Practical Arduino Cool Projects for Open Source Hardware- P17 potx

In this project we'll assemble a speech synthesizer shield that combines a SpeakJet chip with a simple audio amplifier to let you add speech output to a new or existing Arduino project..

Speech Synthesizer

, when a 4MHz cal for software

as to offload the ely used speech chip ophone Speech uters, industrial

ed to increase ourse, it is almost ailable processing came a footnote cause in terms of processing power the ATMega

could possibly

o it and the Arduino

t much good if you just

nt to add voice feedback to an existing project And the demise of the SPO256 means you can't just

ut it, and

t has a much

er than a clunky parallel interface

The result is the SpeakJet, an 18-pin DIP device that can do everything the old SPO256 did plus

more

In this project we'll assemble a speech synthesizer shield that combines a SpeakJet chip with a

simple audio amplifier to let you add speech output to a new or existing Arduino project The required parts are pictured in Figure 9-1, and the complete schematic is in Figure 9-2 (The schematic might be a bit difficult to see, but you can also find it on the Practical Arduino web site.)

Synthesized speech was, for a long time, the Holy Grail of computing Back in the 1980s

CPU made your computer the fastest machine in the neighborhood, it just wasn't practi

to create intelligible speech In those days, the only sensible way to generate speech w

task to dedicated hardware because the CPU simply couldn't keep up The most wid

through the 1980s and early 1990s was the famous General Instrument SPO256A-AL2 All

Processor It was used in toys, external speech synthesizer peripherals for desktop comp

control systems, and all sorts of other unexpected places Then, as CPU power continu

rapidly, speech synthesis was moved to being a software function Nowadays, of c

always done entirely with software in the main CPU, using only a tiny fraction of the av

power As a result the SPO256 became unnecessary, dropped out of production, and be

in the history of technology

This leaves Arduino developers in a quandary, be

chips put us back into the realm of 1980s desktop performance again An ATMega

produce intelligible speech directly, but it would use every available CPU cycle to d

itself would be pretty much useless at doing anything else at the same time—no

wa

link one up to your Arduino and offload speech generation to it

With old stock of the SPO256 drying up Magnevation decided to do something abo

designed their own speech chip that works on the same principles as its predecessor bu

smaller physical package and offers a handy serial interface rath

Parts Required

Main

ield

et speech synthesizer chip (www.magnevation.com)

1 18-pin DIP IC socket

3 1K resistors

lithic ceramic capacitors (may be marked "103") onolithic ceramic capacitors (may be marked "104") trolytic capacitor (6.3V or greater)

1 3mm Ggreen LED

1 3mm red LED

D

r line output option cable)

On-b

ckage

2 10uF electrolytic capacitors (6.3V or greater)

citor (6.3V or greater) 000pF) ceramic capacitor (may be marked "102")

ramic capacitor (may be marked "104")

nal

1 Audio speaker (usually 8 ohms)

1 2-pin, 0.1-inch pitch socket

1 3.5mm stereo line socket 1

1 Length of single-core shielded cable

parts:

1 Arduino Duemilanove or equivalent

1 Prototyping sh

1 SpeakJ

2 10K resistors

2 27K resistors

2 10nF mono

1 100nF m

1 10uF elec

1 3mm blue LE

1 2-pin, 0.1-inch pitch pin header (fo

oard audio amplifier:

1 LM386 audio amplifier IC, DIP8 pa

1 8-pin DIP IC socket

1 220uF electrolytic capacitor (6.3V or greater)

1 100uF electrolytic capa

1 1nF (1

1 100nF monolithic ce

1 10K trimpot

1 2-pin PCB-mount screw termi

Line-level output cable:

1 If you prefer an RCA output, simply swap out the 3.5mm socket for an RCA socket, and use a male-to-male RCA connection cable to reach your connected device

Figure 9-1 Parts required for SpeakJet-based speech synthesizer

Figure 9-2 Schematic of SpeakJet-based speech synthesizer

If you prefer an RCA output simply swap out the 3.5mm socket for an RCA socket, and use a male-to-male RCA connection cable to reach your connected device

