White Paper - Winbond Electronics

PROPRIETARY AND CONFIEDNTIAL

Software considerations for adding

a speech recording system to a Digital

cellular telephone

1.1 Abstract

Explosive growth in the digital cellular market has provided an opportunity and a

challenge to cellular phone manufacturers at the same time. Their customers are

demanding smaller size, lower cost, longer talk times, data modem interface and other

attractive features such as a built-in speech recording system. This article features

software considerations for adding speech recording to GSM cellular telephone using

traditional digital memory storage and an innovative voice recorder chip using multi-level

analog storage. Conclusions are drawn in favor of the innovation for software simplicity,

ease of implementation, time-to-market advantage, and relative power consumption.

2.1 Introduction

Product differentiation has become a necessity for GSM phone manufacturers in a very competitive market

place. Time to market and design cycle time reduction is important to every phone manufacturer to remain

profitable and gain market share. A brief description of the GSM cellular phone architecture and

hardware design considerations for speech recording and playback in digital and analog memory have

been presented in an earlier white paperi. This paper will focus mainly on the software considerations for

the recording and playback of speech in conventional digital memory and the ISD33000 family single-

chip multi-level storage (MLS) analog memory. A simple analysis between the two approaches will be

shown for memo recording, off-the-air recording, and playback modes. Conclusions will be drawn based

on the simplicity and elegance of the analog recording scheme through the analysis of software

requirements and power consumption.

2.2 The GSM Digital Cell Phone

The typical base-band signal processing for a GSM phone is shown in Figure 1.

Speech

Encoder and

Decoder

DSP functions

Voice band

filtering, A/D and

D/A converters

Channel

Equalizer and

digital filter

Channel

Encoder and

Decoder

Figure 1

The DSP (Digital Signal Processor) in a GSM digital cell phone has numerous tasks to perform in a very

short period of time. ItÕs list of functions include digital filtering, Viterbi decoding, channel equalization,

block coding and convolutional coding, interleaving, channel decoding, speech encoding, speech

decoding and voice band filtering including voice activity detection (VAD), comfort noise insertion (CNI),

lost speech frame substitution and muting. In addition to these tasks, data modulation, demodulation,

burst building, tone generation, echo cancellation, etc. are also required which further tend to maximize

the MIPS usage of the DSP. In order to perform these tasks within the required time frame requires a

dedicated DSP with enough capability to conform to the GSM specifications.

There is an economic pressure on the phone manufacturers to squeeze the last ounce of performance

out of a DSP, but at the same time there is pressure to reduce the MIPS requirements for longer battery life.

PROPRIETARY AND CONFIEDNTIAL

Hence, power consumption should be given special attention during the discussions here in this paper.

The GSM (Global System for Mobile Communications) is a Time-Division Multiple Access (TDMA) system for

mobile communication of voice and data information. There are 124 transmit and receive channels in the

system with 200 KHz bandwidth for each channel. The data rate on each channel is 270.83 kb/s. On

each transmit or receive channel, there are frames of 4.615 ms duration, and each frame has eight time

slots. Each time slot is assigned to a single mobile subscriber. Thus it is necessary to complete all transmit

or receive signal processing and data handling requirements in this time slot.

GSM uses RPE-LTP (Regular Pulse Excitation with Long Term Prediction) as the speech coder. The full

rate speech coder has a rate of 13 kb/s for the speech data. The speech coded operates on a 13-bit

speech data sample with a sampling rate of 8 ksps. There are 160 samples in each 20 ms speech

window, which are converted to coefficients of 260 bitsii.

Since we know the time slot duration, channel data rate, speech rate and speech coder requirements for

GSM, we can estimate the MIPS requirements for various functions. DSP performs channel

encoding/decoding (4.5 MIPS), GMSK modem and data filtering (15 MIPS), Viterbi decoding and channel

equalization (6 MIPS), speech encoding/decoding (5 MIPS), voice activity detection (0.5 MIPS), etc.iii Thus

a minimum of 35-40 MIPS is required to implement the GSM. Additionally, the system micro-controller is

active with the GSM protocol implementation, data handling, timing, radio resource management and man-

machine interface.

