WAN-0215 DRC Operation in Wolfson Audio CODECs

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WAN-0215
DRC Operation in Wolfson Audio CODECs
INTRODUCTION
This applications note has been created to explain the operation of the Dynamic Range Controller
(DRC) used in the latest Wolfson audio CODECs. Not all devices will have all of the functions
described in this application note. The devices using the DRC function are shown in Table 1.
WM8903
WM8904
WM8912
WM8948
WM8993
WM8994
WM8944
WM8945
WM8946
Table 1 Devices that use the DRC Function
This list will change as newer devices are introduced using this same technology.
The DRC is a circuit that can be enabled in the playback or digital record path of the CODEC,
depending upon the selected DSP mode, boost quiet signals and attenuate louder signals. The
function of the DRC is to adjust the signal gain in conditions where the input amplitude is unknown or
varies over a wide range, e.g. when recording from microphones built into a handheld system.
The DRC can apply Compression and Automatic Level Control to the signal path and replaces the
ALC used by many Wolfson devices. It incorporates ‘anti-clip’ and ‘quick release’ functions for
handling transients in order to improve intelligibility in the presence of loud impulsive noises.
In some devices, the DRC also incorporates a Noise Gate function, which provides additional
attenuation of very low-level input signals. This means that the signal path is quiet when no signal is
present, giving an improvement in background noise level under these conditions.
DRC COMPRESSION / EXPANSION / LIMITING
The DRC supports two different compression regions, separated by a “Knee” at a specific input
amplitude. In the region above the knee, the compression slope DRC_HI_COMP applies; in the
region below the knee, the compression slope DRC_LO_COMP applies.
The overall DRC compression characteristic in “steady state” (i.e. where the input amplitude is nearconstant) is illustrated in Figure 1.
Figure 1 DRC Response Characteristic
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Note that Figure 1 shows the transfer response for the DRC i.e. the output signal amplitude for a
given input signal amplitude, and not the gain of the DRC. The Gain of the DRC is the difference
between the input signal amplitude in dB and the output amplitude in dB
For additional attenuation of signals in the noise gate region, an additional “knee” can be defined
(shown as “Knee2” in Figure 1). When this knee is enabled, this introduces an infinitely steep dropoff in the DRC response between the DRC_LO_COMP and DRC_NG_EXP regions.
The ADC HPF MUST be enabled when the DRC is used in the record path as dc offsets will
cause erroneous operation.
The DRC also supports a noise gate (NG) region, where low-level input signals below the level set by
DRC_KNEE2_IP are heavily attenuated. This function can be enabled or disabled according to the
application requirements.
COMPRESSION
The basic DRC operation does not use the noise gate (NG) function and Knee2 has no effect as
shown in Figure 2.
Figure 2 DRC Basic Response Characteristic
The “Knee” (Knee1) is determined by the input level DRC_KNEE_IP and DRC_KNEE_OP level. In
the region above the knee, the compression slope DRC_HI_COMP applies; in the region below the
knee, the compression slope DRC_LO_COMP applies.
The value Y0 is calculated from the equation below, where the Knee values are in dB and the Comp
is a scalar value:
Y0 = DRC_KNEE_OP - (DRC_KNEE_IP * DRC_HI_COMP)
For example, DRC_KNEE_IP = -24dB, DRC_KNEE_OP -12dB, DRC_HI_COMP = ¼:
Y0 = -12 - ( -24 * ¼ ) = -6dB
The compression values can be set for different DRC performance. For a compression slope of 1
there is no compression. The output signal level will change by the same amount as the input signal
level changes. This is the same as having a fixed gain between the input and output signals. A
compression slope of 0 results in a constant output amplitude which is the same as using an
automatic level control (ALC) to maintain a constant output signal level for a varying input signal
level. For compression slopes between 0 and 1, the signal level on the output signal level will change
by less than change in the input signal level.
