WM8978 Wolfson Microelectronics Ltd., WM8978 Datasheet - Page 42

• Manufacturer Part Number: WM8978
• Description: The WM8978 is a low power, high quality stereo codec designed for portable applications such as Digital still camera or Digital Camcorde
• Manufacturer: Wolfson Microelectronics Ltd.

WM8978
OUTPUT SIGNAL PATH
w
NOISE GATE
Signal level at ADC [dBFS] < NGTH [dBFS] + PGA gain [dB] + Mic Boost gain [dB]
Table 21 ALC Noise Gate Control
(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 WM8978 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 WM8978 output signal paths consist of digital application filters, up-sampling filters, stereo Hi-Fi
DACs, analogue mixers, speaker, stereo headphone and stereo line/mono/midrail output drivers.
The digital filters and DAC are enabled by register bits DACENL and DACENR. The mixers and
output drivers can be separately enabled by individual control bits (see Analogue Outputs). Thus it is
possible to utilise the analogue mixing and amplification provided by the WM8978, irrespective of
whether the DACs are enabled or not.
The WM8978 DACs receive digital input data on the DACDAT pin. The digital filter block processes
the data to provide the following functions:
High performance sigma-delta 24-bit audio DAC converts the digital data into an analogue signal.
R35
ALC Noise Gate
Control
REGISTER
ADDRESS
§
§
§
§
Digital volume control
Graphic equaliser
Digital peak limiter.
Sigma-Delta Modulation
BIT
2:0
3
NGTH
NGEN
LABEL
000
0
DEFAULT
Noise gate threshold:
000=-39dB
001=-45dB
010=-51db
… (6dB steps)
111=-81dB
Noise gate function enable
1 = enable
0 = disable
PTD Rev 2.6 November 2005
DESCRIPTION
Preliminary Technical Data
42