Now that we understand the critical timing, speech processing, MIPS requirements and the micro-

controller loading, we will analyze digital and analog speech storage schemes in the following sections,

giving special attention to power consumption, software overhead, and implementation time.

GSM

RF section

GSM

Baseband

section

System Flash

memory

System

Microcontroller

Speech Flash

memory

Figure 2

3.1 Digital implementation of speech recording and playback

Figure 2 shows a typical block diagram for voice recording using a traditional digital approach with the

DSP that is being used for all the GSM signal processing requirements. It is assumed that the speech will

be recorded in a serial FLASH memory dedicated to digital speech recording and storage. Since the DSP

may have two serial ports available and one is used for the coded interfaceiv, the other serial port can be

used for

interfacing with the serial FLASH. If a parallel FLASH is used for speech storage, a serial-to-parallel

converter would be needed to interface between the DSP and the FLASH memory, or the speech data will

have to be routed from the DSP into the system micro-controller and then into the memory. Such

processing is not covered in this paper but additional loading on the DSP, the system micro-controller and

PROPRIETARY AND CONFIEDNTIAL

the system buss will occur. A separate speech FLASH memory is shown in the figure, but it is also possible

to use the system FLASH memory for speech storage as long as care is taken to make sure that the system

code cannot be accidentally destroyed during audio recording. Other memory technologies may be used

as well, but nonvolatility may be a design requirement.

3.2 ISD33000 Implementation of speech recording and playback

Figure 3 shows a typical block diagram for voice recording using an MLS analog voice recorder chip such

as the 4 minute storage ISD33240, a member of the ISD33000 Family, in a GSM cellular telephone. Since

the inputs and outputs of ISD33000 are analog, the interface to the microphone and earpiece is very

simplev. It is not necessary to burden the DSP or its ports for any interface needs. The interface between

the system micro-controller and ISD33000 is via an SPI bus.

Figure 3

In the paragraphs that follow, we will investigate the functions required for voice storage in a cellular

phone: (1) recording during telephone standby time, as required for a personal voice memo recorder,

(2) recording off the air during a phone call, and (3) playback of the recorded voice during standby time

so that the user can listen to the recorded messages. Additionally, we will look at (4) playback during a

phone call; this function is necessary to play an outgoing message or OGM to emulate the operation of an

answering machine. And finally, we will look at what is required to do message management in each

technology to maximize use of the available memory.

4.1 Voice memo recording - digital:

In the digital speech recording setup, the voice memo recording function is only feasible when the GSM

phone is not handling a cellular call. It should be noted that in this mode the DSP and system micro-

controller are mostly in the stand-by or power down state. They are only active to the extent necessary to

detect an incoming call. In order to initiate the recording function, the DSP and the system micro-controller

will have to be brought up to the full operating state. Figure 4 shows the data flow path through the DSP

while in the voice memo recording mode. The coded provides digital data at 8k samples/s that are

digitally filtered by the DSP.

Audio In

Audio Out

Microphone

Earpiece

GSM

RF section GSM

Baseband

section

System

Micro-controller

SPI

ISD33K series

SPI

ANA IN-

ANA IN+

AUD OUT

System

Flash

memory

PROPRIETARY AND CONFIEDNTIAL

Data from codec/AFE

Voice filtering

Speech encoding

Send data to the SIO

Interrupt system micro-

controller for message

management

Invoke memo record

Write data to Serial Flash

Figure 4

The DSP will form speech frames of 260 bits based on the RPE-LTP coding that can be stored in the Flash

memory. The serial I/O port on the DSP interfaces with the memory, and message management task is

handled by the system micro-controller. The rest of the operation is very simple and straightforward. This

process will use about 3.5 MIPS in the DSP for speech coding, filtering, etc. and about 300 lines of

software overhead in the system micro-controller for message management. One drawback of this digital

solution is the additional power consumption while invoking the voice memo recording function. The DSP,

Coded, memory and the controller together will consume more than 300 mA of current while in the record

mode. If the system Flash memory is shared with the speech data, then additional code would be

required and a need for increased MIPS in the DSP, resulting in additional power consumption.

