Latest Trends in Radio Microphones
700-MHz-band “specified” radio microphones are
used in music programs, concerts, and the like, but it has
become necessary to shift the frequency used by these
devices into the 1.2GHz band and the white space of
terrestrial television broadcasts. At STRL, we have been
researching and developing low-latency digital specified
radio microphones to contribute to a smooth transition to
the new frequencies. In this article, we explain the latest
trends in radio microphones, which are progressing from
analog to digital and low-latency digital systems.
1. Introduction
Radio microphones, also known as wireless
microphones, are cordless and use one-way radio
transmission technology to send the sounds of voices or
musical instruments without loss of quality. They are used
in a wide range of venues and for various purposes. For
example, they are used to deliver lectures and speeches
in public and in schools and for program production in
broadcast stations, theatres, and concert halls.
Radio communication systems have been rapidly
developing, and all over the world, the bandwidth
used by mobile phones and other radio devices is being
expanded. To accommodate this expansion, in Japan,
the frequencies of specified radio microphones (see Section
2) from 770 to 806 MHz will be migrated to the 1.2-GHz
band and to digital television broadcast white spaces*1.
The revisions necessary for this frequency migration
are currently in progress, and we are conducting R&D
on a low-latency specified radio microphone. This article
describes the radio transmission technology used in this
radio microphone.
Frequencies within the bands used for broadcasting and
communications, which have not yet been used because of
geographical or technical reasons.
2. Specified Radio Microphones
The term specified radio microphone (also called
the type-A radio microphone)4) refers to a type of
professional wireless microphone for broadcasting,
theatre, and concerts. These microphones are classified
as land mobile stations under the Radio Law of Japan,
and as such, they require a radio station licence to
use the bandwidth from 770 to 806 MHz. To avoid
interference between microphones and ensure stable
operation, users must coordinate their operations as to
the location, timeframe, and frequencies used.
In contrast, the Radio Law also classifies some radio
microphones5) as specified low-power radio stations*2.
These include type-B radio microphones, which use
bandwidth from 806 to 810 MHz, type-C microphones,
which use the 322-MHz band, and type-D microphones,
which use the 74-MHz band. These microphones do not
require a license and do not require their operations
to be coordinated. They are not currently subject to
frequency migration.
3. System Revisions for Migration from the 700MHz Band
A report from the Information and Communications
Council in April, 2012 indicated that specified radio
microphones using frequencies from 770 to 806 MHz
(Type-A radio microphones) must migrate to the 1,240 to
1,260 MHz band (excluding 1,252 to 1,253 MHz), digital
terrestrial television broadcast white spaces (470 to 710
MHz), or the 710 to 714 MHz band. System revisions
to accommodate these changes are being made, and
licenses for using microphones in the770- to 806-MHz
range will expire on March 31, 20196).
*1
Application class
Low-power radio stations used for specific purposes. Licensing
is not required.
*2
Table 1: Parameters used in study of analog radio microphones
Pro
General-use
Transmission format
Frequency modulation
linear
Loudspeaker
Frequency modulation, Frequency modulation,
compander
compander
Max. input sound pressure (dBspl*)
130
130
116
Dynamic range (dB)
96
96
82
Transmission system dynamic range (dB)
96
66
52
Max. frequency shift (kHz)
150
40
8
Max. modulation frequency (kHz)
15
15
7
Occupied bandwidth (kHz)
330
110
30
Required receiver input power (dBµ)
51
33
28
*dB sound pressure level: Units for sound pressure level
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4. Wireless Transmission Technologies for Radio
Microphones
Radio microphones can be classified into three types
in terms of their transmission technologies. The first
is the analog radio microphone1), as stipulated in an
R&D report by the Wireless Microphone Development
section of the development committee of the Research
& Development Center for Radio System (RCR) in 1988.
