MT92210 Datasheet, PDF (74/175 Pages) Zarlink Semiconductor Inc – 1023 Channel Voice Over IP Processor

Data Sheet
MT92210
The CN energy is calculated by summing the energies of the most recent samples received. Depending on whether
the current state is voice or silence, the energy will be summed on a different number of samples, using the First
Energy Period and Subsequent Energy Period respectively. This is done because energy on background noise and
silence can be calculated on a much longer period to obtain greater accuracy. The sum of samples is kept in the
Energy Sum and the sample count is kept in the Energy Counter. When the Energy Counter reaches the terminal
count, a linear-to-dB conversion is performed to take the Energy sum divided by the Energy Counter to obtain the
average linear energy, then converted to a logarithmic scale to obtain an energy in dBov. The TX Silence
Suppression Structure contains a dB Correction that allows the dB energy to be converted to any scale, like dBm0
instead of dBov, for example. There is also a Maximum dB Value and a Minimum dB Value that can be set, making
sure that the value in CN packets stays within a certain range.
The silence suppression information can also be fed to the process externally. When this is the case, a special PCM
code is used on the TDM bus. In this mode, 2 TSSTs must be used to feed the MT92210 with the transmit data.
When the last PCM byte of the packet is read off the H.110 bus is 00h and the associated time slot upper bit also
indicates 0, this means that the packet must be suppressed. Note that the TX unsigned PCM magnitude is
transmitted on the lower 7 bits of the associated time slot. If the code received is normal data (upper bit 1), then the
packet is valid. In this case, the external agent makes all the suppress/don't suppress decisions, but the MT92210
still calculates the CN Energy and decides when to send and when not to send CN packets.
The MT92210 also supports many modes in which it can send CN packets. In the most common configuration, it
will suppress silence packets and send CN packets at the beginning of silence periods, or whenever it decides to
update. It is also possible to configure it to suppress silence packets but never to send CN packets at any time. It
also supports two modes in which it does not suppress packets. In the first one, it uses the marker bit in RTP to
indicate the beginning of a voice period; in the second, all silence packets are indicated with a marker bit = ‘1’, as
well as the first packet at the beginning of a voice period. Finally, instead of calculating its own CN energy and
generating the packet itself, the MT92210 can use a preprogrammed CN packet. This preprogrammed packet can
be of variable length, allowing spectral data to be passed along as well as an energy value. Preprogrammed
packets must be written into external SSRAM A by an agent through the CPU interface and may be updated
periodically. Because a pointer is given to the preprogrammed packet, its payload may be changed in a glitch-free,
atomic way. Spectral CN packets may be up to 64 bytes in length.
The silence suppression process also monitors and counts underruns that occur on the RX TDM side. The
underrun information from the RX side is passed along and is written to the TX circular buffer. The silence
suppression process can thus keep a 16-bit Underrun Count of the number of underrun events that have occurred.
The format of the TX Silence Suppression Structure varies according to whether or not the silence suppression
decision is being taken internally or if it is being fed from an outside agent, and according to whether or not the
generated CN packet will be a white energy value or a preprogrammed spectral value.
Zarlink Semiconductor Inc.
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