Depending on how often memo recording is performed, the power consumption could reduce the overall

stand-by and talk time for the GSM phone. In a market where every manufacturer is trying to increase the

stand-by and talk times, the excess power consumption using this method of implementation could have a

negative impact.

4.2 Voice memo recording, ISD33000:

Voice memo record is very easy and simple to implement with the ISD33000 with much less increase in

power consumption. Since the DSP in not required for this function it will remain in stand-by or idle mode.

The system controller software will also be minimal with liberal timing requirements. Figure 5 shows the

flow diagram for voice memo record mode.

DSP and System micro-controller

in standby or idle mode

Interrupt for Pocket

memo record ?

Interrupt system micro-

controller for message

management

Initiate ISD33000 to record

Return to standby or idle mode

Figure 5

During voice memo recording, the ISD33000 will require about 40 mA and will need about 45 lines of

code. This is very easy and quick implementation compared to the digital storage approach and there

are very minimal risks in affecting any GSM system critical timing requirements.

4.3 Voice Memo Recording, summary:

Voice Memo Recording is an easy function to perform for both digital and the MLS analog methods. The

ISD33000 solution has significant advantages, however, since it requires very much less total power and

much less code to be written for the micro-controller. Table 1 summarizes the most important data.

PROPRIETARY AND CONFIEDNTIAL

Digital Memo Record ISD33000 Memo Record

DSP Mips Required 3.5 0

Power 300 mA 40 mA

Lines of code for micro-controller 300 lines 45 lines

Table 1

5.1 Off-the-air recording - Digital:

There are two Off-the-air recording methods in use today in cellular telephones. Some manufacturers

chose to record only the incoming call while others desire to record both sides of the conversation, i.e. full

duplex recording. In the digital storage implementation these two methods are very different. In either

case, several points have to be considered carefully. The DSP is running full speed to perform the data

demodulation, channel equalization, Viterbi decoding, de-interleaving, channel decoding, speech

decoding and other essential tasks within each specific time slot. The system micro-controller is busy with

the protocol timing loops and other housekeeping tasks. We will look at simplex, incoming call only

recording first.

The incoming speech data frame of 260 bits will have to be intercepted from the DSP while all the

processing is taking place. If we assume that a serial FLASH memory is used for speech storage and is

interfaced through a serial port on the DSP, the software for the DSP will need to be modified and

additional instructions added to make the speech data available on the serial port. Figure 6 shows data

flow diagram through the DSP when off-the-air command is invoked.

Data demodulation

Digital filtering

Channel equalization

Channel decoder

Off-the-air record ?

Speech Decoder Send data to the SIO

Interrupt system micro-

controller for message

management

Voice filtering

Data to the codec/AFE

Write data in speech Flash

Figure 6

Either the DSP or the system controller will have to handshake with the speech FLASH memory for storage

of the speech data. As it can be seen from the data flow diagram, every byte from the DSP will have to be

sent out through the serial port to the speech flash memory. Depending on how the GSM protocol is

implemented and resource availability, this could pose some problem with the system timing since all the

processing will have to be done within the confines of 4.615 milliseconds. The system controller software

will have to be modified to add message management functions. About 0.3 MIPS of additional DSP

processing will be required to implement the off-the-air recording and approximately 200-300 lines of

code will be required in the system micro-controller. Depending on the access time and programming

time for the serial Flash, this could pose timing problems in the protocol implementation.

This function will cause some increase in the power consumption in the telephone, but other DSP and RF

processes will dominate power usage. It is of utmost importance to notice that the additional message

management code and the programming time for the 260 bits of serial data into the serial Flash could

PROPRIETARY AND CONFIEDNTIAL

impact the GSM timing requirements.

In our second off-the air recording case, we have a very difficult situation because now we have the

outgoing audio stream to also capture and we are still limited by the critical 4.615 mS window dictated by

the GSM channel requirements. The Coded digitizes the analog speech from the microphone and the

DSP performs the speech encoding. This encoded 260 bits represents the 20mS speech window, which

also has to be stored into the speech memory prior to channel encoding. The two data streams will have

to be mixed, and possibly attenuated. The data from the DSP will now have to be ported out to the

speech memory via the SIO port. This total process now requires a total of 1.2 Mips, 0.9 Mips additional to

record the microphone input and mix the two bit streams. This will increase current consumption still

further, though the RF power amplifier and other DSP functions will still dominate this specification. About

300 lines of code will be required to implement both the modifications to the DSP code and the system

micro-controller code. It should be noted that some DSPs may reach capacity before the implementation

of this function rendering it impossible to accomplish without further changes to the GSM platform.

