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Tele3113 wk9wed Tele3113 wk9wed Presentation Transcript

  • TELE3113 Analogue and Digital Communications – Delta Modulation Wei Zhang w.zhang@unsw.edu.au School of Electrical Engineering and Telecommunications The University of New South WalesTELE3113 - ∆ Modulation 23 Sept 2009 p. -1
  • Delta Modulation (1) The difference signal is encoded into just a single bit (only two levels, +∆ and –∆). ∞ one bit per sampled point DM transmitter ∑ δ (t − nTs ) n = −∞ ε (t ) + - DM signal xDM (t )  ∆ d(t) > 0 ε (t ) = ∆ sgn[d (t )] =  − ∆ d (t ) < 0 ∞ ∞xDM (t ) = ∆ sgn[d (t )] ∑ δ (t − nTs ) = ∆ ∑ sgn[d (nTs )]δ (t − nTs ) n = −∞ n = −∞ ∞ ~ (t ) = xq ∑ ∆ sgn[d (nT )] n = −∞ s (staircase approximation of x(t))TELE3113 – ∆ Modulation 23 Sept 2009 p. -2
  • Delta Modulation (2) DM signal xDM (t ) Demodulation: Integrator + LPF Quantization error/noise: We assume that the quantization error eq(t) in delta modulation is equally likely to lie anywhere in the interval (-∆,∆), then ∆ 1 ∆2 2∆ −∫ e (t ) = 2 q eq (t )deq = 2 ∆ 3TELE3113 – ∆ Modulation 23 Sept 2009 p. -3
  • Delta Modulation (3) Slope overload: d(t) ∆ dx(t ) To avoid slope overload, we require > Ts dt maxTELE3113 – ∆ Modulation 23 Sept 2009 p. -4
  • Adaptive Delta Modulation (1) To resolve the problem of slope overload, the step size is varied according to the level of the input signal (to catch up the change of the input signal x(t)). If a sequence of DM pulses of the same polarity is generated (i.e. x(t) changes rapidly and slope-overload occurs), then the step size ∆ will be increased; if the polarity of the DM pulses tends to alternate, then the step size ∆ will be reduced; ADM transmitter IntegratorTELE3113 – ∆ Modulation 23 Sept 2009 p. -5
  • Adaptive Delta Modulation (2) Demodulation: IntegratorReceived ADM +LPF Recovered signal signal Received ADM signal To get the relative change Recovered envelope in step size ∆, the signal xr(t) information will be used to adjust the amplifier Application: Vocoder (1.2-2.4kbit/s) If the step size is changed continuously instead of discrete levels, it is called continuously variable slope delta modulation (CVSDM).TELE3113 – ∆ Modulation 23 Sept 2009 p. -6
  • Differential Pulse Code Modulation (DPCM) ~ Input message x∆ (kTs ) d (kTs ) = x∆ (kTs ) − x∆ (kTs ) DPCM signal signal + x(t ) Sampler Quantizer Encoder - ~ (kT ) x∆ s Delay ~ (kT ) = ~ ((k − 1)T ) + d ((k − 1)T ) x∆ s x∆ s s Received Recovered d (kTs ) signal DPCM signal Reconstruction Decoder filter ~ (kT ) xq s Delay ~ (kT ) = ~ ((k − 1)T ) + d ((k − 1)T ) xq s xq s sTELE3113 – ∆ Modulation 23 Sept 2009 p. -7