PSB7280 datasheet(18/190 Pages) SIEMENS

18 / 190 page

PSB 7280

Semiconductor Group

Data Sheet 1998-07-01

1.7

System Integration

Example of integration in ISDN/analog videophone:

The first example represents a low-cost solution for a desk-top standalone videophone

that connects to an ISDN S0 bus (ISDN basic access) or an analog telephone line.

The ISDN basic access consists of two 64 kbit/s so-called B-channels to carry user

information (voice, data,

…), and a separate 16 kbit/s D-channel primarily used for

signaling. The video and audio are both compressed so that they are carried, along with

additional control information, in the two B-channels, or 128 kbit/s.

The general aspects of videotelephony are covered by ITU-T H.320 recommendation.

The video is compressed according to the H.261 (sometimes called “p

× 64”) or the

H.263 recommendation.

For the ISDN videophone (H.320) the compressed video and audio signals are

multiplexed together with additional synchronization and control information into two

B-channels, which are separately switched via the network and thus have to be

resynchronized at the other end. The multiplexing and resynchronization of the

B-channels is specified by the H.221 recommendation (see Figure 4).

Figure 4

Using non-parametric compression techniques, audio can be compressed to 64 kbit/s

PCM (logarithmical A- or

µ-law approximation for 3.1-kHz voice acc. to G.711) or

48/56/64 kbit/s sub-band coded adaptive PCM (for 7-kHz audio acc. to G.722). This

leaves, however, only approximately 64 kbit/s for video on the ISDN which, at this rate,

yields only a marginally good picture quality. For the analog videophone it’s not even

possible to transfer only audio at this data rate.

In order to make the best possible use of the total bandwidth and obtain the best possible

video quality, the audio should require only a small fraction of the total data rate. This is

made possible by using parametric compression techniques such as LD-CELP

(16 kbit/s). Above all, the corresponding norms (G.728) are internationally adopted

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