In some cases, a third party software GSM protocol stack is used in the design of the phone. This

implementation is usually already optimized for specific hardware. In this case, it would be very difficult to

modify the code and still maintain the system timing.

5.2 Off-the-air recording - ISD33000:

Off-the-air speech recording using ISD33000 is very simple compared to the digital approach. The

interface between ISD33000 and the system is made through a simple 3-wire serial bus. The analog

interface is also simple between the earpiece output of the coded and the analog input on ISD33000.

While the system is active and in voice conversation mode, and an off-the-air record command is invoked,

initiate the chip into record mode. This operation only requires the serial transfer of 3 bytes of data. In

the simplest case, no further intervention is required until recording is commanded to end, or the chipÕs

memory is exhausted. If full message management is performed using the ISD33000Õs built in control

architecture, the micro-controller needs to be interrupted only once in every 150 mS to 300 mSvi. And

then, only a few lines of code need be executed to update the Message Address Table (MAT) used in the

chip's message management operation. This is very easy since the GSM time slots are 4.615 mS. This will

not affect any critical timing requirements for the GSM protocol. The total additional software overhead

required to service the ISD33000 is estimated to be less than 45 lines of code. The current consumption

of the ISD33000 in the record mode will be only 40 mA. Again, the DSP and RF power usage dominate

during this operation.

If a third party software GSM protocol stack is used in the design of the phone, it would be relatively simple

to add the few lines required for implementing the off-the-air recording function and still maintain the

system timing.

It is very easy to record full-duplex conversation off-the-air by simple analog mixing of the two audio

streams. In addition the ISD33000Õs ANA-IN input configuration allows the combining of two unequal

level signals with little additional circuitry. In fact, there is no extra code or current consumption required

to implement full-duplex record. This is a significant advantage over the digital speech storage in the full

duplex off-the-air recording mode. Figure 7 shows the steps necessary to perform duplex and simplex

recording off-the-air.

PROPRIETARY AND CONFIEDNTIAL

DSP and System micro-controller active in

voice conversation mode

Off-the-air record ?

Continue voice

conversation mode

Interrupt system micro-

controller for message

management

Initiate ISD33K to record

Figure 7

5.3 Off-the-air Recording, summary:

Off the air recording is a task for the digital solution. In fact, the bandwidth required for DSP processing

may be a limiting factor. Additionally, timing constraints must be carefully watched to insure that no data is

lost or required call processing compromised. Existing or third party software may already be optimized

for most efficient operation, and the required modifications to add this feature may be difficult to fit in. The

ISD33000 solution, however, requires minimal microprocessor overhead and no modifications to already

existing DSP code. In both cases, the additional power requirement needed to add this feature is

minimal compared to that required by the call processing and RF components of the phone. Table 2

summarizes the most important data.

Digital Off-the-air record ISD33000 Off-the-air

record

DSP Mips required simplex 0.3 0

DSP Mips required duplex 1.2 0

micro-controller code duplex 200 lines 45 lines

micro-controller code duplex 300 lines 45 lines

Table 2

6.1 Playback of recorded speech during standby - Digital:

Since full access to the DSP is essential in playback of the recorded speech, it is only available during the

standby mode. The system controller and the DSP need to be put in the normal operating mode and the

data from the speech memory needs to be fed into the DSP for speech processing. Figure 8 shows the

data flow through the DSP and system micro-controller. Upon an interrupt to playback, the micro-

controller will enable the DSP to read data from the serial Flash, which will then be decoded and digitally

filtered. The coded converts digital to analog signals for playback through the earpiece. This process

will require about 1.8 MIPS in the DSP and about 200 lines of code overhead in the system controller for

message management.