For example, if the compression slope is ¼, the change in output signal level is ¼ of the change in
the input signal level. So for a 4dB change in input signal level there will be a 1dB change in output
level.
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The compression regions can be set independently to get the desired operation.
The registers associated with the basic DRC operation are shown in Table 2.
REGISTER
ADDRESS
BIT
DRC Control 1
7
DRC Control 4
7:2
DRC_KNEE_IP
000000
Input signal level at the Compressor
‘Knee’.
000000 = 0dB
000001 = -0.75dB
000010 = -1.5dB
… (-0.75dB steps)
111100 = -45dB
111101 = Reserved
11111X = Reserved
DRC Control 5
7:3
DRC_KNEE_OP
00000
Output signal at the Compressor
‘Knee’.
00000 = 0dB
00001 = -0.75dB
00010 = -1.5dB
… (-0.75dB steps)
11110 = -22.5dB
11111 = Reserved
2:0
DRC_HI_COMP
011
Compressor slope (upper region)
000 = 1 (no compression)
001 = 1/2
010 = 1/4
011 = 1/8
100 = 1/16
101 = 0
110 = Reserved
111 = Reserved
7:5
DRC_LO_COMP
000
Compressor slope (lower region)
000 = 1 (no compression)
001 = 1/2
010 = 1/4
011 = 1/8
100 = 0
101 = Reserved
11X = Reserved
DRC Control 7
LABEL
DRC_ENA
DEFAULT
0
DESCRIPTION
DRC Enable
0 = Disabled
1 = Enabled
Table 2 DRC Registers for Basic Operation
NOISE GATE
The DRC also supports a noise gate region, where low-level input signals below the level set by
DRC_KNEE2_IP are heavily attenuated. This is useful for reducing background noise during periods
of silence. The attenuation is controlled by the expansion slope DRC_NG_EXP as shown in Figure 3.
The expansion slope DRC_NG_EXP can be set to rapidly reduce the output signal level when the
input signal reduces. When the expansion slope is set to 1 then there is no expansion and the output
signal level changes by the same as the input signal level change. If the expansion slope is set to a
value greater than 1, then the output signal level changes by more than the change in input signal
level.
For example, if the expansion slope is 4, then the change in output signal level is 4 times larger than
the change in the input signal level. So for a 1 dB change in input signal level the output signal level
will change by 4dB.
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Figure 3 DRC Response Characteristic with Noise Gate
The input signal level where the NG takes affect is set by DRC_KNEE2_IP.
The additional registers associated with the NG function are shown in Table 3. Note that the DRC
should be set for basic operation as described in the previous section.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DRC Control 1
8
DRC_NG_ENA
0
DRC Control 4
12:8
DRC_KNEE2_IP
DRC Control 7
9:8
DRC_NG_EXP
000000
00
DESCRIPTION
DRC Noise Gate Enable
0 = Disabled
1 = Enabled
Input signal level at the Noise Gate
threshold ‘Knee2’.
00000 = -36dB
00001 = -37.5dB
00010 = -39dB
… (-1.5dB steps)
11110 = -81dB
11111 = -82.5dB
Only applicable when
DRC_NG_ENA = 1.
Noise Gate slope
00 = 1 (no expansion)
01 = 2
10 = 4
11 = 8
Table 3 DRC Registers for Noise Gate Operation
NOISE GATE WITH KNEE2
For additional attenuation of output signal levels in the noise gate region, an additional “knee” can be
defined, shown as “Knee2” in Figure 4. When this knee is enabled (DRC_KNEE2_OP_ENA=1), this
introduces an infinitely steep drop-off in the DRC response between the DRC_LO_COMP and
DRC_NG_EXP regions as shown in Figure 4.
For example, if DRC_KNEE2_IP is set to -40dB and DRC_KNEE2_OP is set to -30dB, when the
input signal level reduces to -40dB the output signal level will drop to -30dB. So if the output signal
level is -20dB when the input signal is just above DRC_KNEE2_IP (-40dB), when the input signal
drops to -40dB the output signal will drop to -30dB.
