MT92210 Zarlink Semiconductor, MT92210 Datasheet - Page 126

• Manufacturer Part Number: MT92210
• Description: 1023 Channel Voice Over ip (VoIP) Processor
• Manufacturer: Zarlink Semiconductor

126
9.3.1
When interfacing with external compression or silence suppression agents in xxPCM, the MT92210 uses special
data formats that allow it to pass more than payload information over the H.110 bus. When transmitting ADPCM
data, the position of the highest ‘1’ in the data sample indicates the type of compression used: in this way, all
ADPCM compression types can be encoded within a single payload byte. In addition, one extra piece of information
can be passed to an off-chip ADPCM decompression agent: an ADPCM reset flag. This flag is set on the first byte
following a silence period and it indicates to the ADPCM decompression agent to reset the ADPCM transcoder to
default values. This is a requirement after silence periods because no ADPCM data has been sent during the
silence period, causing the compression and decompression agents to be out of sync. One solution to this problem
is to reset both at the beginning of valid voice.
The implicit compression type as well as the ADPCM reset flag can be coded within an 8-bit sample. The ADCPM
sample may also be contained in the high bits of the byte: in this case, the decoding is reversed, the position of the
lowest ‘1’ in the data sample indicates the compression type.
An extra time slot is needed to encode PCM values along with the ADPCM values. If the compression rate can
change to PCM as well as ADPCM, then 512 new values appear: 256 PCM values, and 256 PCM values with a
reset flag. Despite the data being PCM, the ADPCM decompression agent may use it to synchronize its CODEC
during the period of time where the data is PCM, so that if the decompression rate reverts to ADPCM, its context is
valid. Therefore 2 extra bits on an associated stream are used.
The RX TDM is also capable of passing spectral CN packets on the H.110 bus. The encoding of these packets is
done by passing 80h on the bus as a header, followed by the packet and trailed by a 16-bit CRC for data integrity
purposes. When spectral CN packets are used, no padding is inserted by the chip: invalid bytes are indicated by a
00h code on the bus.
RX TDM Data Formats
16K bytes:
32K bytes:
64K bytes:
256 bytes:
512 bytes:
1K bytes:
2K bytes:
4K bytes:
8K bytes:
Figure 70 - RX Circular Buffer Base and Size
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Zarlink Semiconductor Inc.
RX Circular Buffer Base [20:9]
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