Source code available from www.practicalarduino.com/projects/speech-synthesizer

ech synthesis" to

to understand

as "hello world,"

t speak

al spelling of a word cannot be trivially converted to intelligible sounds

ted by any

as emotion play as well just to

a sound for that

n be long, as in the first tters within the nown as the "y" sound

we actually make called a

a series of phonemes

s of sound A

by a variety of allophones, depending on context, accent, and other

g intelligible: a

d a speaker with a result is that we can the same

sually still It's critical to understand the difference between letters and allophones if you want to be able to generate intelligible speech from an allophone-based speech synthesizer such as the SpeakJet Stop

lly as a series of letters and start thinking about them audibly as a series of

re hundreds of

er, instead you equence of you can make it say pretty much anything

Speech Output Signal

There are a few different ways you could incorporate a speech synthesizer into a larger project, so we've provided different output options in the design You can install all the parts, including a nice on-board audio amplifier with speaker, or stop at just the line output if you're connecting to amplified speakers or another device

The circuit diagram follows a direct signal path from left to right, with the commands from the Arduino coming in from digital pin 3 on the left and ending at the loudspeaker output on the right There

Instructions

The SpeakJet chip used in this project uses a technique called "allophone-based spe

create the necessary sounds that we interpret as intelligible speech It's very important

how it works if you want to get good results from it

You can't simply send the SpeakJet a string of letters spelled out literally such

because the way we write words down and the way we say them is often quite different We don'

phonetically, so the origin

Instead we subconsciously apply dozens of conventions that alter the sound represen

particular letter based on its context within a word or sentence, and even on factors such

being conveyed or whether a sentence is a question or a statement Accents come into

make things really complicated

The result is that it's not possible to take a specific letter, such as "e," and define

letter that will apply in all contexts The letter "e" may be short, as in "set," or it ca

e in "concrete." It can even be silent, but have an effect on the pronunciation of other le

word, as in the last e in "concrete." Letters can also combine to form dipthongs, sometimes k

gliding vowels, such as the "oy" in "boy," where the "o" sound slides smoothly into

Disregard the spelling of words for a moment and think only about the sounds

when we speak those words The smallest meaningful unit of sound in human speech is

"phoneme." Written text consists of a series of letters, but spoken text consists of

Phonemes, in turn, are represented by allophones, which are the smallest audible unit

phoneme can be represented

factors Variation in allophones is what gives people different accents while still bein

speaker with one accent may use a particular allophone to represent a phoneme, an

different accent may use a different allophone to represent that same phoneme The

hear that the sound (allophone) is different, but our brain still maps it conceptually to

phoneme: the word sounds odd when someone has a different accent, but we can u

understand what the speaker means

thinking about words visua

sounds or allophones There are only 26 basic letters in the English alphabet, but there a

different allophones You don't send letters representing words to the speech synthesiz

send allophones that represent the sounds you want it to make By stringing together a s

allophones

is a dashed line drawn vertically through the circuit, so you can stop at that point if yo

level output and don't want an on-board amplifier and speaker

Conversion of commands to an analog audio signal is performed within the Speak

list

u just want a line-Jet chip, which ens on pin 10 for serial communications from the Arduino Several pins on the SpeakJet need to be

be explained

as shown in the

CA connectors are commonly used to connect audio and video inputs and outputs together

ck the ech to the audio are capable of onnect a regular evice that has to operate stand-alone and still provide voice feedback this is probably the best route to take

ome from a larger speaker, or a system without sensitive high-frequency

t the high-frequency digital switching noise of the SpeakJet's underlying PWM

chip and a few other

ite expensive we sing a socket protects the

o use in other

ly to the propriate supply connections on the prototyping shield A 100nF bypass capacitor between every IC's

o GND In our

we took advantage of the prototyping shield's built-in 100nF capacitor mounting points

to be tied to either ino

e active-low reset pin, so use a 10K resistor to pull it up to VCC to allow the SpeakJet to

he chip to enter ing it up to VCC Pin 13 is M0, the "Demo Mode" pin This one is active-high, so you can tie it straight to GND by

putting a jumper wire on the underside of the board to connect pin 13 diagonally across to pin 5, the

