mt91l61asr1 Zarlink Semiconductor, mt91l61asr1 Datasheet - Page 5

• Manufacturer Part Number: mt91l61asr1
• Description: 3 V Multi-featured Phone Codec With Programmable U/a Law Companding
• Manufacturer: Zarlink Semiconductor

MT91L60/61
Data Sheet
Functional Description
Filter/Codec
The Filter/Codec block implements conversion of the analog 0-3.3 kHz speech signals to/from the digital domain
compatible with 64 kb/s PCM B-Channels. Selection of companding curves and digital code assignment are
programmable. These are ITU-T G.711 A-law or µ-Law, with true-sign/Alternate Digit Inversion or true-sign/Inverted
Magnitude coding, respectively. Optionally, sign-magnitude coding may also be selected for proprietary
applications.
The Filter/Codec block also implements transmit and receive audio path gains in the analog domain. A
programmable gain, voice side-tone path is also included to provide proportional transmit speech feedback to the
handset receiver. This side tone path feature is disabled by default. Figure 3 depicts the nominal half-channel and
side-tone gains for the MT91L60/61.
In the event of PWRST, the MT91L60/61 defaults such that the side-tone path is off, all programmable gains are set
to 0dB and ITU-T µ-Law is selected. Further, the digital port is set to SSI mode operation at 2048 kb/s and the FDI
and driver sections are powered up. (See Microport section.)
The internal architecture is fully differential to provide the best possible noise rejection as well as to allow a wide
dynamic range from a single 3 volt supply design. This fully differential architecture is continued into the Transducer
Interface section to provide full chip realization of these capabilities for the handset functions.
A reference voltage (V
), for the conversion requirements of the Codec section, and a bias voltage (V
), for
Ref
Bias
biasing the internal analog sections, are both generated on-chip. V
is also brought to an external pin so that it
Bias
may be used for biasing external gain setting amplifiers. A 0.1µF capacitor must be connected from V
to analog
Bias
ground at all times. Although V
may only be used internally, a 0.1µF capacitor must be connected from V
to
Ref
Ref
ground. The analog ground reference point for these two capacitors must be physically the same point. To facilitate
this the V
and V
pins are situated on adjacent pins.
Ref
Bias
The transmit filter is designed to meet ITU-T G.714 specifications. The nominal gain for this filter is 0 dB (gain
control = 0 dB). Gain control allows the output signal to be increased up to 7 dB. An anti-aliasing filter is included.
This is a second order lowpass implementation with a corner frequency at 25 kHz.
The receive filter is designed to meet ITU-T G.714 specifications. The nominal gain for this filter is 0 dB (gain control
= 0 dB). 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.
Side-tone is derived from the input of the Tx filter 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 gain control
bits located in Gain Control Register 2 (address 01h).
Transmit and receive filter gains are controlled by the TxFG
-TxFG
and RxFG
-RxFG
control bits, respectively.
0
2
0
2
These are located in Gain Control Register 1 (address 00h). Transmit filter gain is adjustable from 0dB to +7dB and
receive filter gain from 0dB to -7dB, both in 1dB increments.
Side-tone filter gain is controlled by the STG
-STG
control bits located in Gain Control Register 2 (address 01h).
0
2
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 Smag/ITU-T control bit. The A/µ control bit is logically OR’ed with the A/µ pin providing
access in both controller and controllerless modes. Both A/µ and Smag/ITU-T reside in Control Register 2 (address
04h). Table 1 illustrates these choices.
5
Zarlink Semiconductor Inc.