DESIGNING THE ISD33000 SERIES INTO
DIGITAL CELLULAR PHONES
by:
Prasanna Shah
President and CEO
Pragmatic Communication Systems, Inc.
and
Phillip Pyo
Product Marketing Manager
ISD
DESIGNING THE ISD33000 SERIES INTO DIGITAL CELLULAR PHONES
Introduction
Cellular phone manufacturers are constantly under pressure to release new models of
phones with compact designs and more user features. This puts tremendous pressure on
their development teams to re-engineer the phone architecture, software development,
hardware development and layout. Time to market with a new product is also crucial
for manufacturers to remain competitive.
An emerging trend in cellular phone handsets is the addition of “voice” features. These
voice features include: 1) the “on-the-fly” recording of telephone conversations, which
eliminates the need for writing down notes and improves driving safety by enabling
hands free usage, 2) recording voice memos, where reminders are left for oneself, and
3) integrating a telephone answering machine, which can screen or capture otherwise
missed calls. The basic function which enables these new features is the ability to reli-
ably record and playback voice messages. To meet the requirements of cellular handset
performance, the solution must be low-power, to conserve battery life, have a small form
factor, to enable a compact design, low-cost to keep the retail pricing down, and repro-
duce high quality sound, to meet the expectations of consumers. Additional factors to
consider are development time and costs, since a quick time-to-market and effective
design investment are critical for new products.
The following design illustrates how a single ISD33000 device can be integrated into a
GSM cellular handset to enable these new “voice” features. With minimal hardware and
software development, this solution provides one to four minutes of high quality, voice
record and playback capability with low-power consumption and minimal board space.
In addition, this implementation of the ISD33000 is easily portable to other analog or
digital cellular, PCS or PHS handsets because it is integrated in a manner which is inde-
pendent of the transmission and processing protocol being utilized.
Integration of an ISD33000 into a GSM Phone
The block diagram of the ISD33000 series voice recorder chip integrated into a typical
GSM digital cellular phone is shown in Figure 1. The GSM architecture is based on a
typical GSM chip-set available from one of the several semiconductor manufacturers in
the U.S.A.
The ISD33000 series chip interfaces directly to the earpiece and microphone signals
without involving the DSP software for speech encoding and decoding or adding code
to the protocol stack, layer 1, layer 2 and layer 3 of the GSM protocol. The ISD33000
series is also connected directly to the system micro-controller’s standard 8-bit general
purpose I/O port. This connection provides the user interface control and message
management.
GSM
RF
Section
ISD33000
Series
Voice Recorder
+
Audio Amplifier
and Switches
System
Microcontroller,
Memory,
Keyboard
Display and Battery
Management
GSM
Baseband
Section
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➤
➤
➤
➤
➤
➤
➤
➤
➤
➤
➤
➤
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➤
Figure 1
The detailed schematic of the ISD33000 series application is as shown in Figure 2.
Besides the voice recorder chip, only two other ICs are needed. Switches S1, S2 and S3
are implemented by using a 74HC4053 as an analog multiplexer switch to select various
audio input and output paths. This comes in a single 16-pin SOIC package.
TDA7050T, which is packaged in a small 8-pin SOIC, is an integrated audio amplifier
with fixed gain to drive the earpiece. A couple of capacitors are used for blocking the
DC voltages in several sections of the design as shown.
The general purpose I/O lines on the system microcontroller are used to control and
interface with the ISD33000 and the 74HC4053 switches. GPIO0 controls the switch-
es S1 and S2 and does the source selection between the audio input / output of the
ISD33000 and the GSM baseband section. Switch S3 is controlled by GPIO1 and mul-
tiplexes the audio output of the GSM baseband and the ISD33000 to the earpiece.
Resistors R1 and R2 provide attenuation to the signal from the GSM baseband to the
analog input of the ISD33000. R3 is used to attenuate signals going to the fixed gain
audio amplifier TDA7050T. GPIO2:7 are used to interface with the ISD33000’s con-
trol signals RAC, /INT, /SS, SCLK, MOSI and MISO. (Note: more information about
these signals and their functions can be obtained from the data sheet for the ISD33000.)
