HFC-4S Cologne Chip AG, HFC-4S Datasheet - Page 196

• Manufacturer Part Number: HFC-4S
• Description: Isdn HDLC Fifo Controller With 8 (4) Integrated S/t Interfaces
• Manufacturer: Cologne Chip AG

HFC-4S
HFC-8S
8.1 Conference unit description
The HFC-4S / 8S has a built in conference unit which allows up to 8 conferences with an arbitrary
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 con-
ference 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-4S / 8S 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 conference
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 probability 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 S/T interface
As the conference unit is located in the PCM transmit data path, some additional explanations for
conference members on the S/T interface have to be made.
Conference members can also be B-channels of the S/T interface. In this case, a pair of trans-
mit / receive PCM time slots have to be configured to loop back the data.
In detail, the conference signal on S/T-channel[
the signal is looped-back from slot[ ,RX] to HFC-channel[
channel[
although the FIFOs are not used to store data (see Section 3.4).
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Ò
,RX] and HFC-channel[
Multiparty audio conferences
Ñ
,TX] require one transmit and one receive FIFO to be enabled,
Data Sheet
Ò
,RX] gets assigned to PCM time slot[ ,TX] and
Ñ
,TX]. The data transmission on HFC-
March 2003 (rev. A)
Cologne
Chip