Preliminary Information
mode. Similarily, the four 2-bit ADPCM words on
ADPCMi are transparently bypassed, with a two
frame delay, to PCMo1 and PCMo2 during the ENB1
or B1 timeslot. Bits 3-8 are don’t care. (See Figures 6
& 9.)
When SEL is set to 1, the same bypass occurs as
described when SEL = 0 except that the ENB2 or B2
timeslots are utilized.
LINEAR, FORMAT and A/µ pins are ignored in
bypass mode.
MT9126
When the decoders are programmed for PCMo1/2
disable (MS4 to MS6 set to 1) the PCMo1/2 outputs
are high impedance during the B Channel timeslots
and also, during ST-BUS operation, the D and C
channel timeslots according to the state of ENB1.
Therefore convergence is maintained. The encode
processing function and data I/O remain active.
Whenever any combination of the encoders or
decoders are set to the disable mode the following
outputs remain active. A) ST-BUS mode: ENB2/
F0od, EN1, EN2 and C2o. Also the “D” and “C”
channels from PCMo1 and ADPCMo remain active if
ENB1 is set to 0. If ENB1 is brought high then
PCMo1 and ADPCMo are fully tri-stated. B) SSI
mode: When used in the 16-bit linear mode, only the
EN1 output remains active. For complete chip power
down see PWRDN.
PCM BYPASS
When SEL is set to 0, the B1 and B2 PCM channels
on PCMi1 are transparently passed, with a two-
frame delay, to the same channels on the ADPCMo.
Simiarily, the two 8-bit words which are on ADPCMi
are transparently passed, with a two-frame delay, to
channels B1 and B2 of PCMo1 while PCMo2 is set
to a high-impedance state.(See Figures 6 & 9.)
When SEL is set to 1, the B3 and B4 channels on
PCMi2 are transparently passed, with a two frame
delay, to the same channels on ADPCMo. Similarily,
the two 8-bit words which are on ADPCMi are
transparently passed, with a two-frame delay, to
channels B3 and B4 of PCMo2. In this case PCMo1
is always high-impedance if ENB1 = 0. If ENB1 = 1
during ST-BUS operation then the D and C channels
are active on PCMo1.
LINEAR, FORMAT and A/µ pins are ignored in
bypass mode.
Other Pin Controls
16 Bit Linear PCM
Setting the LINEAR pin to logic one causes the
device to change to 16-bit linear (uniform) PCM
transmission on the PCMi/o1 and PCMi/o2 ports.
The data rate for both ST-BUS and SSI operation in
this mode is 2048 kbit/s and all decode and encode
functions are affected by this pin. In SSI mode, the
input channel strobes ENB1 and ENB2/F0od remain
active for 8 cycles of BCLK for an ADPCM transfer.
The EN1 output is high for one BCLK period at the
end of the frame (i.e., during the 256
th
BCLK period).
In ST-BUS mode, the output strobes EN1 and ENB2/
F0od are adjusted to accommodate the required
PCM I/O streams. The EN1 output becomes a single
bit high true pulse during the last clock period of the
frame (i.e., the 256
th
bit period) while ENB2/F0od
becomes a delayed, low true frame-pulse (F0od)
output occuring during the 64
th
bit period after the
EN1 rising edge.
Linear PCM on PCMi1 and PCMi2, are received as
14-bit, two’s complement data with three bits of sign
extension in the most significant positions (i.e.,
S,S,S,12,...1,0) for a total of 16 bits. The linear PCM
data transmitted from PCMo1 and PCmo2 are 16-bit,
two’s complement data with one sign bit in the most
significant position (i.e., S,14,13,...1,0)
Algorithm Reset Mode
While an algorithmic reset is asserted the device will
incrementally converge its internal variables to the
'Optional reset values' stated in G.726. Algorithmic
reset requires that the master clock (MCLK) and
frame pulse (ENB1/2 or F0i) remain active and that
the reset condition be valid for at least four frames.
Note that this is not a power down mode; see
PWRDN for this function.
ADPCMo & PCMo1/2 Disable
When the encoders are programmed for ADPCMo
disable (MS1 to MS3 set to 1) the ADPCMo output is
set to a high impedance state and the internal
encode
function
remains
active.
Therefore
convergence is maintained. The decode processing
function and data I/O remain active.
32 and 24 kbit/s ADPCM mode
In 32 kbit/s and 24 kbit/s linear mode, the 16-bit
uniform PCM dual-octets of the B1, B2, B3 and B4
channels (from PCMi1 and PCMi2) are compressed
into four 4-bit words on ADPCMo. The four 4-bit
ADPCM words of the B1, B2, B3 and B4 channels
8-39
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