05-07-2012, 12:11 PM
Study of Time Division Multiplexing Pulse Code Modulation.
[attachment=26813]
Theory :
In PCM the speech signal is converted from analogue to digital form. In quantization the levels are assigned a binary codeword. All sample values falling between two quantization levels are considered to be located at the centre of the quantization interval. In this manner the quantization process introduces a certain amount of error or distortion into the signal samples. This error known as quantization noise is minimized by establishing a large number of small quantization intervals. Of course, as the number of quantization intervals increase, so must the number or bits increase to uniquely identify the quantization intervals. For example, if an analogue voltage level is to be converted to a digital system with 8 discrete levels or quantization steps three bits are required. In the ITU-T version there are 256 quantization steps, 128 positive and 128 negative, requiring 8 bits. A positive level is
represented by having bit 8 (MSB) at 0, and for a negative level the MSB is 1.Pulse code modulation, more popularly known as PCM is the most widely used digital modulation system. It is a widely known fact that the analog modulation systems are most prone to the noise present in the channel and receiver. As we will see further that the digital modulation systems are far less sensitive to noise as compared to analog modulation. The basis of digital modulation systems lies on pulse modulation i.e. a particular characteristic of the pulse is varied in accordance with the information signal. In PCM, modulation process is executed in three steps:
[attachment=26813]
Theory :
In PCM the speech signal is converted from analogue to digital form. In quantization the levels are assigned a binary codeword. All sample values falling between two quantization levels are considered to be located at the centre of the quantization interval. In this manner the quantization process introduces a certain amount of error or distortion into the signal samples. This error known as quantization noise is minimized by establishing a large number of small quantization intervals. Of course, as the number of quantization intervals increase, so must the number or bits increase to uniquely identify the quantization intervals. For example, if an analogue voltage level is to be converted to a digital system with 8 discrete levels or quantization steps three bits are required. In the ITU-T version there are 256 quantization steps, 128 positive and 128 negative, requiring 8 bits. A positive level is
represented by having bit 8 (MSB) at 0, and for a negative level the MSB is 1.Pulse code modulation, more popularly known as PCM is the most widely used digital modulation system. It is a widely known fact that the analog modulation systems are most prone to the noise present in the channel and receiver. As we will see further that the digital modulation systems are far less sensitive to noise as compared to analog modulation. The basis of digital modulation systems lies on pulse modulation i.e. a particular characteristic of the pulse is varied in accordance with the information signal. In PCM, modulation process is executed in three steps: