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WM9711 Datasheet(PDF) 23 Page - Wolfson Microelectronics plc |
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WM9711 Datasheet(HTML) 23 Page - Wolfson Microelectronics plc |
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23 / 66 page  Production Data WM9711L w PD Rev 4.1 April 2004 23 MAXIMUM GAIN The MAXGAIN register sets the maximum gain value that the PGA can be set to whilst under the control of the ALC. This has no effect on the PGA when ALC is not enabled. PEAK LIMITER 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 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 limiter 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 WM9711L has a noise gate function that prevents noise pumping by comparing the signal level at the input pins (i.e. before the record PGA) against a noise gate threshold, NGTH. Provided that the noise gate function is enabled (NGAT = 1), 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). If the NGG bit is set, the ADC output is also muted when the noise gate cuts in. 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 1.5dB 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, and always operates on the same channel(s) as the ALC (left, right, both, or none). REGISTER ADDRESS BIT LABEL DEFAULT DESCRIPTION 7 NGAT 0 Noise gate function enable 1 = enable 0 = disable 5 NGG 0 Noise gate type 0 = PGA gain held constant 1 = mute ADC output 62h ALC / Noise Gate Control 4:0 NGTH(4:0) 00000 Noise gate threshold -76.5dBfs -75dBfs … 1.5 dB steps 11110 -31.5dBfs 11111 -30dBfs Table 10 Noise Gate Control |
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