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Figure 4 DRC Response Characteristic with NG and Knee2
The additional registers associated with the Knee2 function are shown in Table 4. Note that the DRC
should be set for NG operation as described in the previous section.
Setting DRC_KNEE2_OP_ENA to 1 when DRC_NG_ENA=0 will have no effect.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DRC Control 5
13
DRC_KNEE2_OP
_ENA
0
12:8
DRC_KNEE2_OP
00000
DESCRIPTION
DRC_KNEE2_OP Enable
0 = Disabled
1 = Enabled
Output signal at the Noise Gate
threshold ‘Knee2’.
00000 = -30dB
00001 = -31.5dB
00010 = -33dB
… (-1.5dB steps)
11110 = -75dB
11111 = -76.5dB
Only applicable when
DRC_KNEE2_OP_ENA = 1.
Table 4 DRC Registers for Noise Gate with Knee2 Operation
GAIN LIMITS
The minimum and maximum gain applied by the DRC is set by registers DRC_MINGAIN,
DRC_MAXGAIN and DRC_NG_MINGAIN. These limits can be used to alter the DRC response from
that illustrated in Figure 1 to Figure 4. If the range between maximum and minimum gain is reduced,
as shown in Figure 5, then the perceived loudness/intelligibility generally improves, at the expense of
reduced dynamic range.
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Figure 5 DRC Response Characteristic with Max and Min Gain Limits
The minimum gain in the Compression regions of the DRC response is set by DRC_MINGAIN. The
minimum gain in the Noise Gate region is set by DRC_NG_MINGAIN. The minimum gain limit
prevents excessive attenuation of the signal path.
The maximum gain limit set by DRC_MAXGAIN prevents quiet signals (or silence) from being
excessively amplified. The registers associated with the gain limits are shown in Table 5
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REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
DRC Control 2
12:9
DRC_NG_MING
AIN [3:0]
0110
Minimum gain the DRC can use to
attenuate audio signals when the
noise gate is active.
0000 = -36dB
0001 = -30dB
0010 = -24dB
0011 = -18dB
0100 = -12dB
0101 = -6dB
0110 = 0dB
0111 = 6dB
1000 = 12dB
1001 = 18dB
1010 = 24dB
1011 = 30dB
1100 = 36dB
1101 to 1111 = Reserved
4:2
DRC_MINGAIN
[2:0]
001
Minimum gain the DRC can use to
attenuate audio signals
000 = 0dB
001 = -12dB (default)
010 = -18dB
011 = -24dB
100 = -36dB
101 = Reserved
11X = Reserved
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REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
1:0
DRC_MAXGAIN
[1:0]
01
Maximum gain the DRC can use to
boost audio signals (dB)
00 = 12dB
01 = 18dB
10 = 24dB
11 = 36dB
Table 5 DRC Gain Limits
GAIN READBACK
The gain applied by the DRC can be read from the DRC_GAIN register. This is a 16-bit, fixed-point
value, which expresses the DRC gain as a voltage multiplier.
DRC_GAIN is coded as a fixed-point quantity, with an MSB weighting of 64. The first 7 bits represent
the integer portion; the remaining bits represent the fractional portion. If desired, the value of this field
may be interpreted by treating DRC_GAIN as an integer value, and dividing the result by 512, as
illustrated in the following examples:
DRC_GAIN = 05D4 (hex) = 1380 (decimal)
Divide by 512 gives 2.914 voltage gain, or 4.645dB
DRC_GAIN = 0100 (hex) = 256 (decimal)
Divide by 512 gives 0.5 voltage gain, or -3.01dB
The DRC_GAIN register is defined in Table 6.
REGISTER
ADDRESS
DRC Status
BIT
LABEL
15:0
DRC_GAIN
[15:0]
DEFAULT
DESCRIPTION
DRC Gain value.
