wm8775 Wolfson Microelectronics plc, wm8775 Datasheet - Page 29

• Manufacturer Part Number: wm8775
• Description: 24-bit, 96khz Adc With 4 Channel I/p Multiplexer
• Manufacturer: Wolfson Microelectronics plc

Production Data
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HOLD TIME (ALC ONLY)
The ALC also has a hold time, which is the time delay between the peak level detected being below
target and the PGA gain beginning to ramp up. It can be programmed in power-of-two (2
2.67ms, 5.33ms, 10.67ms etc. up to 43.7ms. Alternatively, the hold time can also be set to zero. The
hold time only applies to gain ramp-up, there is no delay before ramping the gain down when the
signal level is above target.
OVERLOAD DETECTOR (ALC ONLY)
To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes an
overload detector. If the ADC input signal exceeds 87.5% of full scale (–1.16dB), the PGA gain is
ramped down at the maximum attack rate (as when ATK = 0000), until the signal level falls below
87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.
(Note: If ATK = 0000, then the overload detector makes no difference to the operation of the ALC. It
is designed to prevent clipping when long attack times are used).
NOISE GATE (ALC ONLY)
When the signal is very quiet and consists mainly of noise, the ALC function may cause “noise
pumping”, i.e. loud hissing noise during silence periods. The WM8775 has a noise gate function that
prevents noise pumping by comparing the signal level at the AINL1/2/3/4 and/or AINR1/2/3/4 pins
against a noise gate threshold, NGTH. The noise gate cuts in when:
•
This is equivalent to:
•
Limiter Control
ALC Control 1
ALC Control 2
REGISTER
REGISTER
ADDRESS
ADDRESS
R16(10h)
R20(14h)
R17(11h)
0010000
0010100
0010001
Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]
Signal level at input pin [dB] < NGTH [dB]
BIT
BIT
6:4
3:0
3:0
MAXATTEN
MAXGAIN
LABEL
HLD[3:0]
LABEL
111 (+24dB)
DEFAULT
DEFAULT
0110
0000
Set maximum gain for the PGA (ALC
only)
111 : +24dB
110 : +20dB
…..(-4dB steps)
010 : +4dB
001 : 0dB
000 : 0dB
Maximum attenuation of PGA
Limiter
(attenuation
below static)
0011 or lower:
-3dB
0100: -4dB
…. (-1dB
steps)
1100: -12dB
ALC hold time before gain is
increased.
0000: 0ms
0001: 2.67ms
0010: 5.33ms
… (time doubles with every step)
1111: 43.691s
DESCRIPTION
DESCRIPTION
PD, Rev 4.4, October 2008
ALC (lower PGA
gain limit)
1010 or lower
: -1dB
1011 : -5dB
….. (-4dB steps)
1110 : -17dB
1111 : -21dB
WM8775
n
) steps, e.g.
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