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INTRODUCTION
India, which
is one of the most populous nations of the world, also ranks
for being amongst the poorer countries. With diagnostic equipment
being so expensive, doctors are unable to attend to a vast majority
of rural population, thus denying them of basic health care.
We took this opportunity to design this low cost yet highly
competitive diagnostic audiometer.
Hearing
loss is a common problem among old people and it is increasingly
affecting younger and younger people. People living in urban
areas are specially affected as the cities get crowded and people
are getting exposed to increasing ambient sound levels. To determine
this hearing loss pattern doctors use diagnostic equipment known
as the Audiometer.
The audiometers
used by the doctors generally requires a period of training
before they can fully use the facilities provided by the equipment,
also the tests conducted using these audiometers requires much
time. Doctors have always been faced with the challenging task
of determining the hearing threshold level of individuals. Many
effective instruments have been developed over the past decade
for determining hearing loss, but the advent of semiconductor
devices has introduced a host of new and more accurate audiometric
instruments.
Today as
most of the audiometers offer many common functions and share
the same level of accuracy, the competitive viability is based
solely on cost. Thus the AT90S8535 micro controller based audiometer
provides the same accuracy and functions as the commercially
available audiometers but at a much-reduced cost.
Since, the
hearing loss in an individual is not the same in all frequencies,
doctors felt the need of determining the hearing loss of an
individual at different frequencies throughout the hearing spectrum.
Thus an instrument called the Pure Tone Audiometer was developed.
At the same
time, some of the doctors argued that pure tone audiometry could
not simulate the hearing loss an individual experiences for
speech. Thus a new concept of Speech Audiometry was introduced.
The AT90S8535
based audiometer offers both Speech and Tone Audiometry. It
also allows printouts of the audiogram, showing the hearing
loss pattern of the individual in the available range of frequencies.
Picture
of Audiometer front panel.
Picture
of Audiometer connections.
SALIENT
FEATURES OF THE AUDIOMETER
1> Complete
digital calibration of frequencies and Hearing Level from front
panel.
2> 16x2
line LCD used to display all necessary information logically.
3> Noise
used is digitally generated using Pseudo Random Binary Sequence
algorithm.
4> Serial
port for interface to PC.
5> Can
print Audiogram to any EPSON LX-300 compatible printer.
6> Designed
to be low cost using minimum components.
7> Lightweight
and extremely portable.
8> Hearing
Level attenuator accurate within 0.5db and Frequency generated
is accurate within 2%.
9> Can
be calibrated to operate with a wide range of headphones and
bone conductors.
10> Battery
operation possible.
Direct printout
from Epson LX-300 printer.
SALIENT
FEATURES OF HARDWARE
1) Optimised
for low cost.
2) Just
one I/O expander used (I.CU4).
3) Data
bus (input to I.CU4) and the keyboard share the same I/O port
of the micro controller.
4) L.C.D
is interfaced in 4-bit mode, using a total of 7 I/O lines only.
5) The serial
port doubles as both P.C and printer interface.
6) Voltage
control for frequency and output amplitude of XR2206 tone generation
is provided using on-chip PWMs.
7) Noise
is digitally synthesized using PRBS algorithm.
8) HTL attenuator
and interrupters uses I.C TDA7052 and SSM2211, which makes tone
attenuation and interruption click less and smooth. The attenuator
voltage control is again provided using PWM from micro controller.
9) The noise
attenuator uses I.C TDA8551, whose attenuation can be increased
/decreased using just one pin.
HARDWARE
MICRO
CONTROLLER LOGIC
The micro
controller used is 90S8535. A 6.144 MHz crystal clocks it. The
reset signal is generated via a Brown out detector built in
accordance with Atmel application note AVR180
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