MT9126
three of the serial I/O ports. BCLK can be any
frequency between 128 kHz and 4096 kHz
synchronized to the input strobes. BCLK may be
discontinuous outside of the strobe boundaries
except when LINEAR=1. In LINEAR mode, BCLK
must be 2048 kHz and continuous for 64 cycles after
the ENB1 rising edge and for the duration of ENB2/
F0od.
Preliminary Information
16 kbit/s ADPCM Mode
When SEL is set to 0, the 8-bit PCM octets of the B1,
B2, B3 and B4 channels (PCMi1 and PCMi2) are
compressed into four 2-bit ADPCM words on
ADPCMo during the ENB1 timeslot in SSI mode and
during the B1 timeslot in ST-BUS mode. Similarily,
the four 2-bit ADPCM words on ADPCMi are
expanded into four 8-bit PCM octets (on PCMo1 and
PCMo2) during the ENB1/B1 timeslot. (See Figures 4
& 7.)
When SEL is set to 1, The same conversion takes
place as described when SEL = 0 except that the
ENB2/B2 timeslots are utilized.
A-Law or
µ-Law
8-bit PCM are received and
transmitted most significant bit first starting with b7
and ending with b0. ADPCM data are most
significant bit first starting with I1 and ending with I2.
Mode Select Operation (MS1, MS2, MS3,
MS4, MS5, MS6)
Mode Select pins MS1, MS2 and MS3 program
different bit rate ADPCM coding, bypass, algorithmic
reset and disable modes for all four encoder
functions simultaneously. When 24 kbit/s ADPCM
mode is selected bit 4 is unused while in 16 kb/s
ADPCM mode all ADPCM channels are packed
contiguously into one 8-bit octet. Mode Select pins
MS4, MS5 and MS6 operate in the same manner for
the four decode functions. The mode selects must be
set up according to the timing constraints illustrated
in Figures 16 and 17.
ADPCM BYPASS (32 and 24 kbit/s)
In ADPCM bypass mode the B1 and B2 channel
ADPCM words are bypassed (with a two-frame
delay) to/from the ADPCM port and placed into the
most significant nibbles of the PCM1/2 port octets.
Note that the SEL pin performs no function for these
two modes (See Figures 6 & 9). LINEAR, FORMAT
and A/µ pins are ignored in bypass mode.
In 32 kb/s ADPCM bypass mode, Bits 1 to 4 of the
B1, B2, B3 and B4 channels from PCMi1 and PCMi2
are transparently passed, with a two frame delay, to
the same channels on ADPCMo. In the same
manner, the B1, B2, B3 and B4 channels from
ADPCMi are transparently passed, with a two frame
delay, to the same channels on PCMo1 and PCMo2
pins. Bits 5 to 8 are don’t care. This feature allows
two voice terminals, which utilize ADPCM
transcoding, to communicate through a system
without
incurring
unnecessary
transcode
conversions. This arrangement allows byte-wide or
nibble-wide transport through a switching matrix.
24 kb/s ADPCM bypass mode is the same as 32 kb/s
mode bypass excepting that only bits 1 to 3 are
bypassed and bits 4 to 8 are don’t care.
32 kbit/s ADPCM Mode
In 32 kbit/s ADPCM mode, the 8-bit PCM octets of
the B1, B2, B3 and B4 channels (PCMi1 and PCMi2)
are compressed into four 4-bit ADPCM words on
ADPCMo. Conversely, the 4-bit ADPCM words of the
B1, B2, B3 and B4 channels from ADPCMi are
expanded into four 8-bit PCM octets on PCMo1 and
PCMo2. The 8-bit PCM octets (A-Law or
µ-Law)
are
transferred most significant bit first starting with b7
and ending with b0. ADPCM words are transferred
most significant bit first starting with I1 and ending
with I4 (See Figures 4 & 7). Reference ITU-T G.726
for I-bit definitions.
24 kbit/s ADPCM Mode
In 24 kbit/s mode PCM octets are transcoded into 3-
bit words rather than the 4-bit words utilized in 32
kbit/s ADPCM. This is useful in situations where
lower bandwidth transmission is required. Dynamic
operation of the mode select control pins will allow
switching from 32 kbit/s mode to 24 kbit/s mode on a
frame by frame basis. The 8 bit PCM octets (A-Law
or
µ-Law)
are transferred most significant bit first
starting with b7 and ending with b0. ADPCM words
are transferred most significant bit first starting with
I1 and ending with I3 (I4 becomes don’t care). (See
Figures 4 & 7.)
ADPCM BYPASS (16 kbit/s)
When SEL is set to 0, only bits 1 and 2 of the B1, B2,
B3 and B4 PCM octets (on PCMi1 and PCMi2) are
bypassed, with a two frame delay, to the same
channels on ADPCMo during the ENB1 timeslot in
SSI mode and during the B1 timeslot in ST-BUS
8-38
MITEL [ MITEL NETWORKS CORPORATION ]
