MC33219A Datasheet, PDF (21/28 Pages) Motorola, Inc – Voice Switched Speakerphone

Freescale SMeCm33i2c1o9Anductor, Inc.
experience has shown to be a “comfortable response” by
the circuit.
2) The distortion in the receive attenuator will increase as
the CT capacitor value is decreased. The extra THD will be
most noticeable at the lower frequencies and at the lower
ampitudes. Table 1 provides a guideline for this issue.
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{Table 1. THD versus CT Capacitor}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ^CT
Idle – Rx
Capacitor Transition
Input
@ RAI
Freq.
THD
@ RAO
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{15 µF}
25 ms
20 mVrms 300 Hz 1.2%
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0 KHz 0.25%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{100 mVrms 300 Hz 0.5%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0KHz 0.2%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{6.8 µF}
12 ms
20 mVrms 300 Hz 5.0%
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0KHz 0.7%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{100 mVrms 300 Hz 1.3%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0 KHz 0.35%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{3.0 µF}
5.0 ms
20 mVrms 300 Hz 11%
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0KHz 1.8%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{100 mVrms 300 Hz 2.6%}
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ _{1.0KHz 0.7%}
Considerations in the Design of a Speakerphone
The design and adjustment of a speakerphone involves
human interface issues as well as proper signal levels.
Because of this fact, it is not practical to do all of the design
mathematically. Certain parts of the design must be done by
trial and error, most notably the switching response and the
“How does it sound?” part of the testing. Among the
recommendations for a successful design are:
1) Design the enclosure concurrently with the
electronics. Do not leave the case design to the end as its
properties are just as important (just as equally important) as
the electronics. One of the major issues involved in a
speakerphone design is the acoustic coupling of the speaker
to the microphone, which must be minimized. This
parameter is dependent entirely on the design of the
enclosure, the mounting of the speaker and the microphone,
and their characteristics.
2) Ensure the speaker is optimally mounted. This fact
alone can make a difference of several dB in the sound level
from the speaker, as well as the sound quality. The speaker
manufacturer should be consulted for this information.
3) Do not breadboard the circuit with the microphone and
speaker hanging out in midair. It will not work. The speaker
and microphone must be in a suitable enclosure, preferably
one resembling the end product. If this is not feasible,
temporarily use some other properly designed enclosure,
such as one of the many speakerphones on the market.
4) Do not breadboard the circuit on a wirewrapped board
or a plug–in prototyping board. Use a PC board, preferably
with a ground plane. Proper filtering of the supply voltage at
the VCC pin is essential.
5) The speakerphone must be tested with the intended
hybrid and connected to a phone line or phone line simulator.
The performance of the hybrid is just as important as the
enclosure and the speakerphone IC.
6) When testing the speakerphone, be conscious of the
environment. If the speakerphone is in a room with large
windows and tile floors, it will sound different than if it is in a
carpeted room with drapes. Additionally, be conscious of the
background noise in a room.
7) When testing the speakerphone on a phone line, make
sure the person at the other end of the phone line is not in the
same room as the speakerphone.
Design Procedure
A recommended sequence follows in Figure 31,
assuming the end product enclosure is available, with the
intended production microphone and speaker installed, and
the PC boards or temporary substitutes installed.
Figure 31. Basic Block Diagram for Design Purposes
VM
Mike
MCI ^Amp MCO TAI
TAO
Tx Attenuator
Microphone
V1
R1
I1
Acoustic
Coupling
(GAC)
Control
Speaker
Speaker
Amp
Rx Attenuator
RAO
RAI
TLI
RLI
I2
R2
V2
RXO
Tip
GST Hybrid
Ring
RXI
MOTOROLA ANALOG IC DEVICE^FD^oA^rT^MA ^{orGeoIntfoo:rwmwatwio.fnreOenscTahlies.cPoromduct,}
21