Special Feature

Ultrasmall Audio Receiving Unit Using
InfraredYasuyuki Matsuya†, Takako Ishihara, Ryusuke Kawano,
Junichi Inoue, and Yuichi Kado
Abstract
Most of the IT services offered to date have provided an image-based communication platform. We
have now endowed ultrasmall sensors and actuators with short-range wireless communication capability and developed devices that can be attached to all sorts of everyday things, embedded in the environment, or even worn. We have investigated some of the potential applications made possible by communication between people and all sorts of objects. This article presents an overview of VoiceUbique, anultrasmall audio receiver that has a tiny dynamic speaker for the actuator and uses infrared waves for
short-range wireless communication.

1. Introduction
Internet and email-centered information technology (IT) has primarily been concerned with the transmission of information in the form of text and images(corresponding to the graphical user interface (GUI)),
so little attention has been paid recently to the conveyance of information by plain-old human speech.
To some extent this can be attributed to the fact thatmodern-day data communication has come to rely on
packet communication in which realtime delivery is
not required, while speech needs the time sequence ofthe data to be preserved, so packet-delivery techniques are not very applicable. Yet for the widespreaddissemination of IT, the age-old medium of humanspeech for providing information (voice user interface (VUI)) is as important as ever, and the implementation of an interface that is extensively available
and can be used by anyone is a critical means of providing information. More particularly, the ability touse such an interface virtually anywhere becomes
important as the need to convey information ubiquitously increases [1], [2], and this raises the issue ofmaking terminals wearable. We have developed† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: mats@aecl.ntt.co.jp

Vol. 2 No. 2 Feb. 2004

VoiceUbique as an example of a VUI terminal implemented in a wearable form. This article provides an
overview of VoiceUbique and discusses its performance.
2. System construction of VoiceUbique
VoiceUbique is an ultrasmall audio receiver implemented in the form of a ring that we developed as awearable VUI device that could be used in ordinaryand everyday living spaces. Going beyond the usual
ubiquitous access attributes of anytime and anywhere, VoiceUbique can also aim audio messages at asingle person—such as just you or just me. In orderfor the device to be available for use anytime and anywhere, the audio receiver must be worn constantly
either on the hand or in the ear, which means that its
size and weight must enable it to be worn the entire
day without causing any discomfort or awkwardness
to the wearer. And to enable one to listen to the device
for extended periods without getting tired, the soundreproduction should be as natural as possible, noise
should be screened out, and the sound quality should
be good enough that one can listen to and enjoy
music. And finally, to achieve an interface that is so
simple that anybody can use the device, we had to get
rid of buttons for selecting channels and other hardto-figure-out controls. We came up with the59Special Feature

following short list of requirements for a wearable
VoiceUbique receiver:
1. Small enough to be implemented as a ring (total
volume: a few cubic centimeters)
2. Capable of at least ten hours of continuous use
on one small lithium coin cell
3. Standby power consumption of less than 10 µW
4. A user interface involving completely naturaland inconspicuous actions
5. Good sound quality equivalent to FM-radio
broadcast quality (signal bandwidth of 16 kHz
and signal-to-noise ratio of 70 dB).
The tiny receiving units that are currently available
for micropower, Bluetooth, and other low-power
schemes are about the size of a postage stamp andconsume only 30 mW of power, but these specifications are still not close to satisfying our requirements#1 and 2. A digital transmission scheme that is robust
against transmission noise would be preferable toprovide the sound quality stipulated in #5, so wechose to use infrared carrier communication and a(a) Transmitter

digital transmission scheme based on 1-bit quantization technology for the VoiceUbique system.
Figure 1 shows the main components of the
VoiceUbique system. The transmitter (a) is 70 mm ×
40 mm × 15 mm. The infrared emitter (b) consists ofIrDA (Infrared Data Association) units of the same
type as used in personal computers, each 12 mm indiameter × 65 mm long. The ring-shaped receiver (c)has a hemispherical receiver in the middle 16 mm indiameter × 15 mm thick (2.5 cm3); the volume of theentire device including the band of the ring is 3 cm3,
including a lithium non-rechargeable cell supporting
continuous use for up to 35 hours, so it easily satisfies
requirements #1 and 2.
Figure 2 shows a typical situation in which VoiceUbique might be used. Pausing in front of an exhibit ata museum or exhibition hall, the user nonchalantly
brings her hand bearing the VoiceUbique receiverring up to her ear, and is then able to hear a detailed
description of the exhibit, background music, or othercontent. This allows the user to obtain information(b) Infrared emitter(c) ReceiverFig. 1. VoiceUbique system.

