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MC145505 Datasheet(PDF) 8 Page - Motorola, Inc |
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MC145505 Datasheet(HTML) 8 Page - Motorola, Inc |
8 / 28 page MC145500 •MC145501•MC145502•MC145503•MC145505 MOTOROLA 8 DEVICE DESCRIPTIONS A codec–filter is a device which is used for digitizing and reconstructing the human voice. These devices were devel- oped primarily for the telephone network to facilitate voice switching and transmission. Once the voice is digitized, it may be switched by digital switching methods or transmitted long distance (T1, microwave, satellites, etc.) without degra- dation. The name codec is an acronym from “Coder” for the A/D used to digitize voice, and “Decoder” for the D/A used for reconstructing voice. A codec is a single device that does both the A/D and D/A conversions. To digitize intelligible voice requires a signal to distortion of about 30 dB for a dynamic range of about 40 dB. This may be accomplished with a linear 13–bit A/D and D/A, but will far exceed the required signal to distortion at amplitudes greater than 40 dB below the peak amplitude. This excess perform- ance is at the expense of data per sample. Two methods of data reduction are implemented by compressing the 13–bit linear scheme to companded 8–bit schemes. These com- panding schemes follow a segmented or “piecewise–linear” curve formatted as sign bit, three chord bits, and four step bits. For a given chord, all 16 of the steps have the same volt- age weighting. As the voltage of the analog input increases, the four step bits increment and carry to the three chord bits which increment. With the chord bits incremented, the step bits double their voltage weighting. This results in an effec- tive resolution of 6–bits (sign + chord + four step bits) across a 42 dB dynamic range (7 chords above zero, by 6 dB per chord). There are two companding schemes used; Mu–255 Law specifically in North America, and A–Law specifically in Europe. These companding schemes are accepted world wide. The tables show the linear quantization levels to PCM words for the two companding schemes. 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 inband 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 A/D converter. The D/A process reconstructs a staircase version of the desired inband signal which has spectral images of the in- band signal modulated about the sample frequency and its harmonics. These spectral images are called aliasing com- ponents which need to be attenuated to obtain the desired signal. The low–pass filter used to attenuate filter aliasing components is typically called a reconstruction or smoothing filter. The MC145500 series PCM Codec–Filters have the co- dec, both presampling and reconstruction filters, a precision voltage reference on chip, and require no external compo- nents. There are five distinct versions of the Motorola MC145500 Series. MC145500 The MC145500 PCM Codec–Filter is intended for stan- dard byte interleaved synchronous and asynchronous appli- cations. The TDC pin on this device is the input to both the TDC and CCI functions in the pin description. Consequently, for MSI = 8 kHz, TDC can be one of five discrete frequencies. These are 128 kHz (40 to 60% duty cycle) 1.536, 1.544, 2.048, or 2.56 MHz. (For other data clock frequencies, see MC145502 or MC145505.) The internal reference is set for 3.15 V peak full scale, and the full scale input level at Txl and output level at RxO is 6.3 V peak–to–peak. This is the + 3 dBm0 level of the PCM Codec–Filter. All other functions are described in the pin description. MC145501 The MC145501 PCM Codec–Filter offers the same fea- tures and is for the same application as the MC145500, but offers two additional pins and features. The reference select input allows the full scale level of the device to be set at 2.5 Vp, 3.15 Vp, or 3.78 Vp. The –Tx pin allows for external transmit gain adjust and simplifies the interface to the MC3419 SLIC. Otherwise, it is identical to MC145500. MC145502 The MC145502 PCM Codec–Filter is the full feature 22–pin device. It is intended for use in applications requiring maximum flexibility. The MC145502 contains all the features of the MC145500 and MC145501. The MC145502 is in- tended for bit interleaved or byte interleaved applications with data clock frequencies which are nonstandard or time varying. One of the five standard frequencies (listed above) is applied to the CCI input, and the data clock inputs can be any frequency between 64 kHz and 4.096 MHz. The Vref pin allows for use of an external shared reference or selection of the internal reference. The RxG pin accommodates gain ad- justments for the inverted analog output. All three pins of the input gain–setting operational amplifier are present, provid- ing maximum flexibility for the analog interface. MC145503 The MC145503 PCM Codec–Filter is intended for stan- dard byte interleaved synchronous or asynchronous applica- tions. TDC can be one of five discrete frequencies. These are 128 kHz (40 to 60% duty cycle), 1.536, 1.544, 2.048, or 2.56 MHz. (For other data clock frequencies, see MC145502 or MC145505.) The internal reference is set for 3.15 V peak full scale, and the full scale input level at Txl and output level at RxO is 6.3 V peak–to–peak. This is the + 3 dBm0 level of the PCM Codec–Filter. The +Tx and –Tx inputs provide max- imum flexibility for analog interface. All other functions are described in the pin description. MC145505 The MC145505 PCM Codec–Filter is intended for byte in- terleaved synchronous applications. The MC145505 has all the features of the MC145503 but internally connects TDC and RDC (see pin description) to the DC pin. One of the five standard frequencies (listed above) should be applied to |
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