GND pin on the IC

connected either to low (GND) or high (VCC) levels to put it into the correct mode as will

later

A line-level output can be brought out to a 3.5mm stereo jack or RCA connector,

schematic R

in home entertainment systems, so if you want to really give your Arduino a voice to ro

neighborhood you can use the line-level output to drive an amplifier or send the spe

input of your TV

For a self-contained speaking device the audio amplifier components in the project

driving a speaker directly to a fairly respectable volume With this output you can c

speaker from a sound system to the screw terminals on the shield and get both good volume and decent audio quality If you're building a d

The best sound will c

response, as they won't le

(pulse-width modulation) carrier through

Beginning Assembly

No matter which output method you choose you'll definitely need the SpeakJet

supporting parts, so begin by fitting those to the shield Because the SpeakJet itself is qu

fitted an 18-pin IC socket to the shield rather than solder the chip in directly U

chip from possible thermal damage during soldering and also allows you to remove it t

projects later if you want to

The SpeakJet GND connection is on pin 5, and VCC is on pin 14 Link them direct

ap

VCC pin and GND is always good practice, so connect the 100nF capacitor from pin 14 t

case

Apart from the power supply pins, the SpeakJet has several other pins that need

GND or VCC to force it to run in a mode that can be controlled externally by the Ardu

Pin 11 is th

run

Pin 12 is M1, the "Baud Configure" pin, which is also active-low We don't want t

automatic baud-rate configuration mode so use another 10K resistor to disable it by pull

as well

The only other SpeakJet pin that absolutely must be connected is pin 10, the RCX (serial input) pin That's the pin the Arduino will use to send data to the SpeakJet chip In this project we use a software

serial communications library to connect the SpeakJet to one of the general-purpose digital I/O lines

rather than tie up the hardware USART on digital pins 0/1, so use a short jumper lead and a 1K resistor to connect from Arduino digital pin 3 to SpeakJet pin 10 (see Figure 9-3)

with basic connections in place

The prototyping shield we used has mounting points for surface-mount parts including two 100nF

(C1 and C2 on the shield) and active-low (GND-to-illuminate)

unt LED to Arduino, but in order to hear the

SpeakJet PWM "Audio" Output

The SpeakJet works by varying the duty cycle of a fixed 32KHz frequency pulse-width–modulated (PWM)

"carrier" from pin 18 into an external two-pole low-pass filter, successfully converting the PWM digital signal into an analog voltage waveform suitable for line output or audio amplifiers

Done fast enough, what this means is that the duty cycle percentage of the PWM signal converts to the same percentage of DC voltage output from the filter, so a 50% 0 to 5V PWM duty cycle will convert

to about 2.5 volts DC out of the filter

Figure 9-3 SpeakJet chip mounted on shield

bypass capacitors mentioned previously

status LEDs We fitted surface-mount bypass capacitors and linked the green surface-mo

GND, providing a handy power-on indicator

At this point the SpeakJet is ready to receive instructions from the

result you need to connect something to the output

This is a great, inexpensive way to generate analog voltages and waveforms from any

tal-Analog-t intal-Analog-telligible can still be heard There is a 120-ohm minimum load and 25mA maximum current specified for this pin; be careful not to connect anything that may load the output pin more than this and risk damage

wered computer this purpose,

gh this quick test and the header can be skipped if you'd like to move straight on to the more useful

o line output It's only a few more parts, and the same cable can be used

For the quick test, connect a 3.5mm stereo headphone socket to the pin header as shown in Figure 9-4 The SpeakJet is a single mono output, so we connect it to both the left and right channels on the

socket

microcontroller PWM output, and is used in many projects to get a DAC voltage (Digi

Conversion) output without special hardware

The raw PWM from SpeakJet output pin 18 is digital and sounds somewhat noisy, bu

speech

to the IC

Quick Test

A simple way to test that everything is working up to this point is to connect a pair of po

speakers directly to the SpeakJet output pin 18 We placed a header on pin 18 just for

thou

filtered audi

Figure 9-4 Connection from header to powered computer speakers

speakers or

Fit Status Indicators

You certainly don't need status indicators for the speech synthesizer to operate, but it can be handy to have visual feedback of what the chip is doing Pins 15, 16, and 17 operate as status outputs when the

SpeakJet is in normal operation, so by connecting three LEDs with matching dropper resistors we can

see exactly what it's up to These pins can also be connected to the Arduino so that the software can

monitor the SpeakJet speech operation

The status pins, as shown from left to right in Figure 9-5, are described in Table 9-1