A more advanced type, the digital radio microphone2),
is described in a report issued by the Low-power Radio
Systems ICT subcommittee of the Information and
Communications Council in 2008. The latest type is a
low-latency specified digital radio microphone3) described
in a report by the same mobile communications systems
committee as a measure against frequency crowding in
2013.
4.1 Analog Radio Microphones
Research and development on analog radio
microphones goes back to the 1980s. At the time, radio
microphones were treated as weak radio stations, and
most used the 200- or 400-MHz bands. A ministerial
ordinance in 1986 placed limitations on the electrical
field strength at a distance of 3 m from a weak radio
station. In particular, the permitted values for weak
radio waves from 322 MHz to 10 GHz could be no more
than 35 μV/m. Thus, it became difficult to use radio
microphones over 322 MHz as weak radio stations, and
for that reason, serious R&D on 800-MHz-band radio
microphones began.
Radio microphones were studied in terms of three
applications, i.e., professional use, general use, and
loudspeaker use, and on analog frequency modulation
(FM). The parameters used in these studies are shown in
Table 11).
The use of a compander, which compresses and then
expands the amplitude of the audio signal, was studied
for general and loudspeaker radio microphones. A
compander puts a log compression amplifier*3 at the
An amplifier that compresses the input/output amplitude
based on a log curve.
*3
Table 2: Main technical requirements of analog radio microphones
Specified low-power stations
radio microphone
(Type-B radio microphone) Specified radio microphone
(Type-A radio microphone)
Item
Frequency bands used
779 to 788 MHz（till Mar. 31, 2019）
797 to 806 MHz（till Mar. 31, 2019）
470 to 710 MHz（TV white space）
710 to 714 MHz
1,240 to 1,252 MHz
1,253 to 1,260 MHz
806 to 810 MHz
10 mW or less
10 mW or less
50 mW or less (1,200 MHz band)
Antenna power
Communication scheme
Modulation
Compander
Occupied bandwidth
Permitted values
Simplex/Duplex
Simplex/Duplex
Frequency modulation
Frequency modulation
No (linear)
Yes
Yes or No
330 kHz
110 kHz
160 kHz
250 kHz
(stereo)
110 kHz
Station permit
Required
Not required
Operational regulation
Required
Not required
Audio signal
Log compression amplifier
FM Modulator
Power amplifier
Transmitter system
Receiver amplifier
FM Demodulator
Antilog expansion amplifier
Audio signal
Receiver system
Figure 1: Compander overview diagram
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transmitter in order to reduce the dynamic range of the
modulator input and an antilog expansion amplifier at
the receiver, as shown in Figure 1. This results in linear
amplification of the overall characteristic.
A compander reduces the bandwidth of radio waves
needed to transmit the audio signal and the required input
power at the receiver. It also produces an improvement in
perceived sound quality. However, its transient response
can distort audio quality. For this reason, some type-A
radio microphones do not use companders7).
The main technical requirements for most current
analog radio microphones are shown in Table 2. In the
2012 system revisions, March 31, 2019 is the deadline for
switching from the 770-to-806-MHz band to the 710-to714-MHz and 1.2-GHz bands. Antenna powers up to 50
mW are allowed in the 1.2-GHz band. The 2012 system
revisions also permit an occupied bandwidth of 160
kHz8).
4.2 Digital Radio Microphones
Research and development on digital radio
microphones that efficiently use bandwidth began in the
2000s. It was prompted by the growing use of advanced
sound effects at concerts and large numbers of radio
microphones at big events. An overview of digital radio
microphones is shown in Figure 2. Single-carrier phaseshift keying (PSK) modulations such as QPSK and 8PSK
were initially studied with the intention of transmitting
an audio signal compressed to approximately 1/5th of its
original size at bit rates of 384 to 576 kbps and applying
digital processing such as error-correction coding. The
occupied bandwidth for phase modulation transmission
is equivalent to approximately half of the transmitted
bit rate (192 kHz for type B microphones and 288 kHz
for type A).
