WM8985_07 WOLFSON [Wolfson Microelectronics plc], WM8985_07 Datasheet - Page 52

• Manufacturer Part Number: WM8985_07
• Description: Multimedia CODEC With Class D Headphone and Line Out
• Manufacturer: WOLFSON [Wolfson Microelectronics plc]

WM8985
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NOISE GATE (NORMAL MODE ONLY)
Signal level at ADC [dBFS] < NGTH [dBFS] + PGA gain [dB] + Mic Boost gain [dB]
Table 28 ALC Noise Gate Control
To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes a
limiter function. 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 ALCATK = 0000), until the signal level falls
below 87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.
Note: If ALCATK = 0000, then the limiter makes no difference to the operation of the ALC. It is
designed to prevent clipping when long attack times are used.
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 WM8985 has a noise gate function that
prevents noise pumping by comparing the signal level at the input pins against a noise gate
threshold, NGTH. The noise gate cuts in when:
This is equivalent to:
Signal level at input pin [dBFS] < NGTH [dBFS]
The PGA gain is then held constant (preventing it from ramping up as it normally would when the
signal is quiet).
The table below summarises the noise gate control register. The NGTH control bits set the noise
gate threshold with respect to the ADC full-scale range. The threshold is adjusted in 6dB steps.
Levels at the extremes of the range may cause inappropriate operation, so care should be taken with
set–up of the function. The noise gate only operates in conjunction with the ALC and cannot be used
in limiter mode.
The diagrams below show the response of the system to the same signal with and without noise
gate.
PEAK LIMITER
R35 (23h)
ALC Noise Gate
Control
REGISTER
ADDRESS
BIT
2:0
3
NGTH
NGATEN
LABEL
000
0
DEFAULT
000 = -39dB
001 = -45dB
010 = -51db
011 = -57dB
100 = -63dB
101 = -70dB
110 = -76dB
111 = -81dB
Noise gate function enable
1 = enable
0 = disable
Noise gate threshold:
DESCRIPTION
PP, Rev 3.5, March 2007
Pre-Production
52