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參數(shù)資料
| 型號： | LM4935WL |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | ADC |
| 英文描述： | Audio Sub-System with Dual-Mode Stereo Headphone and Mono High Efficiency Loudspeaker Amplifiers and Multi-Purpose ADC |
| 中文描述： | 音頻子系統(tǒng)的雙模單聲道立體聲耳機和揚聲器放大器的高效率和多功能模數(shù)轉(zhuǎn)換器 |
| 文件頁數(shù)： | 36/112頁 |
| 文件大小： | 5017K |
| 代理商： | LM4935WL |
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12.0 Status & Control Registers
(Continued)
12.15 AGC OVERVIEW
The Automatic Gain Control (AGC) system can be used to optimize the dynamic range of the ADC for voice data when the level
of the source is unknown. A target level for the output is set so that any transients on the input won’t clip during normal operation.
The AGC circuit then compares the output of the ADC to this level and increases or decreases the gain of the microphone
preamplifier to compensate. If the audio from the microphone is to be output digitally through theADC then the full dynamic range
of the ADC can be used automatically. If the output is through the analog mixer then the ADC is used to monitor the microphone
level. In this case, the analog dynamic range is less important than the absolute level, so
AGC_TIGHT
should be set to tie
transients closely to the target level.
To ensure that the system doesn’t reduce the quality of the speech by constantly modulating the microphone preamplifier gain,
the ADC output is passed through an envelope detector. This frames the output of the ADC into time segments roughly equal to
the phonemes found in speech
(AGC_FRAME_TIME)
. To calculate this, the circuit must also know the sample rate of the data
from the ADC
(ADC_SAMPLERATE)
. If after a programmable number of these segments
(AGC_HOLDTIME)
, the level is
consistently below target, the gain will be increased at a programmable rate
(AGC_DECAY)
. If the signal ever exceeds the target
level (AGC_TARGET) then the gain of the microphone is reduced immediately at a programmable rate
(AGC_ATTACK)
. This is
demonstrated below:
20134112
AGC Operation Example
The signal in the above example starts with a small analog input which, after the hold time has timed out, triggers a rise in the
gain ((1)
→
(2)).After some time the real analog input increases and it reaches the threshold for a gain reduction which decreases
the gain at a faster rate ((2)
→
(3)) to allow the elimination of typical popping noises.
Only ADC outputs that are considered signal (rather than noise) are used to adjust the microphone preamplifier gain. The signal
to noise ratio of the expected input signal is set by
NOISE_GATE_THRESHOLD
. In some situations it is preferable to remove
audio considered to be consisting solely of background noise from the audio output; for example conference calls. This can be
done by setting
NOISE_GATE_ON
. This does not affect the performance of the AGC algorithm.
The AGC algorithm should not be used where very large background noise is present. If the type of input data, application and
microphone is known then the AGC will typically not be required for good performance, it is intended for use with inputs with a
large dynamic range or unknown nominal level. When setting
NOISE_GATE_THRESHOLD
be aware that in some mobile phone
scenarios the ADC SNR will be dictated by the microphone performance rather than the ADC or the signal. Gain changes to the
microphone are performed on zero crossings. To eliminate DC offsets, wind noise, and pop sounds from the output of the ADC,
the ADC’s HPF should always be enabled.
L
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相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM4935WLX | Audio Sub-System with Dual-Mode Stereo Headphone and Mono High Efficiency Loudspeaker Amplifiers and Multi-Purpose ADC |
| LM4936 | Stereo 2W Audio Power Amplifiers with Volume Control and Selectable Control Interface (SPI or I2C) |
| LM4951 | Wide Voltage Range 1.8 Watt Audio Amplifier |
| LM4953 | Ground-Referenced, Ultra Low Noise, Ceramic Speaker Driver |
| LM4953SD | Ground-Referenced, Ultra Low Noise, Ceramic Speaker Driver |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4935WLX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Audio Sub-System with Dual-Mode Stereo Headphone and Mono High Efficiency Loudspeaker Amplifiers and Multi-Purpose ADC |
| LM4936 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Stereo 2W Audio Power Amplifiers with Volume Control and Selectable Control Interface (SPI or I2C) |
| LM49360 | 制造商:TI 制造商全稱:Texas Instruments 功能描述:Mono Class D Audio Codec PMU with Ground Referenced Headphone Amplifiers, Earpiece Driver, Audio DSP, 2 Step-Down DC-DC Converters, and 7 LDO Regulators |
| LM49360RL | 制造商:TI 制造商全稱:Texas Instruments 功能描述:Mono Class D Audio Codec PMU with Ground Referenced Headphone Amplifiers, Earpiece Driver, Audio DSP, 2 Step-Down DC-DC Converters, and 7 LDO Regulators |
| LM49360RLX | 制造商:TI 制造商全稱:Texas Instruments 功能描述:Mono Class D Audio Codec PMU with Ground Referenced Headphone Amplifiers, Earpiece Driver, Audio DSP, 2 Step-Down DC-DC Converters, and 7 LDO Regulators |
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