Journal of Signal and Information Processing, 2012, 3, 421-426
http://dx.doi.org/10.4236/jsip.2012.34055 Published Online November 2012 (http://www.SciRP.org/journal/jsip)
Development of Digital-Controlled Measuring Instrument
for Hearing
Longcong Chen1, Gaiqin Liu2, Bin Gao1, Ping Chen1, Xingliang Xiong1*
1Laboratory of Forensic Medicine and Biomedical Information, Chongqing Medical University, Chongqing, China; 2School of Opto-
electronic Information, Chongqing University of Technology, Chongqing, China.
Received July 21st, 2012; revised August 27th, 2012; accepted September 5th, 2012
ABSTRACT
A digital-controlled measuring instrument for hearing, which is mainly composed of STC11F16EX, AD9833, ISD4004-
16MS and LM1972, is introduced in this paper. It may output highly accurate and purified sine signal whose frequency
and amplitude are controlled by digital data. Specially, the displaying number of decibel is equal to actual number of
decibel because that frequency response of earphone is corrected through software of microcontroller. Additionally,
touching buttons, which is simple and convenience to use, is selected. Hence this instrument is convenient to measure
and teach about hearing specially, research and study on frequency characteristic of human ear and impedance charac-
teristic of human body in medical science.
Keywords: STC11F16EX; Digital-Controlled; Hearing; AD9833
1. Introduction
As is known to all, sounds waves that human can hear
have range not only in frequency from 20 Hz to 20 KHz,
but also on intensity. Usually, hearing threshold of inten-
sity includes two limited value: one is the maximum
value named pain threshold, and the other is the mini-
mum value called threshold of audibility [1]. The inten-
sity between audibility and pain threshold is audibility
range, and to same frequency sound waves, the larger
sound intensity must sound louder, but for different fre-
quencies of those, it is not necessarily sound stronger.
Therefore, in order to obtain loudness level curve, we
have to measure the sound intensity level of different
frequency under the same loudness. And different per-
sonal loudness curve is not completely same, that is to
say, different people have different hearing. In practice,
to measure personal hearing, different frequencies of
pure sine and other signal are used to drive sound
equipments, such as speakers, and earphone. The size of
sound intensity level is usually utilized to measure hear-
ing with decibels. Certainly, the instruments, which can
output single frequent sound and other sound, are de-
manded. In additionally, the auditory tests in the com-
prehensive check-up for students, who prepare for higher
school, college and university, are needed, and in the
physical and medical teaching and research, the human
hearing and impedance characteristics [2] are often de-
manded. As a result, the needed generator should have
the function of adjusting frequency and amplitude, and
also can produce high precision and purity sine and other
sound signals. Based on above, a novel instrument,
which is mainly composed of STC11F16EX, AD9833,
ISD4004-16MS and LM1972, is put forwarded.
2. Instrument Struction
Design of the instrument includes two sides, hardware
and software. Hardware is divided into nine parts: Micro-
controller, voice signal generator, single frequency gen-
erator, signal selector, audio fader amplifier, stereo audio
power amplifier, keyboard, LCD monitor and power
source circuit. The instrument block diagram is shown in
Figure 1.
This instrument mainly based on microcontroller and
high-performance integrated chips, such as STC11-
F16EX, ISD4004-16MS, AD9833 and so on, which not
only can increase its intelligence, precision, stability, but
also reduce its power consumption and volume. The mi-
crocontroller, STC11F16EX, output all kinds of com-
mand and data signal to control relative chips for gener-
ating needed sound. In this instrument, ISD4004-16MS,
which is act as voice signal generator and can record
various sound under management of microprocessor, is
applied to produce recorded audio sound, while AD9833,
single frequency signal, is ued to generate single fre- s
*Corresponding author.