InfraredFig. 2. Typical situation using VoiceUbique.

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about the objects on display as she moves through theexhibit, and the interface for accessing the information is perfectly natural and inconspicuous.

reduced to 3 mW or less. Below we discuss the system configuration that enabled us to achieve thisremarkably low power consumption.
Figure 3 shows a block diagram of the circuits of
the VoiceUbique system. The sound source that stores
messages or other types of audio content might be thesame type of device as used in an analog signal output terminal or headphone output terminal. The transmitter receives the analog signal from the soundsource, adjusts the gain with an amplifier, and thenconverts the analog signal to a digital signal througha bit-stream conversion process using a noise-shaping
analog-to-digital (A/D) converter [3]. This digital
signal is then converted to an ON-OFF keying *1
infrared signal by an IrDA transmission device, andtransmitted. The receiver converts the incoming
infrared carrier to an electrical signal, adjusts theshape of the wave using an inverter, and recreates theoriginal sound just by driving the speaker. An ultrasmall dynamic speaker was used to achieve an ultrasmall size.
The significant thing to note about this configuration is that it enabled us to omit some of the circuitry
usually required for digital transmission from the
VoiceUbique receiver (clock playback, frame synchronization circuitry, etc.) and also eliminated theneed for a digital-to-analog (D/A) converter, so wewere able to reduce both the size and the power consumption of the device at the same time. Indeed, wewere able to bring the current consumption of the
VoiceUbique receiver down to 1 mA, including thereceiver operating current and the speaker drive current. Figure 4 shows the actual measured continued
usage time versus battery voltage for two different
lithium batteries: CR1220 and CR1616. At the lower
limit voltage of 2.2 V, the receiver could be operated

3. Miniaturization and reduction of power
consumption
The most problematic issues in miniaturizing thereceiver were the continuous operating time requirement and the volume and weight of the battery. Table
1 lists the volume, power, and weight specifications
for some of the lithium non-rechargeable coin cells
that are currently available. The four-digit numbers in
the “battery type” column have the following meaning: the first two digits specify the diameter of thebattery and the second two digits indicate ten times
the thickness of the battery in millimeters.
Considering the thickness of the battery casing (2mm) and our ring-shaped implementation, we settled
on the CR1220 cell (diameter: 12 mm). An added
advantage of using this size cell is that there are bothrechargeable and non-rechargeable batteries available in this size, so it would be easy to implement aversion of VoiceUbique using either type of battery.
We calculated that to satisfy requirement #2 (tenhours of continuous use) using the CR1220 battery,the power consumed by the receiver had to be
Table 1. Size versus capacity for typical commercial
lithium cells.
Battery type

Volume (cm3)Power (mAh)

Weight (g)CR1216

0.18

25

0.7

CR1220

0.23

35

1.2

CR1616

0.32

55

1.2

CR1620

0.40

75

1.3

CR2016

0.50

90

1.6

CR2025

0.75

165

2.5

CR2032

1.00

220

3.1

*1 ON-OFF keying sends data by switching the light beam on andoff.

IrDA unit

Audio
sourceAmp.

Noise-shaping
A/D converter

InfraredLED
driverTransmitter

IrDA unit

InfraredLED
receiver

Receiver

LED: light emitting diodeFig. 3. VoiceUbique circuit block diagram.

Vol. 2 No. 2 Feb. 2004

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3.5

300

Distance [Y] (cm)Battery voltage (V)250
3CR1616
2.5
CR12202

1.5

2 units, same
direction

200
150
1 unit100

2 units,
differentorientation(±10°)

50
0
–150
0

10

20
30
Time (hour)4050Fig. 4. Continued usage time vs. battery voltage.