In 2008, the results of the study were issued by the lowpower radio systems committee of the Information and
Communications Council2), and commercialization of
digital radio microphone products using frequencies from
770 to 806 MHz began. The digital radio microphones
produced good sound quality and many could be used
simultaneously, but compression and expansion of the
data produced a latency of 3 to 5 ms in the audio, and
this caused insurmountable difficulties for performers in
Audio signal
AD
Conversion
Digital
compression
Phase
modulator
Power
amplifier
Transmitter system
Receiver
amplifier
Phase
demodulator
Digital
expansion
DA
Conversion
Audio signal
Receiver system
Figure 2: Digital radio microphone overview
Table 3: Digital radio microphone main technical requirements
Specified radio microphone
(A-type radio microphone))
Item
Frequency bands used
770 to 806 MHz (till Mar. 31, 2019)
470 to 710 MHz (telephone white space)
710 to 714 MHz
1,240 to 1,252 MHz
1,253 to 1,260 MHz
50 mW or less
Antenna power
806 to 810 MHz
10 mW or less
Simplex/broadcast
Communication scheme
Specified low-power stations
radio microphone
(B-type radio microphone) Simplex/broadcast
Modulation
Phase modulation, frequency modulation, quadrature
amplitude modulation, orthogonal frequency division
multiplexing
Occupied bandwidth
Permitted values
600 kHz
(1,200 MHz band only)
288 kHz
Phase modulation, frequency
modulation, quadrature
amplitude modulation
192 kHz
Station permit
Required
Not required
Operational regulation
Required
Not required
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music programs and concerts. However, the technology
has advanced, and some manufacturers have released
digital radio microphones with latencies of only 1 to 2
ms.
The Ministry of Internal Affairs and Communications
(MIC) in Japan announced a frequency reorganization
plan in 2011, in which specified radio microphones
using frequencies from 770 to 806 MHz to would be
“migrated” to frequencies in the digital terrestrial
television broadcast white spaces and the 1.2 GHz band.
The reason for doing so is the continued expansion
of wireless communications systems and bandwidth
used by their devices such as mobile phones. The
destination frequencies will also be used for other tasks,
so establishing a digital transmission scheme for radio
microphones that is tolerant of interference and has
extremely low latency has become an urgent issue.
In order to transmit the microphone audio signal
with stability, high quality, and low latency on the new
frequencies, we began R&D on a transmission scheme
using orthogonal frequency division multiplexing
(OFDM). OFDM is resistant to fading and promised to
increase transmission capacity and reduce latency.
Testing to verify the low-latency specified digital radio
microphone transmission scheme that we developed
in FY 2011 and FY2012 was done as part of the “Study
of technical requirements for migration of specified
radio microphone frequencies for more efficient use of
the 700 to 900 MHz band” conducted by MIC. In May,
2013, the Information and Communications Council
released the report of the mobile communications
systems sub-committee and this was quickly followed by
system revisions for low-latency specified digital radio
microphones in August, 20139). The main technical
requirements for digital radio microphones are shown
in Table 3. The system revisions of August, 2013 added
OFDM as a modulation method, as well as a permitted
occupied bandwidth of 600 kHz in the 1.2 GHz band
(underlined parts in Table 3).
4.3 Audio Latency
Audio latency has an effect on musical performances.
The sound from the mixing board is actively sent back to
the performers through the in-ear monitors, making the
delay requirements much more stringent. Performers are
able to hear their own voices or instruments directly and
when this live sound is mixed with delayed sound from
the in-ear monitors, it has an effect on the sound quality
felt by the performer. As a preliminary part of developing
the low-latency specified digital radio microphone, we
checked this effect by evaluating the detectable limits of
delay in the audio monitors and their effects. We found
that if the delay of the entire system is 3 ms or less, there
is generally no problem, but if the delay exceeds 5 ms,
80% of performers noticed the delay10).
In real environments, a radio microphone, mixing
board and in-ear monitor are used together, as shown
in Figure 3. Most modern mixing boards are also digital,
so they introduce a delay of approximately 2 to 3 ms.
