Practical Arduino Cool Projects for Open Source Hardware- P17 potx

C H A P T E R 9

  


139
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 you
r 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, speec
h 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 desk
top 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.
Wi
th 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
CHAPTER 9  SPEECH SYNTHESIZER
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.
140
CHAPTER 9  SPEECH SYNTHESIZER

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

Figure 9-2. Schematic of SpeakJet-based speech synthesizer
141
CHAPTER 9  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
142
CHAPTER 9  SPEECH SYNTHESIZER
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).

143
CHAPTER 9  SPEECH SYNTHESIZER

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.
144
CHAPTER 9  SPEECH SYNTHESIZER
This is a great, inexpensive way to generate analog voltages and waveforms from any
tal-Analog-
t intelligible
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.
145
CHAPTER 9  SPEECH SYNTHESIZER
Table 9-1. Sp atus outputs
Pin Name
eakJet st
Function
Read17 D0 y
16 D1 Speaking
15 D2 Buffer Half Full

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
146
CHAPTER 9  SPEECH SYNTHESIZER
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
147
CHAPTER 9  SPEECH SYNTHESIZER

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.
148