GSM
RF
section
ANA IN-
ANA+
AUD OUT
RAC
INT
SS
SCLK
MOSI
MISO
System
Microcontroller
➤
➤
➤
➤
➤
➤
➤
➤
➤
➤
GSM
RF Section
GSM
Baseband
Section
GP1O0
GP1O1
GP1O2
GP1O3
GP1O4
GP1O5
GP1O6
GP1O7
ISD33000
Series
➤
➤
Microphone
Earpiece
Audio
IN
Audio
OUT
S1
R3
TDA7050T
S3 R2
S2
R1
TDA7050T
➤
Figure 2
Figure 3
Initialize ISD33000
Select S2 to route signal from DSP
Audio output to ANA IN
Record the incoming voice
“on-the-fly” in the ISD33000
Store the “on-the-fly”
message record pointer
Exit
➤ ➤➤ ➤
Design Considerations for Optimum Performance
To optimize device performance, care should be exercised with the printed circuit board
layout. Power supply decoupling capacitors should be placed as close to the ISD33000
as possible. A good ground plane around the device enhances the noise performance
and audio reproduction. All the peripheral components should be placed as close to the
ISD33000 as possible and lead lengths and trace lengths should be minimized. If possi-
ble, the RF and IF sections of the GSM radio section should be isolated from the base-
band section of the phone to avoid noise, spurious injection or pickup from the proxim-
ity of resonating components.
Software Considerations
The software development for message management and control of the ISD33000 chip
is straight forward. Figures 3, 4, 5 and 6 show basic flowcharts for in system recording
“on-the-fly” conversations, announcement or memo recording, automatic answer and
message recording, and message playback.
FLOWCHART TO RECORD “ON-THE-FLY”
Initialize ISD33000
Select S1 and S2 to route signal
from Microphone to ANA IN
Record anouncement or
memo in the ISD33000
Playback the recorded message
Is the
message
correct?
Store the announcement
message pointer
Exit
➤ ➤➤ ➤ ➤ ➤
➤
No
Yes
Figure 4
FLOWCHART TO RECORD ANNOUNCEMENT OR A REMINDER MEMO
Enter auto answer mode
Select S1 to route signal from
ISD33000 to DSP to Audio Input
Select S2 to route Audio Output
from the DSP to ANA IN
Play the outgoing message
Is there
a message
to record?
Record the incoming message and
increment the message counter
Reset S1 and S2 to the
original state and Exit
➤ ➤➤ ➤ ➤ ➤
➤
No
Yes
Figure 5
FLOWCHART FOR THE AUTO ANSWER AND RECORD MODE
Playback mode
Select S3 to route signal from AUD OUT
of ISD33000 to the earpiece
Set the message counter
to the first message
Play the recorded message
Was this
the last
message
played?
Reset S3 to its original state
and reset the message counter
Exit
➤ ➤➤ ➤ ➤ ➤
➤
No
Yes
Increment
message
counter
➤
Figure 6
FLOWCHART TO PLAYBACK RECORDED MESSAGES
Power Consumption
Because of the ISD33000’s 3V supply and low current consumption characteristics,
adding these new “voice” features does not heavily impact battery life. Table 1 displays
the total power consumption for recording, playback and standby. For instance, the
ISD33000 in standby mode contributes less than 10 uA. When in active mode it adds
less than 45 mA to the system’s power consumption.
Advantages of Using the ISD33000 for Voice Storage
There are several advantages inherent in this implementation of the ISD33000 solution.
First and foremost is the development time required. The ease-of-use of the ISD33000
series product enables not only a straight forward design, but also a relatively simple
software development. In Table 2 overall development time has been estimated to take
only a matter of weeks to integrate the ISD33000 into this GSM phone.
DEVICE RECORD (mA) PLAYBACK (mA) STANDBY
DSP Engine / Core 65 mA 6 mA 6 mA
ISD33000 Series 40 mA 30 mA 10 uA
Micro-controller 30-45 mA 30-45 mA 2 mA
Switches / Amp 8 mA 20 mA 8 mA
Total ~150 mA ~90 mA ~ 16 mA
DEVELOPMENT TASK ISD33000 SERIES
Hardware Design 3 days
Printed Circuit Board Layout 1 day
Software Development 10 days
Storage Memory Management 2 Days
Message Management 4 days
Total Est. Development time 3 ~ 4 Weeks
Table 1
Table 2
This short development time accelerates to market these new “voice” features. Also the
single-chip form factor and low-power performance insures the final cellular design will
remain compact and provide long battery life. Because the ISD33000 is integrated in
the analog audio path and not in the digital baseband section, this solution provides
flexible cross-platform solution for digital or analog cellular phones. Since most digital
cellular telephone architectures are similar regardless of the protocol standards they
implement (IS-54, IS-136, CDMA, etc.), one can easily use this same type of design and
expect similar results.
Conclusion
New cost effective features will continue to be required in cellular phones to increase the
utility and value to the end-user and to help differentiate cellular phones between manu-
facturers. The ISD33000 was designed to enable new “voice” features to be added and
brought to market quickly, independent of the cellular protocol. Its low-power, ease-of-
use, and single-chip form factor all contribute to its value of any cellular phone.