Thus, if the delay of both the radio microphone and the
earphone monitor can be kept to 1 ms or less, the overall
system delay can be kept to 5 ms or less.
4.4 Transmission Format for Low-latency Specified
Digital Radio Microphone
If the sound signal is transmitted uncompressed in
order to ensure low latency, the data rate must be at least
four times higher than when using data compression.
To achieve this, we studied a method combining multivalue modulation techniques to use frequency efficiently,
with OFDM, which is resistant to reflections and other
interfering signals.
With this method, we hoped to reduce delay by
manipulating the transmission parameters. We first
reduced the time needed for fast Fourier transform
(FFT) and error correction buffering by using a short
OFDM symbol length and using an error correction
code (convolution code) compatible with a short code
length. Then, we minimized the buffer time for signal
processing by making the audio signal sample length
an integral multiple of the OFDM symbol length and
error correction code symbol length. The transmission
parameters, taking these results into consideration, are
shown in Table 4 11).
The data rate for transmitting an uncompressed audio
signal sampled at 48 kHz with 24-bit quantization,
which is the standard quality in studio environments,
is 1,152 kbps. Parity coding able to detect errors in the
audio signal in single-word (24-bit) units was added to
this, assuming errors would be corrected in the receiver.
Convolution coding and Viterbi decoding, which have
low latency, were used for error-correction coding and
decoding.
The low-latency specified digital radio microphone
Radio microphone
receiver
Audio
mixing board
In-ear monitor
receiver
In-ear monitor
transmitter
Figure 3: Radio microphone and (earphone) monitor
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transmission method has three transmission modes, as
shown in Table 4. In the standard microphone mode, the
uncompressed audio signal is transmitted with 16QAMOFDM modulation. The interference-tolerant mode uses a
method called instantaneous companding12) to compress
the information by 50%, and then uses QPSK-OFDM
for transmission to improve performance when there is
noise or other interference. The in-ear monitor mode uses
instantaneous companding to reduce the information by
50%, before sending a two-channel stereo audio signal.
Table 4: Low-latency specified digital radio microphone transmission parameters
Data coding
Mode
Standard Mic.
Interference
tolerant mic.
Analog audio signal
Mono
Mono
Stereo
Quantization (bits)
24
24
24 (2 channels)
48
Sampling freq. (kHz)
Uncompressed
Data compression
Instantaneous comp./decomp. Instantaneous comp./decomp/
24
Transmitted data (bits)
1,152
Data rate (kbps)
12
12 (2 channels)
576
1,152
CRC*1-2
Parity coding
Convolution coding, 2/3
Error coding, coding rate
16QAM
Primary (carrier) modulation
QPSK
16QAM
OFDM
Secondary modulation
Effective symbol length (µs)
78.4
Symbol length (µs)
83.3
Guard interval (µs)
4.9
12.75
Carrier interval (kHz)
Total
No. Carriers
Transmission path coding
In-ear monitor
46
Data
39
SP*2
3
TMCC*3
3
CP*4
1
586.5
Transmission bandwidth (kHz)
*1 Cyclic Redundancy Check.
*2 Scattered Pilot.
*3 Transmission and Multiplexing Configuration Control.
*4 Continual Pilot.
Transmitter
Pilot signal
Transmitter antenna
Error correction
coding
Carrier
modulation
OFDM
Framing
IFFT *1
Add
GI *2
*1 Inverse Fast Fourier Transform
*2 Guard Interval
Receiver antenna
Sync
detect
Receiver
FFT
Max. val.
combining
Sync
detect
Data carrier
extraction
FFT
Carrier
demodulation
Error correction
decoding
DA
Conversion
Mixing console, etc.
