HFC-S2M Cologne Chip AG, HFC-S2M Datasheet - Page 200

• Manufacturer Part Number: HFC-S2M
• Description: Isdn HDLC Fifo Controller With Primary Rate Interface
• Manufacturer: Cologne Chip AG

Cologne
HFC-E1
Multiparty audio conferences
Chip
8.1 Conference unit description
The HFC-E1 has a built in conference unit which allows up to 8 conferences with an arbi-
trary number of members each. The conference unit is located in the data stream going out
to the PCM interface. So the normal outgoing data is replaced by the conference data. The
number of conference members that can be combined to one conference is only limited by
the number of the PCM time slots (maximum 64 members with 128 PCM time slots). Each
time slot can only be part of one conference.
All PCM values combined to a conference are added in one 125 s time intervall. Then
for every conference member the added value for this member is substracted so that every
member of a conference hears all the others but not himself. This is done on a alternating
buffer scheme for every 125 s time intervall.
To enable the conference unit the bit V_CONF_EN in the register R_CONF_EN must be
set. If this is done there are additional accesses to the SRAM of HFC-E1 which reduces
performance of the on-chip processor on the other hand. Thus conference cannot be used
with 8 Mbit/s PCM data rate where 128 slots are used, except the chip operates with doubled
input frequency.
To add a PCM time slot to a conference the slot number must be written into the register
R_SLOT. If the time slot has not yet been linked to a HFC-channel this can be done by
writing the HFC-channel number and the channels source / destination (input / output pins)
to the A_SL_CFG register. Afterwards the conference number must be written into the
A_CONF register. Noise suppression threshold and input attenuation level can be configured
independently for each time slot.
To remove a time slot from a conference the time slot must be selected by writing its number
to the R_SLOT register. Then 0x00 must be written into the A_CONF register.
8.2 Overflow handling
The data summation of the conference HFC-channels can cause signal overflows. The con-
ference unit internally works with signed 16 bit words. In case of an overflow the amplitude
value is limited to the maximum amplitude value.
Overflow conditions can be checked with the R_CONF_OFLOW register. Every bit of this
register indicates that an overflow has occured in one of the eight corresponding conferences.
The more conference members are involved in a conference, the higher is the probabil-
ity of signal overflows. In this case the signal attenuation can be reduced by the bitmap
V_ATT_LEV in the register A_CONF. This can be done on-the-fly to improve the signal
quality of a conference.
8.3 Conference including the E1 interface
As the conference unit is located in the PCM transmit data path, some additional explanations
for conference members on the E1 interface have to be made.
200 of 272
Data Sheet
March 2003 (rev. A)