capítulo 5
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McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001
Capítulo 5
Codificación y
modulación
McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001
Codificación
• Es necesario convertir los datos en forma de bits a señales antes de enviarlos a través del medio de transmisión.
• La conversión de una señal digital a analógica también se denomina modulación de una señal digital.
• La conversión de una señal analógica a otra analógica también se denomina modulación de una señal analógica.
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Figure 5-1
Different Conversion Schemes
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Figure 5-2
Digital to Digital Encoding
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Figure 5-3
Types of Digital to Digital Encoding
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Conversión digital a digital
• Unipolar, usa un único valor de nivel, que generalmente representa el ‘1’ y el ‘0’ mantiene la señal a 0.
• Polar, usa dos niveles de amplitud. Hay varias codificaciones: NRZ, RZ, bifásica, manchester y manchester diferencial.
• Bipolar, usa 3 niveles: positivo, cero y negativo. Tipos: Bipolar con Inversión de marca alternada (AMI), Bipolar con sustitución de 8 ceros (B8ZS) y Bipolar 3 de alta densidad (HDB3).
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Figure 5-4
Unipolar Encoding
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Figure 5-5
Types of Polar Encoding
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Figure 5-6
NRZ-L and NRZ-I Encoding
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Figure 5-7
RZ Encoding
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Figure 5-8Manchester and Diff. Manchester Encoding
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Figure 5-9
Types of Bipolar Encoding
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Figure 5-10
Bipolar AMI Encoding
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Figure 5-11
B8ZS Encoding
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Figure 5-12
HDB3 Encoding
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Resumen Digital a Digital
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Figure 5-13
Solution to Example 5.1
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Figure 5-14
Solution to Example 5.2
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Conversión de analógico a digital
• Se trata de digitalizar una señal analógica y consiste en muestrear periódicamente la señal analógica con infinitos valores continuos y transformar su valor en valores enteros discretos que posteriormente se convertirán en ceros y unos.
• De esta manera la señal analógica se transforma en pulsos digitales.
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Figure 5-15
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Analog to Digital Encoding
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Modulación por amplitud de pulsos (PAM)
• La señal analógica se muestrea con una determinada frecuencia y se genera una señal de pulsos discretos.
• PAM es el primer paso del método Modulación por codificación en pulsos (PCM)
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Figure 5-16
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PAM
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Figure 5-17
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Quantized PAM Signal
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Modulación por codificación en pulsos (PCM)
• PCM modifica los pulsos generados por PAM para obtener una señal completamente digital.
• PCM consta de 4 procesos:– PAM– Cuantificación– Cuantificación binaria, y– Codificación digital a digital.
• Según el teorema de Nyquist la tasa del muestreo debe ser al menos doble de la frecuencia más alta de la onda para no perder información significativa.
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Figure 5-18
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Quantizing Using Sign and Magnitude
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Figure 5-19
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PCM
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Figure 5-20
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From Analog to PCM
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Figure 5-20-continued
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From Analog to PCM
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Figure 5-20-continued
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From Analog to PCM
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Figure 5-20-continued
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From Analog to PCM
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Figure 5-21
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Nyquist Theorem
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Conversión de digital a analógico
• Los datos digitales deben ser modulados sobre una señal analógica (portadora) que ha sido manipulada para mostrar los valores correspondientes a los ceros y los unos de la señal digital. La modulación puede ser:– Modulación por desplazamiento de amplitud (ASK).
– Modulación por desplazamiento de frecuencia (FSK).
– Modulación por desplazamiento de fase (PSK).
– Modulación de amplitud en cuadratura (QAM), que combina cambios en fase y en amplitud.
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Figure 5-22
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Digital to Analog Encoding
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Figure 5-23
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Modulación para datos digitales
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Figure 5-24
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ASK
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Figure 5-25
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Bandwidth for ASK
BW=(1+d) x Nbaudio
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FSKFigure 5-27
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Bandwidth for FSKFigure 5-28
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BW=(fc1-fc0) + Nbaudio
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PSKFigure 5-29
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PSKConstellation
Figure 5-30
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4-PSKFigure 5-31
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4-PSKCharacteristics
Figure 5-32
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8-PSKCharacteristics
Figure 5-33
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PSKBandwidth
Figure 5-34
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BW=(1+d) x Nbaudio
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4-QAM and 8-QAMConstellations
Figure 5-35
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8-QAM SignalFigure 5-36
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16-QAMConstellation
Figure 5-37
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Bit Rate and Baud Rate
Figure 5-38
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Figure 5-38-continued
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Bit Rate and Baud Rate
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Conversión de analógico a analógico
• La modulación analógica permite cambiar las características de una señal analógica. Hay tres modos de modulación:– Modulación en amplitud (AM)– Modulación en frecuencia (FM)– Modulación en fase (PM)
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Figure 5-39
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Analog to Analog Modulation
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Figure 5-40
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Figure 5-41
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Amplitude Modulation
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Figure 5-42
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AM Bandwidth
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Figure 5-43
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AM Band Allocation
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Figure 5-44
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Frequency Modulation
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Figure 5-45
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FM Bandwidth
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Figure 5-46
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FM Band Allocation
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