MT9196
The receive filter is designed to meet CCITT G.714
specifications. The nominal gain for this filter path is
0 dB (gain control = 0dB). Gain control allows the
output signal to be attenuated up to 7 dB. Filter
response is peaked to compensate for the sinx/x
attenuation caused by the 8 kHz sampling rate.
The Rx filter function can be altered by enabling the
Dial EN control bit in Control Register 1 (address
0Eh). This causes another low-pass function to be
added with a 3 dB point at 1000 Hz. This function is
intended to improve the sound quality of digitally
generated dial tone received as PCM.
Side-tone is derived from the Tx filter before the LP/
HP filter section and is not subject to the gain control
of the Tx filter section. Side-tone is summed into the
receive handset transducer driver path after the Rx
filter gain control section so that Rx gain adjustment
will not affect side-tone levels. The side-tone path
may be enabled/disabled with the Voice sidetone bit
located in the Receive Path Control Register
(address 13h).
Transmit and receive filter gains are controlled by the
TxFG
0
-TxFG
2
and RxFG
0
-RxFG
2
control bits,
respectively. These are located in the FCODEC
Control Register 1 (address 0Ah). Transmit filter gain
is adjustable from 0 dB to +7 dB and receive filter
gain from 0 dB to -7 dB, both in 1 dB increments.
Side-tone filter gain is controlled by the STG
0
-STG
2
control bits located in the FCODEC Control Register
2 (address 0Bh). Side-tone gain is adjustable from
-9.96 dB to +9.96 dB in 3.32 dB increments.
Companding law selection for the Filter/CODEC is
provided by the A/µ companding control bit while
the coding scheme is controlled by the sign-mag/
CCITT control bit. Both of these reside in Control
Register 2 (address 0Fh). Table 1 illustrates these
choices.
Sign/
Magnitude
1111 1111
1000 0000
0000 0000
0111 1111
Preliminary Information
Digital Gain and Tone Generation
The Digital gain and Tone generator block is located,
functionally, between the serial FDI port and the
Filter/CODEC block. Its main function is to provide
digital gain control of the transmit and receive audio
signals and to generate digital patterns for DTMF
and tone ringer signals.
Gain Control
Gain control is performed on linear code for both the
receive and the transmit PCM. Gain control is set via
the Digital Gain Control Register at address 19h.
Gain, in 3.0 dB increments, is available within a
range of +21.0 dB to -24 dB.
DTMF Generator
The digital DTMF circuit generates a dual sine-wave
pattern which may be routed into the receive path as
comfort tones or into the transmit path as network
signalling. In both cases the digitally generated
signal will undergo gain adjustment as programmed
into the transmit and receive gain control registers.
Gain control is assigned automatically as functions
are selected via the transmit and receive path control
registers.
The composite signal output level in the transmit
direction is -4 dBm0 (µ-Law) and -10 dBm0 (A-law)
with programmable gains at zero dB. Pre-twist of 2.0
dB is incorporated into the composite signal resulting
in a low tone output level of -8.12 dBm0 and a high
group level of -6.12dBm0 (for
µ-Law,
6 dB lower for
A-Law). Note that these levels will be influenced by
the Anti-Howling circuit when it is enabled (see Anti-
Howling section for more details). DTMF side-tone
levels are set to -28 dBm0 from the generator circuit.
Other receive path gains must be included when
calculating the analog output signal levels.
Adjustments to these levels may be made by altering
the settings of the Gain Control register (address
19h).
The frequency of the low group tone is programmed
by writing an 8-bit coefficient into the Low Tone
Coefficient Register (address 1Ah) while the high
group tone frequency uses the 8-bit coefficient
programmed into the High Tone Coefficient Register
(address 1Bh). Both coefficients are determined by
the following equation:
Code
+ Full Scale
+ Zero
-Zero
(quiet code)
- Full Scale
CCITT (G.711)
µ-Law
1000 0000
1111 1111
0111 1111
0000 0000
A-Law
1010 1010
1101 0101
0101 0101
0010 1010
Table 1
The Filter/CODEC autonull circuit ensures that
transmit PCM will contain no more than
±1
bit of
offset due to internal circuitry.
Frequency (in Hz) = 7.8125 x COEFF
Where COEFF is an integer between 0 and 255.
Frequency resolution is 7.8125 Hz in the range 0 to
1992 Hz.
7-132
MITEL [ MITEL NETWORKS CORPORATION ]
