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CS4237B Datasheet(PDF) 81 Page - Cirrus Logic |
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CS4237B Datasheet(HTML) 81 Page - Cirrus Logic |
81 / 114 page with the rest of the bits in I22, are used to set the sample frequency. Once enabled, these bits can be changed without doing an MCE cycle. The third method supports independent sample frequencies (Fs) for capture and playback. The independent sample frequency mode is enabled by setting IFSE in X11. Once enabled, the other two methods for setting Fs (I8, I10, and I22) are disabled. The capture (ADC) Fs is set in X12 and the playback (DAC) Fs is set in X13. Changing Audio Data Formats In MODE 1, MCE must be used to select the audio data format in I8. Since MCE causes a calibration cycle, it is not ideal for full-duplex operation. In MODE 2 and 3, individual Mode Change Enable bits for capture and playback are provided in register I16. MCE (R0) must still be used to select the sample frequency, but PMCE (playback) and CMCE (capture) allow changing the respective data formats without causing a calibration to occur. Setting PMCE (I16) clears the playback FIFO and allows the upper four bits of I8 to be changed. Setting CMCE (I16) clears the capture FIFO and allows the upper four bits of I28 to be changed. Audio Data Formats In MODE 1 operation, all data formats of the WSS Codec are in "little endian" format. This format defines the byte ordering of a multibyte word as having the least significant byte occupy- ing the lowest memory address. Likewise, the most significant byte of a little endian word oc- cupies the highest memory address. The sample frequency is always selected in the Fs & Playback Data Format register (I8). In MODE 1 the same register, I8, determines the audio data format for both playback and capture; however, in MODE 2 and 3, I8 only selects the playback data format and the capture data format is independently selectable in the Capture Data Format register (I28). The WSS Codec always orders the left channel data before the right channel. Note that these definitions apply regardless of the specific for- mat of the data. For example, 8-bit linear data streams look exactly like 8-bit companded data streams. Also, the left sample always comes first in the data stream regardless of whether the sam- ple is 16-bit or 8-bit in size. There are four data formats supported by the WSS Codec during MODE 1 operation: 16-bit signed (little endian), 8-bit unsigned, 8-bit com- panded µ-Law, and 8-bit companded A-Law. See Figures 14-17. Additional data formats are supported in MODE 2 and 3: 4-bit ADPCM, and 16-bit signed Big Endian. See Figures 18 through 21. With the ad- dition of the Big Endian and ADPCM audio data formats, the WSS Codec is compliant with the IMA recommendations for digital audio data for- mats (and sample frequencies). 16-BIT SIGNED The 16-bit signed format (also called 16-bit 2’s complement) is the standard method of repre- senting 16-bit digital audio. This format gives 96 dB theoretical dynamic range and is the standard for compact disk audio players. This format uses the value -32768 (8000h) to repre- sent maximum negative analog amplitude, 0 for center scale, and 32767 (7FFFh) to represent maximum positive analog amplitude. 8-BIT UNSIGNED The 8-bit unsigned format is commonly used in the personal computer industry. This format de- livers a theoretical dynamic range of 48 dB. This format uses the value 0 (00h) to represent maxi- mum negative analog amplitude, 128 for center scale, and 255 (FFh) to represent maximum positive analog amplitude. The 16-bit signed and 8-bit unsigned transfer functions are shown in Figure 22. DS213PP4 CS4237B 81 |
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