This is the DRC gain, expressed as
a voltage multiplier. Fixed point
coding, MSB = 64.
The first 7 bits are the integer
portion; the remaining bits are the
fractional part.
Table 6 DRC Gain Readback
DYNAMIC CHARACTERISTICS
The dynamic behaviour determines how quickly the DRC responds to changing signal levels. If the
output amplitude were to follow the compression characteristics instantaneously, the waveform
shape would be altered and distortion would be produced.
Note that the DRC responds to the peak signal amplitude over a period of time.
When the DRC is operating as a compressor, the gain reduces when the input signal increases. The
DRC utilises attack and decay rates to control the dynamic behaviour of the gain. When the gain
reduces, the DRC_ATK rate controls the rate of decrease in gain. When the gain increases due to a
decrease in signal level, the DRC_DCY rate controls the rate of increase in gain as shown in Figure
6.
Note that the actual levels that the DRC settles to depend on the input signal and the DRC response.
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Figure 6 Attack and Decay Rates
Generally a fast attack rate is preferred to allow the system to respond quickly to transients to
prevent clipping, and a slow decay rate is preferred to prevent the gain fluctuating in the presence of
high amplitude low-frequency signals. These register fields are described in Table 7.
Note that the register defaults are suitable for general purpose microphone use. For high quality
music recording it is recommended that a longer decay rate is used.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
DRC Control 3
7:4
DRC_ATK [3:0]
0100
Gain attack rate (seconds/6dB)
0000 = Reserved
0001 = 181us
0010 = 363us
0011 = 726us
0100 = 1.45ms
0101 = 2.9ms
0110 = 5.8ms
0111 = 11.6ms
1000 = 23.2ms
1001 = 46.4ms
1010 = 92.8ms
1011 = 185.6ms
1100-1111 = Reserved
3:0
DRC_DCY [3:0]
0010
Gain decay rate (seconds/6dB)
0000 = 186ms
0001 = 372ms
0010 = 743ms
0011 = 1.49s
0100 = 2.97s
0101 = 5.94s
0110 = 11.89s
0111 = 23.78s
1000 = 47.56s
1001-1111 = Reserved
Table 7 DRC Attack and Decay Rates
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The DRC_ATK and DRC_DCY rates are specified in seconds/6dB step. This means that the time for
the output signal to recover from a change in the input signal level depends on the size of the change
in input signal amplitude.
The DRC_ATK and DRC_DCY rates also increase due to the DRC_HI_COMP setting. The output
rate is given by
Output DRC_ATK = DRC_ATK (Datasheet Value) / (1-R0)
where R0 is the value of the DRC_HI_COMP register.
For example, if the input signal level increases by 15dB with the DRC_ATK rate set at 1.45ms,
sample frequency of 32kHz, and the DRC_HI_COMP is set to ½, the time for the output signal to
recover from the input signal level change will be 15dB / 6dB * 1.45ms = 3.625ms. The
DRC_HI_COMP setting is ½ so allowing for this gives an estimated Attack time of 3.625ms / (1-1/2)
= 7.25ms.
Due to the non-linear behaviour of the peak detector the output attack rate is also affected by
the frequency of the input signal. This is not predictable and can only be estimated at up to
three times the calculated value.
ANTI-CLIP CONTROL
When a small signal is applied to the DRC, a high gain is set. If this is followed by a large signal, the
gain must reduce quickly to prevent the output signal clipping, as shown in Figure 7.
Figure 7 Anti-Clip Control
The DRC includes an Anti-Clip feature to reduce signal clipping when the input amplitude rises very
quickly. This feature uses a feed-forward technique for early detection of a rising signal level. Signal
clipping is minimised by switching to a fast attack rate when required.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DRC Control 1
1
DRC_ANTICLIP
1
DESCRIPTION
DRC Anti-clip Enable
0 = Disabled
1 = Enabled
Table 8 DRC Anti-Clip Control
The Anti-Clip feature will not guarantee that the signal does not clip in all conditions but will reduce
the effect of any clipping that does occur.