Development of Digital-Controlled Measuring Instrument for Hearing 422
Figure 1. Blo ck diagram of the instrument.
quency sound with high accuracy and purified sine. Un-
der management of microcontroller, above two kind of
sound signal is offered to signal selector, which is mainly
composed of ADG1421 that contains two independent
analogy switches, to choice one needed type signal for
audio fader amplifier which primarily includes LM1972
and OP2177. By controlling of single-chip microcom-
puter, STC11F16EX, the double audio digital potenti-
ometers, LM1972, in audio fader amplifier, output the
voice or audio sound signal with special amplitude and
frequency ,which is given to stereo audio power ampli-
fier (APA) after double amplifiers. Finally the needed
signals are input to APA, which includes LM4950, a dual
audio power amplifier with capable of delivering 3.1
watts per channel to a 4 Ω single-end load, to drive head-
phones, speaker and so on. The keyboard for user’s im-
puting command, LCD monitor for display all kinds of
information with FM19264, and power source for pro-
viding other parts with power to work are designed in
this instrument. Hardware of nine parts effectively en-
sures to realize the function of digital-controlled meas-
urement for hearing.
The software of this instrument was designed by Kiel
uVision 3 for STC11F16EX.
The following below, we mainly describe the hard-
ware in four parts, which are microcontroller, voice sig-
nal generator, single frequency generator, and audio
fader amplifier, and software of this instrument in detail.
2.1. The Primary Hardware
The important and vital parts for this instrument are mi-
crocontroller, voice signal generator, single frequency
generator, and audio fader amplifier. Therefore, we only
describe those four parts in detail.
1) Microcontroller
The microcontroller is composed of microprocessor
STC11F16EX [3] and its peripheral components. STC11F16EX
is a single-chip microcontroller based on a high performance 1T
architecture 80C51 CPU, has a fully compatible instruc-
tion set with industrial-standard 80C51 series one, and has
16 K bytes Flash program memory, 1280 byte SRAM
memory and 32 K byte EEPROM. In addition, it has a
6-source, 2-priority-level interrupt structure, on-chip cry-
stal oscillator and a one-time enabled Watchdog Timer.
STC11F16EX play a major role and controls other parts
to realize data exchange and corresponding function in
this instrument.
2) Voice signal generator
The basic goal of voice signal generator is to produce
all kinds of voice that can be recorded in certain conditions
and is of the auditory tests in the comprehensive check-up
for students. This part mainly includes ISD4004-16MS [4],
which provide high quality, single-chip record, or play-
back solutions for 16-minute message applications and
whose block diagram is shown in Figure 2. The speech
samples are stored directly into on-chip nonvolatile me-
mory without the digitization and compression associated
with other solutions. Direct analog provides a natural
sounding reproduction of voice, music, tones, and sound
effects not available with most solid-state solutions. It
provides zero-power message storage and the single or
multiple message is retained for up to 100 years (typi-
cally) without power. In addition, the device can be
re-recorded (typically) over 100,000 times. Address and
control are accomplished through a Serial Peripheral In-
terface (SPI) or Microware Serial Interface to minimize
pin count. By microcontroller STC11F16EX, the needed
voice and audio sound can be recorded and playback at
right time and can meet our aim.
3) Single frequency generator
The primary aim of single frequency generator, in
which AD9833, a programmable waveform generator, is

Development of Digital-Controlled Measuring Instrument for Hearing 423
selected, is to generate single frequency sine signal for
measuring loudness level curve, the human hearing and
impedance characteristics. The AD9833 [5-7],whose func-
tion block diagram is shown in Figure 3, is a low power,
programmable waveform generator capable of producing
sine, triangular, and square wave outputs. The frequency,
phase and waveform of outputting signal are software
programmable. The frequency registers are 28 bits wide.
As the result, the output sine frequency is:
28
outputMCLK 2
f
fK (1)
where is the value loaded into the selected fre-
quency register, means input clock rate.
K
MCLK
Therefore, with a 1 MHz clock rate, the AD9833 can
be tuned to 0.004 Hz resolution, and we can obtain single
f
frequency signal with enough high precision of fre-
quency.
Certainly, after STC11F16EX sends corresponding
data to selected frequency register, we can get the needed
frequency sine signal.
4) Audio fader amplifier
For major target of getting needed signal whose ampli-
tude meets our demand, a digitally controlled 2-channel
audio attenuator—LM1972 [8] and a precision low noise,
low input bias current operational amplifier—OP2177 are
applied. The LM1972, whose function block diagram is
shown in Figure 4, is a digitally controlled 2-channel 78
dB audio attenuator fabricated on a CMOS process.
Figure 2. Function block diagram of ISD4004-16MS.
Figure 3. Function block diagram of AD9833.

Development of Digital-Controlled Measuring Instrument for Hearing 424
Each channel has attenuation steps of 0.5 dB from 0 dB -
47.5 dB, 1.0 dB steps from 48 dB - 78 dB, with a mute
function attenuating 104 dB, which is shown in Figure 5.
Its logarithmic attenuation curve can be Customized
through software to fit the desired application and the
performance of a μPot is demonstrated through its excel-
lent Signal-To-Noise Ratio, extremely low (THD + N),
and high channel separation. Each μPot contains a mute
function that disconnects the input signal from the output,
providing a minimum attenuation of 96 dB. Transitions
between any attenuation settings are pop free. The
LM1972’s 3-wire serial digital interface is TTL and
CMOS compatible. Therefore, with STC11-F16EX send-
ing data that selects a channel and the desired attenuation
level to LM1972, the needed attenuation level of signal
can be obtain.
Figure 4. Function block diagram of LM1972.
Figure 5. Attenuation steps scheme of LM1972.

