successive samples. G.722 does this in two audio frequency sub-bands: 50-4kHz and 4kHz-7kHz.DEEP TECH NOTE!Only two bits are allocated per sample for audio frequencies above 4 kHz Ð

sufficient for conveying the sibilance in voice signals, but not very good for intricate musical sounds. Also, the Òpredictor modelÓ 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 distinct ÔfuzzyÕ quality.

G.722 has a frequency response extending to 7 kHz with fairly poor fidelity. Unless there is no alternative, it should not be used for music.

5-8 CODING