Data from serial Flash

Speech decoding

Voice filtering

Data to codec/AFE

Interrupt system micro-

controller for message

playback

Invoke playback of speech

Figure 8

PROPRIETARY AND CONFIEDNTIAL

In the playback mode about 200 mA of current consumption is estimated for the DSP, Coded, speech

memory, and system controller with its memory. Once again, if the system memory is utilized for speech

storage, the data will have to be routed through the micro-controller into the DSP and would increase

both the current consumption and code requirements.

6.2 Playback of recorded speech during standby - ISD33000:

Since the DSP is not needed for the playback of recorded speech when using the ISD33000, it can

remain in the idle or stand-by mode. The system micro-controller will be used only to initiate the

ISD33000 to playback and provide the addresses from the MAT. Figure 9 shows the simple flow diagram

for speech playback.

DSP and System micro-controller

in standby or idle mode

Interrupt for message

playback ?

Interrupt system micro-

controller for message

management

Initiate ISD33000 to playback Return to standby or idle mode

Figure 9

During the playback mode, the ISD33000 will consume about 30 mA and the controller will need 35 lines

of code. The timing requirement is also very liberal due to the intelligent message management features

of the ISD33000.

6.3 Playback of recorded speech during standby, summary:

Playback of recorded speech during standby is an easy function to perform for both digital and the MLS

analog methods. The ISD33000 solution again has significant advantages, however, since it requires

much less total power and quite a bit less code to be written for the micro-controller. Table 3 summarizes

the most important data.

Digital Playback - Standby ISD33000 Playback - Standby

DSP Mips Required 1.8 0

Power 200 mA 30 mA

Lines of code for micro-controller 150 lines 30 lines

Table 3.

7.1 Playback of Speech during an active phone call - digital:

This function is required to enable the answering machine function in a cell phone. The telephone is put

into a mode such that it automatically answers and plays an outgoing message (OGM) back to the calling

party. As demonstrated in the section on Off-the-air recording, the DSP and system micro-controller are

then heavily involved with call processing, data demodulation, channel equalization etc. The stored

speech data in the Flash memory will be read into itÕs associated serial port and injected into the digital

data stream by the DSP for transmission by the phone. The system micro-controller will have to keep up

with message management functions. and have to routed to the output data stream. About 1.8 Mips will be

required from the DSP and approximately 150 lines of code added to the micro-controller.

7.2 Playback of speech during an active phone call - ISD33000:

PROPRIETARY AND CONFIEDNTIAL

Playback of speech during an active phone call is very simple compared to the digital approach. The

system micro-controller uses the SPI control to initiate the ISD33000 to playback and provide the

addresses from the MAT. Analog gates are added to the hardware to direct the playback to the

telephoneÕs audio input. This function requires no additional Mips from the DSP and requires only 30

lines of additional code for the micro-controller.

7.3 Playback of speech during an active phone call, summary:

As in the previous Off-the-air recording example, the digital approach adds a significant burden to the

DSP and system micro-controller. In fact, the bandwidth required for this function is 0.5 Mips higher. The

ISD33000 example again adds no additional Mips requirements and only a small amount of system micro-

controller code. Table 4 summarizes the most important data.

Digital Playback - Active ISD33000 Playback - Active

DSP Mips Required 1.8 0

Lines of code for micro-controller 150 lines 30 lines

Table 4.

8.1 Message management software - digital:

An additional task is required by the digital implementation of speech storage when full message

management is incorporated into the phone. As recording and erasure of random length messages

proceeds in the phone, occasionally, the Flash memory must be cleaned up and reorganized. This is a

background task that would have to be carried out at a very low priority. The system timers for GSM

protocol would be at the highest priority level. The access to the speech memory is via the DSP SIO port.

The message management tasks become very cumbersome and involve powering up the system

controller and the DSP. Figure 10 shows a simple flow diagram for message management and speech

memory optimization.

Update message pointers

Write to the speech memory

at new pointer location

Repeat above until memory

usage optimized

Return DSP/micro to Idle mode

Read messages from

speech memory via

DSP SIO port

Wake up DSP and system

micro-controller

During Idle mode or Standby

invoke Message management

Figure 10

It is necessary to read message blocks from the speech memory into the DSP and update the message

pointers to new locations, and write back the message blocks in the speech memory.

