PCM CODEC Pulse Code Modulation Pulse Code Modulation

PCM CODEC

Pulse Code Modulation Ø Ø Pulse Code Modulation Analogue to Digital Conversion Quantizing Encoding

Pulse Code Modulation ØDEFINITION: Pulse code modulation (PCM) is essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an analog signal is represented by digital words in a serial bit stream. ØThe advantages of PCM are: • Relatively inexpensive digital circuitry may be used extensively. • PCM signals derived from all types of analog sources may be merged with data signals and transmitted over a common highspeed digital communication system. • In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy PCM waveform. • The noise performance of a digital system can be superior to that of an analog system. • The probability of error for the system output can be reduced even further by the use of appropriate coding techniques.

Sampling, Quantizing, and Encoding Ø The PCM signal is generated by carrying out three basic operations: 1. Sampling 2. Quantizing 3. Encoding 1. 2. Sampling operation generates a flat-top PAM signal. Quantizing operation approximates the analog values by using a finite number of levels. This operation is considered in 3 steps a) b) c) 3. Uniform Quantizer Quantization Error Quantized PAM signal output PCM signal is obtained from the quantized PAM signal by encoding each quantized sample value into a digital word.

Analog to Digital Conversion Analog Input Signal ØThe Analog-to-digital Converter (ADC) performs three functions: Sampling Makes the signal discrete in time. If the analog input has a bandwidth of W Hz, then the minimum sample frequency such that the signal can be reconstructed without distortion. Sample ADC Quantization Quantize Encode Digital Output Signal 111 001 010 011 111 110 101 100 011 010 001 000 Makes the signal discrete in amplitude. Round off to one of q discrete levels. Encode Maps the quantized values to digital words that are bits long. ØIf the (Nyquist) Sampling Theorem is satisfied, then only quantization introduces distortion to the system.

Sampling

Quantisation and Encoding

MC 145484 • The MC 145484 is a general purpose per channel PCM Codec–Filter with pin selectable Mu–Law or A– Law companding, and is offered in 20–pin SOG, • SSOP, and TSSOP packages.

PIN ASSIGNMENT

MC 145484 • This device performs the voice digitization and reconstruction as well as the band limiting and smoothing required for PCM systems. • This device is designed to operate in both synchronous and asynchronous applications and contains an on–chip precision reference voltage.

FEATURES OF MC 145484 The MC 145484 PCM Codec–Filter utilizes CMOS due to its reliable low –power performance and proven capability for complex analog/digital VLSI functions. • Single 5 V Power Supply • Typical Power Dissipation of 15 m. W, Power–Down of 0. 01 m. W • Fully–Differential Analog Circuit Design for Lowest Noise • Transmit Band–Pass and Receive Low–Pass Filters On–Chip • Active R–C Pre–Filtering and Post–Filtering • Mu–Law and A–Law Companding by Pin Selection • On–Chip Precision Reference Voltage of 1. 575 V for a – 0 d. Bm TLP @ 600 W • Push–Pull 300 W Power Drivers with External Gain Adjust • MC 14 LC 5480 EVK is the Evaluation Kit for This Device

MC 145484 5 V PCM Codec–Filter Block Diagram

BLOCK DIAGRAM DESCRIPTION • A PCM Codec–Filter is used for digitizing and reconstructing the human voice. These devices are used 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 (T 1, 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 reconstructing voice. • A codec is a single device that does both the ADC and DAC conversions.

MC 145484 Analog Interface to Handset

Idle channel noise • The typical idle channel noise level of this device is less than one LSB.

Dynamic range • To digitize intelligible voice requires a signal– to–distortion ratio of about 30 d. B over a dynamic range of about 40 d. B. This may be accomplished with a linear 13–bit ADC and DAC, but will far exceed the required signal–to –distortion ratio at larger amplitudes than 40 d. B below the peak amplitude.