PSB7238 SIEMENS [Siemens Semiconductor Group], PSB7238 Datasheet - Page 18

• Manufacturer Part Number: PSB7238
• Description: Joint Audio Decoder-Encoder - Multimode
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

PSB 7238
Introduction
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 analog telephone line can carry up to 33.6 Kbit/s using a V.34+ modem.
The general aspects of videotelephony are covered by ITU-T H.320/H.324
recommendations. 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).
For the analog videophone (H.324) the compressed video and audio signals are
multiplexed together with additional control information into a single communication link.
The multiplexing is specified by the H.223 recommendation (see Figure 4).
Figure 4
Using non-parametric compression techniques, audio can be compressed to 64 Kbit/s
PCM (logarithmical A- or
-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.
Semiconductor Group
18
Data Sheet 1998-07-01