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Pulse code modulation

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This module prefaces the subsequent modules on differential pulse code modulation (DPCM). In this module, the standard technique known as pulse code modulation (PCM) is described.
  • PCM is the “standard” sampling method taught in introductory DSP courses. The input signal is
    1. filtered to prevent aliasing,
    2. discretized in time by a sampler, and
    3. quantized in amplitude by a quantizer
    before transmission (or storage). Finally, the received samples are interpolated in time andamplitude to reconstruct an approximation of the input signal. Note that transmission may employ the use of additionalencoding and decoding, as with entropy codes. PCM is the most widespread and well-understood digital coding systemfor reasons of simplicity, though not efficiency (as we shall see).
    This flowchart begins with an arrow, labeled x(t), pointing to the right at a box labeled anti-alias filter. The box is followed by a broken line pointed upward, and continuing down with an arc pointing back down to the main lines of the flowchart. The arc contains an arrow, and the expression t=nT is shown to the right of the arc. The arc is followed by a second arrow pointing to the right. This arrow is labeled x(n), and it points at a box labeled quantize. The quantize box is followed by another arrow pointing to the right, labeled y(n). This arrow points at a dashed box labeled encode, which is followed by an arrow pointing to the right at a rounded box labeled transmit/store. This box is followed by an arrow pointing at another dashed box, labeled decode. The decode box is followed by an arrow, labeled yhat(n), that points at a final box labeled interp. This final box is followed by a final arrow pointing to the right, labeled xhat(t). This flowchart begins with an arrow, labeled x(t), pointing to the right at a box labeled anti-alias filter. The box is followed by a broken line pointed upward, and continuing down with an arc pointing back down to the main lines of the flowchart. The arc contains an arrow, and the expression t=nT is shown to the right of the arc. The arc is followed by a second arrow pointing to the right. This arrow is labeled x(n), and it points at a box labeled quantize. The quantize box is followed by another arrow pointing to the right, labeled y(n). This arrow points at a dashed box labeled encode, which is followed by an arrow pointing to the right at a rounded box labeled transmit/store. This box is followed by an arrow pointing at another dashed box, labeled decode. The decode box is followed by an arrow, labeled yhat(n), that points at a final box labeled interp. This final box is followed by a final arrow pointing to the right, labeled xhat(t).
    Standard PCM system
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Source:  OpenStax, An introduction to source-coding: quantization, dpcm, transform coding, and sub-band coding. OpenStax CNX. Sep 25, 2009 Download for free at http://cnx.org/content/col11121/1.2
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