WM8978 Datasheet, PDF (31/78 Pages) Wolfson Microelectronics plc – STEREO CODEC WITH SPEAKER DRIVER

Product Preview
WM8978
ALC CLIP PROTECTION
To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes a
clip protection 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 ATK = 0000, then the clip protection circuit makes no difference to the operation of the ALC. It is
designed to prevent clipping when long attack times are used.
NOISE GATE
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:
Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]
This is equivalent to:
Signal level at input pin [dB] < NGTH [dB]
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. Note that the noise gate only works in conjunction with the ALC function.
REGISTER
ADDRESS
R35
ALC Noise Gate
Control
BIT
LABEL
2:0 NGTH
3 NGATEN
Table 17 ALC Noise Gate Control
DEFAULT
DESCRIPTION
000
Noise gate threshold:
000=-39dB
001=-45dB
010=-51db
… (6dB steps)
111=-81dB
0
Noise gate function enable
1 = enable
0 = disable
OUTPUT SIGNAL PATH
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 running 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:
§ Digital volume control
§ Graphic equaliser
§ A digital peak limiter.
§ Sigma-Delta Modulation
High performance sigma-delta audio DAC converts the digital data into an analogue signal.
w
PP Rev 1.7 January 2005
31