Development of Digital-Controlled Measuring Instrument for Hearing 425
The operational amplifier—OP2177 consists of very
high precision, dual amplifiers featuring extremely low
offset voltage (60 μV maximum )and drift, low input bias
current (2 nA maximum), low noise, and low power
consumption. OP2177, together with LM1972, can out-
put needed amplitude signal and have enough ability of
driving stereo audio power amplifier.
2.2. Software
To realize the function, the software of this instrument
must be designed, and we completed it by Kiel uVision3
for STC11F16EX with C language. The software, whose
control flow is shown in Figure 6, is primarily composed
of one main program and six subprograms which are
ISD4004-16MS control, AD9833 control, audio selection,
volume control, display and read key subprograms. Main
program mainly calls read key subprogram to obtain the
user input information and calls the corresponding sub-
routine to display frequency, amplitude and to choose
right audio source and channels output. Read key sub-
program mainly gets the input keys’ value and inform the
main program. Display subprogram controls LCD—
FM19264 to display information, such as frequency,
volume, and sound source. ISD4004-16MS control sub-
program realizes sending data and command to ISD4004-
16MS for producing right audio sound or disabling it.
AD9833 control subroutine makes AD9833 generate
corresponding frequency sine signal or disable it. Audio
selection subprogram controls ADG1421 for selecting
right audio signal source. The volume control subroutine
mainly makes audio logarithmic digital potentiometer
LM1972 choose corresponding position, that is to say,
output right amplitude audio signal.
3. Operation Mode
This instrument mainly are two audio signal sources
which one comes from ISD4004-16MS and the other is
generated by AD9833. The former can get various natu-
ral sound such as voice, music, tones, while the latter can
only produce single frequency signal. For the latter, two
working mode, continuous and discontinuous frequency
mode, is designed. Two working mode can be directly
switched through the key “work mode choice”. In suc-
cessive frequency work mode, its output frequencies of
signal have 0.1 Hz steps from 20 Hz to 20 KHz. In dis-
continuous frequency mode, only often used 75 fre-
quency points are given to be selected. The frequency
points are 20 Hz, 25 Hz, 32 Hz, 40 Hz, 50 Hz, 60 Hz, 64
Hz, 80 Hz, 100 Hz, 128 Hz, 160 Hz, 192 Hz, 200 Hz,
256 Hz, 288 Hz, 300 Hz, 350 Hz, 360 Hz, 400 Hz, 450
Hz, 480 Hz, 500 Hz, 512 Hz, 550 Hz, 600 Hz, 640 Hz,
700 Hz, 720 Hz, 750 Hz, 800 Hz, 850 Hz, 900 Hz, 960 Hz,
1000 Hz, 1024 Hz, 1050 Hz, 1100 Hz, 1200 Hz, 1300 Hz,
1400 Hz, 1500 Hz, 1600 Hz, 1800 Hz, 1900 Hz, 2000 Hz,
2048 Hz, 2500 Hz, 3000 Hz, 3500 Hz, 4000 Hz, 4096 Hz,
4500 Hz, 5000 Hz, 5500 Hz, 6000 Hz, 6500 Hz, 7000 Hz,
7500 Hz, 8000 Hz, 8192 Hz, 8500 Hz, 9000 Hz, 9500 Hz,
10,000 Hz, 11,000 Hz, 12,000 Hz, 13,000 Hz, 14,000 Hz,
15,000 Hz, 16,000 Hz, 16,384 Hz, 17,000 Hz, 18,000 Hz,
19,000 Hz, 20,000 Hz. No matter which kind of
Figure 6. Software control flow.

Development of Digital-Controlled Measuring Instrument for Hearing 426
work mode, it is no influence for volume adjustment, but
for different frequencies, the ranges of output sound
volume have different decibels values, this is to say, the
scope is different for different frequencies because head-
set of audio has not same response to different ones, and
microcontroller program made a corresponding modifi-
cation.
4. Disussions and Conclusions
This instrument adopts single-chip control and high-
performance sine waveform, so, it can output high preci-
sion frequency (frequency error < 0.004 Hz) and high
purity pure single audio sound, and applied LCD to dis-
play all kinds of information. This instrument equipped
with two kinds of sound sources, therefore, it is conven-
ient for the audio test and experiment. In addition, all
kinds of control and choices are application of electronic
switch without mechanical contact, by using in practical,
it is not only convenient operation, simple, and working
stability and reliability is extremely high.
Certainly, this instrument can be used as high-preci-
sion frequency and high-drive sinusoidal signal source
for research and teaching in testing human impedance
characteristics.
In a word, this instrument has very good novelty and
practicability. Additionally, touching buttons, which is
simple and convenience to use, is selected. Hence this
instrument is convenient to measure and teach about
hearing specially, research and study on frequency char-
acteristic of human ear and impedance characteristic of
human body in medical science.
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