At this point you could mount the shield on your Arduino, plug in some powered

earbuds, and proceed to the software section to test that it works

Table 9-1 Sp atus outputs

Pin Name

eakJet st

Function Read

We use a 1K resistor and an LED connected from each output to GND as you can see in Figure 9-5 Green indicates the SpeakJet is ready, blue indicates that it is currently speaking, and red indicates that its input buffer more than half full

Figure 9-5 SpeakJet status outputs connected to LEDs

Because the SpeakJet has a 64-byte input buffer and the command size is one

doesn't take many words to fill up the input buffer on the SpeakJet Of course it also take

the SpeakJet

byte per allophone, it

s more time for

to sound out the allophones than it takes for the Arduino to send them to it, so using

more allophones tional bytes sent to it are simply ignored If you try to send a

ly The red LED on

o the SpeakJet ter than it can keep up with speaking it

rduino so the ore sending any

in Figure 9-5 a small 100nF power supply decoupling capacitor near the top center,

e again that's optional,

nexplained when

ch as a ones Line level is

s, or other pieces ecibel volts

t has a nominal low-pass filter we can generate a clean output from the SpeakJet that

gnals below a nals above that ters don't have a hard cutoff frequency but instead tend to roll off gradually around

n those output along with a

e of the resistors The resulting circuit filters the SpeakJet's digital PWM output into a smooth voltage waveform, removing the carrier and induced noise

Fit the pairs of resistors and capacitors as shown in Figure 9-6 You can do this even with the pin 18 direct connection still in place

The output from the second 27K resistor then connects to the positive side of a 10uF electrolytic

capacitor (not yet fitted in Figure 9-6) with the negative side of the electro going to the signal

(non-ground) pin of the Line Out connector

appropriate delay times or waiting for the Buffer Half Full signal to clear before sending

to it is important

Once the SpeakJet buffer is full any addi

large sequence of allophones to it very fast you may find the buffer fills up quite quick

D2 (pin 15) can be handy to give you a quick visual indicator that you're sending data t

fas

At the end of the project we'll discuss use of this output to provide feedback to the A

software can automatically detect when to stop sending data and wait a little while bef

more

You can also see

mounted between the +5V rail and the GND connection used by the LEDs Onc

but there's no harm in having additional decoupling capacitors They can help prevent u

glitches and noise caused by supply fluctuations, so it's always good practice to fit them

possible.Line-Level Output

A "line-level" signal is generally a larger signal than you would get from a device su

microphone, but a lower level than would be used to directly drive a speaker or headph

the connection that is typically fed into a mixer, amplifier, powered computer speaker

of audio equipment Without going into all the technicalities of unusual units such as d

(dBV), as a general rule of thumb a line-level signal in consumer-grade audio equipmen

amplitude of about 0.5V

By creating a simple two-pole

can be fed straight to other audio equipment A "low-pass" filter is a circuit that allows si

certain nominated frequency to pass through, but attenuates (decreases the level) of sig

frequency Simple fil

the cutoff point, with signals below that frequency passing through the filter more easily tha

above it

The two-pole filter consists of a pair of 27K resistors in series with the SpeakJet's

pair of 10nF capacitors, each connected between ground and the output side of on

use a 3.5mm

y is perfect ductor connects

If you use a male RCA plug you have the convenience of being able to plug your speech synthesizer shield directly into a piece of audio equipment, such as an amplifier or TV Using a female RCA socket as shown in Figure 9-7 allows you to use a common male-to-male RCA extension cable to do the same thing Which gender you decide to use depends on what you want to connect your speech synthesizer to and how far away it is Keep in mind that line-level signals should be kept as short as possible, so if you connect a 10-foot length of cable to your shield and put an RCA plug on the other end, the sound quality might not be very good Shorter is always better when it comes to maintaining audio signal quality

Figure 9-6 Shield with two-pole output filter in place

Making a Line-Level Output Cable

Solder a short length of shielded cable to your chosen line plug or socket If you'd like to

stereo socket for powered computer speakers the Quick Test cable described previousl

For RCA, the braid (shield) conductor connects to the outer shell, and the inner con

to the center pin Choose a male or female RCA connector to suit your needs