Project Analog - February 2008 - (Page 10) where the user may be in a windy area, a high pass filter can be employed to remove the wind noise. Notch filters may be used to eliminate motor noise in camera applications, or GSM noise in mobile phone applications. audiO and cOntrOl interface Most portable audio codec’s operate with I2S audio data, and either an SPI or I2C control interface. Some legacy codec’s use an AC-Link interface, used for AC-97 codecs. The speed of the control interface is not critical, since no audio sample data is passed over the control bus. For the digital audio interface, I2S is the most common format, and most codecs will support MSB- or LSB justified as well. If I2S is selected, virtually every codec will support it. For resolution, the majority of codecs support 16 bits, although there are some higher resolution parts, up to 24-bits. In reality, there is no need to go beyond 16 bits, since the audio performance will not need to be better than 90 dB. clOcKing A standard digital audio clock inter10 · ProjeCT ANALog · feb 08 face includes a master clock (controls the modulator), a bit clock, a left-right (frame or word) clock and a data in/out line. Providing high-accuracy low-jitter clocks may be difficult, so consider a codec with an integrated PLL. High-performance analog PLLs are available that operate off of a wide variety of clock sources, including audio, video, USB, and DSP clocks. A codec with an integrated PLL eliminates concerns about accuracy and coupling from high-speed clocks. dac perfOrmance and featureS As previously mentioned, it is a waste to use high-performance DACs for portable applications. Audio sources such as MP-3 have limited dynamic range depending on the compression ratio, so a rule of thumb for the DAC performance is 90 dB. Some codecs include audio enhancement algorithms, such as 3-D stereo enhancement, bass boost, EQ, and ALC. The 3-D stereo enhancement algorithms are usually simple sum-anddifference types, which creates a wider soundfield. A bass-boost filter can be used to boost low frequen- cies, either programmable or with a pre-determined response. The signal amplitude provided to these digital filters is always below full scale, since it is not possible to boost digital signals beyond full scale. A typical bass-boost filter can provide 15 dB of gain for frequencies below 80 Hz, rolling off to zero boost around 1 kHz. The bass boost filter also compensates for the high-pass characteristic of the output ac-coupling cap, allowing for a smaller dc-blocking capacitor. After the bass boost and EQ filters, the signal may be normalized with an ALC algorithm. All of these filter features are important to enhance playback quality for the user experience. playing the muSic Most portable appliances offer several types of output: line level, headphones, and speakers. The line output is usually designed to drive a high-impedance load (about 10 k) through an ac-coupling capacitor. The headphone driver can also be configured for the line output. For the headphone amplifier, the output power runs anywhere from 15 mW to 50 mW, although 30 mW is usually more than adequate for driving the most inexpensive headphones. The important parameter is speaker power level (SPL), not necessarily the raw output power. These outputs have been typically ac-coupled, although the trend toward very small appliances is driving capless outputs to eliminate large and expensive dc-blocking capacitors. This kind of output can be accomplished by having differential BTL drive, a charge pump that generates a negative rail with a ground reference, or by creating a virtual ground at Vcc/2. The virtual ground method can work for a closed-loop system (operating under battery power only), but when connected to an ac supply (as in a docking station), ground must be ground, not biased up, or the device may have catastrophic failure. Also, biasing up the headphone ground prevents the wire from being used as an FM antenna, if a radio is included in the appliance. Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training • 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
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.