Microphone
AD
Conversion
Figure 4: Low-latency specified digital microphone architecture
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Handheld receiver
Digital radio
microphone
(two-piece)
Receiver antennas
Digital radio
(shared)
microphone receiver
Digital radio microphone
(handheld)
Two-piece receiver
Figure 5: Low-latency specified digital radio microphone prototype
Here, instantaneous companding refers to a method
that digitally operates on the amplitude characteristic
of the input audio using a curve that approximates
the log characteristic to compress the information. The
method is similar to companders used with analog
radio microphones, and although it degrades the sound
quality slightly, it achieves data compression with very
little delay.
The architecture of the low-latency specified
digital radio microphone is shown in Figure 4. OFDM
modulation and spatial diversity technology are used
to improve the reliability of the transmission. At the
receiver, maximal-ratio combining*4 on each OFDM
subcarrier is applied before the error correction decoding.
This enables adequate performance to be gained from
spatial diversity and achieves highly reliable radio
transmission without time interleaving, which is another
cause of audio latency. The prototype digital low-latency
specified digital radio microphone is shown in Figure 5.
5. Conclusion
We have given an overview of radio microphone
transmission methods, described the state of frequency
migration for specified radio microphones, and
introduced the work being done on a new low-latency
specified digital radio microphone.
Till now, the 770 to 806 MHz band could be used by
specified radio microphones anywhere in the country
without interference. Analog microphones never had an
issue with latency. Such operation will still be possible
anywhere in Japan after the switchover to the 1.2-GHz
band, and linear PCM audio transmission with high
quality and low latency will be able to be achieved.
However, the new bandwidth must be shared with other
radio-standardized tasks*5, so the new digital radio
microphones will have to be tolerant to interference and
have very low latency.
A method which applies weightings to two input signals
before combining them in such a way as to maximize the
signal-to-noise ratio (SN ratio) of the combined signal.
*5
Tasks using radio waves to measure position, other than for
the purpose of navigation for ships and aircraft.
*4
12
To migrate smoothly from the 700-MHz band,
a 1.2-GHz-band low-latency specified digital radio
microphone must be implemented quickly. Accordingly,
we hope incorporate the results of our studies on these
issues into the standardization work at the Association
of Radio Industries and Businesses (ARIB).
(Hiroyuki Hamazumi)
References
1) Research & Development Center for Radio Systems:
“Wireless Microphone Development Section R&D
Report (June, 1988),” RCR TR-15 (Japanese)
2) Information and Communications Council: “Lowpower Radio Systems ICT Subcommittee Report (Oct.
9, 2008) Ref. 61-2-2” (Japanese)
3) Information and Communications Council: “Mobile
Communication Systems ICT Subcommittee Report
(May 17, 2013) Ref. 94-1-1, Ref. 94-1-2” (Japanese)
4) Specified Radio Microphone User’s Federation (Ed.):
Wireless Microphone Handbook, Kenrokukan
Publishing (Japanese)
5) ARIB: “Radio-Microphone for Specified Low Power
Radio Station,” ARIB RCR STD-15
6) National Printing Bureau: “Official Gazette (Extra)
No. 141 (June 28, 2012)”
7) ARIB: “Specified Radio-Microphone for Land Mobile
Radio Station,” ARIB RCR STD-22
8) ARIB: “Specified Radio Microphone for Land Mobile
Radio Station (TV White Space Band, 1.2GHz Band)
Standard,” ARIB STD-T112
9) National Printing Bureau: “Official Gazette No. 6110
(Aug. 15, 2013)”
10)Kamekawa, Marui, Abe, Hamazumi, Kohchi: “The
shortest delay time detectable during musical
performance and its influence,” The Japanese Society
for Music Perception and Cognition Spring Meeting,
Vol. 5, pp. 25-30 (2012) (Japanese)
11)Taguchi, Nakamura, Iai, Okano and Hamazumi: “A
Study of Low Delay Digital Transmission for Specified
Radio Microphone,” ITE Technical Report, BCT2012101, pp.39-42 (2012) (Japanese)
12)Nikaido, Yamazaki: Digital Audio for the Sound
Engineer, Kenrokukan Publishing (Japanese)
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