continuously for up to 35 hours on one CR1220 battery.
If the IrDA receiving unit is enabled, it consumes
up to about 0.4 mA of current even when it is notreceiving an infrared signal. This led us to implement
a standby circuit that reduces the current to 0 mA
when the device is in standby mode. Essentially, it
works as follows: The IrDA receiver is enabled for 30
seconds after it is turned on, and if an infrared carrier
is received during that 30-s interval, it remains
enabled. However, if no signal is received during the
30-s interval, the IrDA receiver is automatically disabled. The circuit uses only a few nanoamperes ofcurrent when the IrDA receiver is disabled, whichmeans that the power consumption of the VoiceUbique in standby mode is close to 0 mW. A uniaxial
slanted switch was used for the standby circuit
switch, so that the user can automatically activate theswitch without being aware of doing so, just by moving his hand bearing the VoiceUbique receiver up to
his ear.
We thus succeeded in implementing a ring-shaped
wearable audio receiver that satisfied our first threerequirements: 1) ultrasmall size was achieved by
implementing a tiny 3-cm3 receiver powered by aneven smaller lithium battery, 2) the device can becontinually operated for up to 35 hours using infraredtransmission based on 1-bit quantization and a standby circuit, and 3) the standby power consumption was
reduced to a few nanowatts. Although the implementation created in this work was in the shape of ring,these same technologies could just as easily be used
to develop ultrasmall earphones, a strip-shaped listening device, or tiny audio receivers in all sorts ofshapes.

Transmitter
–100

–50

0

50

100

150

Distance [X] (cm)Fig. 5. Reception range of the receiver.

4. Communications distance and optical radiation
safety concerns
Figure 5 shows the coverage of infrared rays when
the infrared transmitter is set at Y = 0 and X = 0. The
three lines show fields of error-free reception for an
infrared digital carrier sent from an IrDA transmitter.
Good audio reception can be received up to a distance[Y] of 150 cm and radius [X] of ±30 cm in the case of
one receiver, and for a distance [Y] of 250 cm and aradius [X] of ±50 cm in the case of two units. When
two units are set at an angle of ±10°, good soundquality can be received up to a distance [Y] of 150 cm
and radius [X] of ±60 cm. It is thus clear that by using
infrared transmission, the reception range can be easily adjusted or modified by using a different numberof receiving units or by deploying them differently.
This is a potentially useful feature that cannot be
obtained using radio waves to transmit audio, andcould for example be exploited to transmit audio content to a select audience within a specified area.
Because VoiceUbique uses invisible infrared signals, we must verify that they do not have any adverse
health effects on people. The most vulnerable part of
the human body is the eyes. The infrared carrier usedby IrDA is dispersed light from a light emitting diode(LED) not the collimated energy of a laser, so it will
be perfectly safe if there is sufficient distance
between the light source and the eye. According toJapanese standard JIS-C-6802, a value of 180
mW/sr *2 means that one could stare into a lightsource (a 2.7-mm diode) positioned 10 cm away foreight straight hours and not sustain any ill effects*2 mW/sr: milliwatts per steradian (solid angle)

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Measurement pointdIntensity distribution in a plane 100 mm away from two sources
2

100 mm

1.8
Intensity (/unit)

1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
–100 –80 –60 –40 –20

0

20

40

60

80

g Light
Light gsourcesourceMeasurement conditions
g=0 mmg=10 mmg=20 mmg=30 mm
g=40 mmg=50 mm
g=60 mmg=70 mmg=80 mmg=90 mm
g=100 mm100

Distance from measurement point to midpoint between emitters [d] (mm)Fig. 6. Light intensity as a function of distance between units.

(class 1 safety). The IrDA emitters used in the
VoiceUbique system emit 89 mW/sr, so although asingle IrDA emitter would be perfectly safe, theabove safety criterion could be exceeded when several units are used at the same time. We found, however, that if the units are set a certain distance apart, thenthe safety standard is satisfied. Figure 6 shows thelight intensity measured at a point located 100 mm
from the midpoint between two transmitters that are
emitting simultaneously as a function of distance
between the two emitters. When they are far apart, thelight intensity approaches the value for a single unit.
When they are close together, the light intensity is
twice that of a single unit, but the intensity weakens
and safety standards are met when the units are atleast 100 mm apart and the midpoint between thelight sources is at least 100 mm away.
This ability to allow us to freely design the communication distance and range to ensure eye safety by
specifying the number and positioning of units is aunique attribute of the VoiceUbique system.
5. Built-in ID transmission function
Miniature RF-ID (radio frequency identification)
tags could be built into VoiceUbique, so that audio
messages targeting just specific individuals could be
easily implemented by incorporating RF-ID tag readers in transmitters. For example, if we assume passive
RF-ID tags (64 bit, 125 kHz) that can only be readand are only viable for transmission ranges up to afew centimeters, then miniature tags (2 mm in diameter × 12 mm long) could be built into VoiceUbique.
Figure 7 illustrates that, with these miniature tags
embedded in the system, a user could pass her handVol. 2 No. 2 Feb. 2004

Content sent to
VoiceUbiqueUser merely
moves her hand

ID sent to PC

Pass hand over
RF-ID reader

Tag reader

Fig. 7. Typical use of VoiceUbique with RF-ID tags.