The Anti-Clip feature is enabled using the DRC_ANTICLIP bit (see Table 8). The feed-forward
processing increases the latency in the input signal path.
Note that the Anti-Clip feature operates entirely in the digital domain. It cannot be used to prevent
signal clipping in the analogue domain nor in the source signal. Analogue clipping can only be
prevented by reducing the analogue signal gain or by adjusting the source signal.
QUICK-RELEASE CONTROL
When a short transient signal is applied to the DRC, it will normally attack (reduce the gain) quickly,
then decay (increase the gain) slowly, as shown in Figure 8. As a consequence, audible drop-outs in
the output signal can be detected.
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The DRC includes a Quick-Release (QR) feature to handle short transient peaks that are not related
to the intended source signal. For example, in handheld microphone recording, transient signal
peaks sometimes occur due to user handling, key presses or accidental tapping against the
microphone. The Quick Release feature ensures that these transients do not cause the intended
signal to be masked by the longer rates of DRC_DCY.
Figure 8 Quick Release Control
The QR feature is enabled by setting the DRC_QR bit. When this bit is enabled, the DRC monitors
the input signal. If a transient peak is detected it may not be related to the intended source signal. If
the transient exceeds the level set by DRC_QR_THR, then the normal decay rate DRC_DCY is
ignored and a faster decay rate DRC_QR_DCY is used instead.
A separate Quick-Release feature is provided for the Noise Gate response. In the case of the signal
level rising after a period of silence, the Noise Gate Quick-Release enables the DRC to transition out
of the noise gate attenuation region at a faster rate than the normal decay rate. The Noise Gate
Quick-Release feature is enabled by setting the DRC_NG_QR bit.
The DRC Quick-Release control bits are described in Table 9.
REGISTER
ADDRESS
BIT
DRC Control 1
5
DRC_NG_QR
0
DRC Noise Gate quick-release
Enable
0 = Disabled
1 = Enabled
2
DRC_QR
1
DRC Quick-release Enable
0 = Disabled
1 = Enabled
3:2
DRC_QR_THR
[1:0]
00
DRC Quick-release threshold (crest
factor in dB)
00 = 12dB
01 = 18dB
10 = 24dB
11 = 30dB
1:0
DRC_QR_DCY
[1:0]
00
DRC Quick-release decay rate
(seconds/6dB)
00 = 0.725ms
01 = 1.45ms
10 = 5.8ms
11 = reserved
DRC Control 6
LABEL
DEFAULT
DESCRIPTION
Table 9 DRC Quick-Release Control
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APPLICATIONS
This section discusses some examples of using the DRC in different applications.
PEAK LIMITER
In a limiter, the signal level is unchanged for amplitudes below the knee, but sharply reduced for
amplitudes above the knee. Normally the knee will be at a high amplitude e.g. around -6dB, so that
the majority of the dynamic range is unchanged.
For example, if a microphone is “distant” from the sound source the output signal from the
microphone may be around -54dBV. If the signal is amplified by the microphone PGA (typically
+30dB) the input level to the ADC is -24dBV. With the limiter configuration below, the signal
amplitude will be boosted digitally (by +18dB) to -6dB. When the sound source is 1cm from the
microphone, the output signal from the microphone will be higher and may be around -34dBV. After
amplification (+30dB) the signal level to the ADC is -4dBV. If the same amount of gain were applied
digitally (+18dB) the signal would clip. By applying less gain the limiter configuration below will
ensure that the signal does not clip (in the steady state).
PARAMETER
DRC_KNEE_IP
VALUE
-24
DRC_KNEE_OP
-6
DRC_HI_COMP
1/4
DRC_LO_COMP
1
ALC
An ALC is used to equalise volume settings so that quiet small-amplitude signals are boosted to
achieve the same amplitude as high-amplitude signals.