It is estimated that the message management and memory optimization could take more than 0.3 MIPS of

the DSP and more than 400 lines of code to implement. The current consumption during this process

could be as much as 300 mA while active and this would be too much drain during Idle or Standby modes.

Considering the importance of talk times and standby times for a given battery capacity, it may b e

PROPRIETARY AND CONFIEDNTIAL

desirable not to implement the memory optimization process and just have a larger capacity memory for

the messages.

8.2 Message management software - ISD33000:

The hooks for message management functionality are built into the into the ISD33000. When message

management is used, it is part of the micro-controller system software used to control the device. The

code sizes quoted in the other sections of this paper assume message management is implemented.

8. Message management software, summary:

Table 5 summarizes the most important data.

Digital Playback - Standby ISD33000 Playback - Standby

DSP Mips Required 0.3 0

Power 200 mA n/a

Lines of code for micro-controller 400 lines n/a

Table 5.

9.0 Comparison of digital speech storage verses the ISD33000.

In the previous few sections, digital and analog approaches to off-the-air recording, memo recording and

speech playback have been discussed. The digital implementation required more attention to the GSM

system timing issues, more usage of the DSP and system micro-controller and more memory for storage.

Tables in each section show a comparison between the two approaches. They show the additional

incremental MIPS required to implement various record and playback modes for both the digital approach

and the ISD33000. In the off-the-air record and OGM modes, the DSP is already in the active mode and

so the additional MIPS or the current consumption is insignificant compared to the MIPS used for GSM a n d

the current for RF power amplifier. In the ISD33000 implementation, there is no additional MIPS

requirement and the current consumption is insignificant as well. It is most important to notice the

increased current consumption and MIPS requirements in the Memo record and playback modes because

the DSP and system micro-controller have to be activated from the standby or idle mode and there is no

consumption from the RF power amplifier. Since both the micro-controller and DSP code have to be

modified in the digital implementation, about 200 Ð 300 lines of additional code would be required to

implement these various modes. The ISD33000 interfaces with the micro-controller via an SPI bus and a

couple of hardware lines for the interrupt, RAC, etc., it requires very minor modification to the system code

and has no impact on the DSP code.

Message management can be cumbersome with the digital approach depending on the block write/erase

or page write/erase requirements for the Flash memories. If every 20 mS speech frame (260 bits of

information) is individually addressed in the memory, the MAT could need as many as 500 bytes for the

pointers, which is not practical. On the other hand, if the block sizes are fixed to 64 bytes, fewer pointers

may be required for MAT but the total memory requirement for 120 seconds of speech storage could be

more than 320 kbytes! There does not seem to be significant difference in the power consumption

between the two implementations during the off-the-air record mode because it will dominated by the DSP

and RF power amplifier. But there is a significant difference in the consumption during the voice memo

function and speech playback indicating that the digital approach will need more power for the DSP,

system micro-controller, and the flash memory compared to the consumption by ISD33000. The software

requirement for digital implementation indicates more than a 1000 lines of additional code whereas the

ISD33000 needs less than 200 lines. The timing requirement for implementing digital storage is

significantly critical as compared to the ISD33000. And the overall time to implement the digital speech

storage could be significantly longer than a simpler ISD33000 based approach.

10.0 Conclusion :

PROPRIETARY AND CONFIEDNTIAL

Speech storage and playback in a GSM cellular phone with digital and ISD33000 approach were

discussed. Based on the above analysis it can be concluded that the ISD33000 approach is much better

than the digital approach. It is a lower power consumption solution, less software overhead, less timing

critical, easier and quicker to implement, and therefore the right choice for speech storage and playback

in GSM cellular phones.

i Prasanna Shah and Phillip Pyo, ÒDesigning the ISD33000 series into Digital Cellular Phones,Ó

ISD Corporation, 1997

ii GSM Specification series 03, 04, 05, 06

iii Wireless Communications by T. S. Rappaport.

iv Data Sheet Ð TMS320C54X family Texas Instruments Inc.

v Data Sheet - ISD33K family Information Storage Devices, Inc

vi ISD Application Note # 2 Ð Joe Jarrett, Field Applications.