bearing the VoiceUbique ring over a tag reader, andthen by bringing her hand up to her ear, could hearmessages intended just for that one individual.
This capability might be put to good use at an exhibition. For example, based on information collectedin advance about the people attending the exhibition,a message system tailored to individual preferences
and interests could be set up that provides moredetailed information about the objects that people areinterested in and less information about other objects.
6. Conclusion
In seeking to devise a voice user interface that canbe used to gain ubiquitous access to information in
everyday public places, we have developed an audio
receiving unit that we have dubbed VoiceUbique.
Implemented in the form of an ordinary ring, VoiceUbique uses infrared signals, consumes only 3 mW
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Special Featureof power, and can be used continuously for up to 35
hours. Leveraging the unique characteristics ofinfrared digital transmission, the system permits
high-quality audio content to be delivered to certain
specified users within a specified area. If RF-ID tags
are embedded into the receivers, content could bedelivered just to certain specific individuals. Buildingon this approach, we plan to promote even more
ubiquitous access using this voice user interface byimplementing the baseband signal processing part asan LSI to make the transmitter smaller, by further
reducing the power consumption of the device, andby embedding sound sources in all sorts of everyday
living places.
References
[1]

[2]
[3]

F. Morisawa, S. Mutoh, J. Terada, Y. Sato, and Y. Kado, “Remote
Hits and Stick-on Communicator That Touches Objects,” Interaction
2003, Digest of Technical Papers, p. 223, Feb. 2003.
“NTT Research and Development at the Forefront of Ubiquitous
Communication,” Nikkei Electronics, Dec. 16, pp. 32-33, 2002.
Y. Matsuya, K. Uchimura, and A. Iwata, IEEE JSSC, Vol. SC-22, No.
6, pp. 921-929, 1987.

Yasuyuki Matsuya

Junichi Inoue

Senior Research Engineer, Supervisor, Smart
Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S. in electronic engineeringfrom Iwate University, Iwate in 1978, andDr.Eng. degree in electronic engineering from
Tokyo Institute and Technology, Tokyo in 1996.
He joined Musashino Electrical Communications Laboratories, Nippon Telegraph and Telephone Public Corporation (now NTT) in 1978.
His primary research interests include advanced
analog circuit design and low-voltage circuit
technology. He is currently involved in R&D ofwearable communications devices. He is a member of the Institute of Electronics, Information,
and Communication Engineers (IEICE) andIEEE.

Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E. degree in electronic engineering from The University of Electro-Communications, Tokyo in 1977. Since joining the Electrical Communication Laboratories, Nippon
Telegraph and Telephone Public Corporation(now NTT), Tokyo in 1977, he has worked on
R&D of memory LSI circuits. He is currently
studying voice interface systems for ubiquitouscommunication. He is a member of IEICE.

Yuichi Kado
Takako Ishihara
Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
She joined Musashino Electrical Communications Laboratories, Nippon Telegraph and Telephone Public Corporation (now NTT) in 1976.
She has been engaged in the testing of memory
LSIs and the design of the low-power high-speed
communication LSIs. She is currently engaged in
R&D of voice interfaces for ubiquitous communications. She is a member of IEICE.

Executive Manager, Smart Device Laboratory,
NTT Microsystem Integration Laboratories.
He received the M.E. and Ph. D. degrees inelectronics from Tohoku University, Sendai,
Miyagi in 1983 and 1998, respectively. In 1983,
he joined NTT Electrical Communications Laboratories, where he engaged in research on SOI
structure formation by heteroepitaxial growth.
From 1989 to 1998, he worked on the development of fully-depleted CMOS/SIMOX LSIs andultralow-power CMOS circuits. From 1999, heengaged in R&D on compact network appliances
using ultralow-power CMOS circuit technologies for ubiquitous communications. He hasserved as a program committee member of the
ISSCC (International Solid-State Circuits Conference) since 1999 and a technical committee
member of IEEE Computer Elements since 2003.
He is a member of IEEE and IEICE.

Ryusuke Kawano
Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E., and M.E. degree in electronic engineering from the University of Osaka
Prefecture, Osaka in 1987, and 1989, respectively. He joined NTT in 1989. His primary researchinterests include designing high-speed digital circuits and R&D of high-speed hardware for communications. He is currently working on hardware for short-range communications. He is amember of IEICE, IEEE, and Japan Institute ofElectronics Packaging (JIEP).

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