Project Analog - February 2008 - (Page 9) sidering that the fixed-gain for the microphone is 20 to 30 dB, that level of noise can be very strong. By using a differential signal path, the noise is cancelled through common mode rejection, and the signal is boosted by 6 dB. Most codecs provide power for electret microphones, but an important feature to look for is a buffered output. fig. 1 fEaturE-rich audio codEcs, such as thE aKm aK4643, can addrEss thE nEEds of comPlEx systEms, alc and input filtering ALC is applicable to both input and output signals. For the input, ALC is used to maintain a constant recording level, as the user’s voice level changes either through inflection or distance. ALC can be realized in either the analog or digital domain. For portable applications, digital ALC is often used, saving cost and providing more flexibility in determining ALC parameters such as attack, threshold, and release. Output ALC is used to normalize the output volume, as well as provide some pre-amplification to the signal. When a user creates a mix of songs from multiple sources, the recording levels can vary widely. A typical level may be at –12 dB. If this signal is sent through the DAC with no normalization, the power amp must provide extra amplification, burning excess power. By normalizing the signal in the digital domain, the power amp is much more efficient, producing more sound level for less power (see Fig. 2). Some specific applications can benefit from special digital filtering. For camera and phone applications Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news fig. 2 for thE inPut, alc is usEd to maintain a constant rEcording lEvEl, as thE usEr’s voicE lEvEl changEs EithEr through inflEction or distancE. alc can bE rEalizEd. shown abovE: (a) thE signal bEforE inPut alc. (b) thE signal aftEr inPut alc. A buffer prevents power supply noise from coupling into the microphone. There is usually a fixed gain block, offering gain from 15 to 40 dB. It may also be useful to have a programmable gain amplifier, enabling fine-tuning for a particular microphone source or environmental condition. Microphone gain should be done in the analog domain. If the gain is realized in the digital domain after the ADC, the constant quantization noise of the ADC is multiplied during quiet input passages. It is generally not important to have a high-quality ADC, since the microphone element is usually the limiting factor for signal-to-noise performance. For most consumer devices, anything over 80 dB should be adequate. Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training 9 · ProjeCT ANALog · feb 08 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mixedsignal http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423
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