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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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MC145483 Datenblatt(PDF) 2 Page - Motorola, Inc |
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MC145483 Datenblatt(HTML) 2 Page - Motorola, Inc |
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2 / 19 page  MC145483 MOTOROLA 2 FREQ FREQ RO – PI PO – PO + VDD VSS VAG TG TI – TI + – + – 1 1 – + SHARED DAC DAC 0.886 V REF ADC TRANSMIT SHIFT REGISTER SEQUENCE AND CONTROL DR FSR BCLKR PDI MCLK BCLKT FST DT RECEIVE SHIFT REGISTER HB VAG Ref VDD VSS R* R* Figure 1. MC145483 13–Bit Linear PCM Codec–Filter Block Diagram DEVICE DESCRIPTION A PCM Codec–Filter is used for digitizing and reconstruct- ing the human voice. These devices are used primarily for the telephone network to facilitate voice switching and trans- mission. Once the voice is digitized, it may be switched by digital switching methods or transmitted long distance (T1, microwave, satellites, etc.) without degradation. The name codec is an acronym from ‘‘COder’’ for the analog–to–digital converter (ADC) used to digitize voice, and ‘‘DECoder’’ for the digital–to–analog converter (DAC) used for reconstruct- ing voice. A codec is a single device that does both the ADC and DAC conversions. To digitize intelligible voice requires a signal–to–distortion ratio of about 30 dB over a dynamic range of about 40 dB. This may be accomplished with a linear 13–bit ADC and DAC. The MC145483 satisfies these requirements and may be used as the analog front–end for voice coders using DSP technology to further compress the digital data stream. In a sampling environment, Nyquist theory says that to properly sample a continuous signal, it must be sampled at a frequency higher than twice the signal’s highest frequency component. Voice contains spectral energy above 3 kHz, but its absence is not detrimental to intelligibility. To reduce the digital data rate, which is proportional to the sampling rate, a sample rate of 8 kHz was adopted, consistent with a band- width of 3 kHz. This sampling requires a low–pass filter to limit the high frequency energy above 3 kHz from distorting the in–band signal. The telephone line is also subject to 50/60 Hz power line coupling, which must be attenuated from the signal by a high–pass filter before the analog–to– digital converter. The MC145483 includes a high–pass filter for compatibility with existing telephone applications, but it may be removed from the analog input signal path by the high–pass bypass pin. The digital–to–analog conversion process reconstructs a staircase version of the desired in–band signal, which has spectral images of the in–band signal modulated about the sample frequency and its harmonics. These spectral images are called aliasing components, which need to be attenuated to obtain the desired signal. The low–pass filter used to at- tenuate these aliasing components is typically called a re- construction or smoothing filter. The MC145483 PCM Codec–Filter has the codec, both presampling and reconstruction filters, and a precision volt- age reference on–chip. |
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