A typical application for this is Digital Still-Cameras (DSC) , for record applications, where the source
that is being recorded is a variable distance from the microphone, but must be recorded at more or
less the same output level to maintain intelligibility of the signal. Another key application is line-level
recording, where different input sources have different signal levels, but should be equalised to the
same level automatically.
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TRADITIONAL ALC
A typical traditional ALC characteristic is shown below. A compression slope of zero (constant
amplitude) is used for signals above the knee, and a slope of 1 (constant gain) is used below the
knee. The latter limits the gain for very small signals to reduce the amplification of noise from the
input source. One of the disadvantages of an ALC is that a very high gain can be produced even for
relatively low signal amplitudes. Side effects such as gain-pumping can become very apparent with
this gain characteristic, making the ALC unsuitable for music recording, unless very long decay times
are used.
Note that in the example below a threshold of -3dB is used to allow for some overshoot of the input
signal which allows the ALC some time to respond before clipping occurs.
PARAMETER
DRC_KNEE_IP
VALUE
-42
DRC_KNEE_OP
-3
DRC_HI_COMP
0
DRC_LO_COMP
1
‘SOFT’ ALC
A ‘soft’ ALC is used in applications where a gentler ALC characteristic is required, for example where
both speech and music recording is required without reconfiguring compressor parameters. An
additional advantage of this configuration is that some of the dynamic range properties of the original
signal is preserved, i.e. the loudness of the signal is still proportional to the distance from the
microphone (although the dynamic range is still squashed), which makes recorded conversation
more natural.
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PARAMETER
DRC_KNEE_IP
VALUE
-45
DRC_KNEE_OP
-9
DRC_HI_COMP
1/8
DRC_LO_COMP
1
MUSIC ALC
This uses even gentler compression characteristics and uses a higher knee threshold to limit the
gain to around 20dB.
PARAMETER
DRC_KNEE_IP
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VALUE
-30
DRC_KNEE_OP
-10.5
DRC_HI_COMP
1/4
DRC_LO_COMP
1
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SUMMARY
The DRC used in the latest Wolfson CODECs can be enabled in the digital playback or digital record
path of the CODEC, depending upon the selected DSP mode. The function of the DRC is to adjust
the signal gain in conditions where the input amplitude is unknown or varies over a wide range, e.g.
when recording from microphones built into a handheld system.
The DRC can apply Compression and Automatic Level Control to the signal path. It incorporates
‘anti-clip’ and ‘quick release’ functions for handling transients in order to improve intelligibility in the
presence of loud impulsive noises.
The DRC also incorporates a Noise Gate function, which provides additional attenuation of very lowlevel input signals. This means that the signal path is quiet when no signal is present, giving an
improvement in background noise level under these conditions.
The operation of the DRC used has been discussed and the registers associated with the DRC
functions have been detailed. There are numerous possible settings that can be implemented with
the DRC and a few of the main application areas have been highlighted.
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APPLICATION SUPPORT
If you require more information or require technical support please contact Wolfson Microelectronics
Applications group through the following channels:
Email:
Telephone:
Fax:
Mail:
[email protected]
+44 (0)131 272 7070
+44 (0)131 272 7001
Applications at the address on last page.
or contact your local Wolfson representative.
Additional information may be made available from time to time on our web site at
http://www.wolfsonmicro.com
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IMPORTANT NOTICE
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Testing and other quality control techniques are utilised to the extent Wolfson deems necessary to support its warranty. Specific
testing of all parameters of each device is not necessarily performed unless required by law or regulation.
In order to minimise risks associated with customer applications, the customer must use adequate design and operating
safeguards to minimise inherent or procedural hazards. Wolfson is not liable for applications assistance or customer product
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or use nor for use of any circuitry other than circuitry entirely embodied in a Wolfson product.
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Tel :: +44 (0)131 272 7000
Fax :: +44 (0)131 272 7001
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