
PSB 7280
Semiconductor Group
20
Data Sheet 1998-07-01
The video is captured by a PAL/NTSC camera and digitized and demodulated e.g. by a
standard SAA 7110 which is directly connected to the video processor. Alternatively, a
digital camera may be used, which can be connected directly to the video processor.
The video processor compresses and decompresses video according to the ITU-T
standards H.261 and multiplexes/demultiplexes video, audio and data according to
H.221/223. The video processor uses DRAMs and SRAMs to store data and program
code.
When operating in the ISDN mode, the H.221 multiplexed data stream is sent via the two
B-channels of the IOM-2 interface to the ISAC-S-TE (ISDN Subscriber Access Controller
for S-interface) which transmits them to the ISDN according to I.430 S0 interface
recommendation. The ISAC-S-TE also handles, together with the attached
microcontroller (e.g. SAB 83C515), D-channel layer-2 and layer-3 call control signaling.
The reverse functions are performed on the B-channels received from the network.
Instead of an S0, it is conceivable to implement any other layer-1 interface just by
replacing the ISAC-S TE by an appropriate transceiver, e.g. by a transceiver for
2-wire
digital transmission ISAC-P PSB 2196 or ISDN echo canceller for 2B1Q.
To achieve “l(fā)ip synchronization”, the audio may be delayed with respect to the video.
This is necessary because of the higher transmission delay suffered by the video signal,
due to the elaborate H.261/263 video compression. A delay of approximately
0.5 seconds is enough in most practical cases. To make maximum use of the existing
memory in the system, the delay is performed by the video processor with its external
RAMs.
In videophone applications calling for high quality, 7-kHz wide-band audio, the PSB 7280
can be switched to G.722 mode. In this case the rate of the compressed audio is 48, 56
or 64 kbit/s.
When decoding MPEG bitstreams, the audio D/A conversion is provided by a stereo
audio DAC.
Demonstration board designs for H.320 PC based videophones containing the chip set
from Siemens AG and 8
×
8 Inc. are available and can be ordered from Siemens/8
×
8.