Analog Transmission

Definition of Terms

• Analog transmission refers to the transmission of analog signals using a band-pass channel. Baseband digital or analog signals are converted to a complex analog signal with a range of frequencies suitable for the channel.
• Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in the digital data.  It is also called modulation of a digital signal. The baseband digital signal representing the digital data modulates the carrier to create a broadband analog signal.
•  In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes.
• In frequency shift keying, the frequency of the carrier signal is varied to represent
  data. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. Both peak amplitude and phase remain constant for all signal elements.
• In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude and frequency remain constant as the phase changes.
• Quadrature amplitude modulation (QAM) is a combination of ASK and PSK. QAM uses two carriers, one in-phase and the other quadrature, with different amplitude levels for each carrier.
• QAM enables a higher data transmission rate than other digital-to-analog methods.
• Baud rate and bit rate are not synonymous. Bit rate is the number of bits transmitted per second. Baud rate is the number of signal units transmitted per second. One signal unit can represent one or more bits.
• The minimum required bandwidth for ASK and PSK is the baud rate.
• The minimum required bandwidth (BW) for FSK modulation is BW = f c1 .f c0 + N baud , where f c1 is the frequency representing a 1 bit, f c0 is the frequency representing a 0 bit, and N baud is the baud rate. 
• A regular telephone line uses frequencies between 600 and 3000 Hz for data communication. 
• ASK modulation is especially susceptible to noise because the amplitude is more affected by noise than the phase or frequency. 
• Because it uses two carrier frequencies, FSK modulation requires more bandwidth than ASK and PSK.  
• Trellis coding is a technique that uses redundancy to provide a lower error rate. 
• The 56K modems are asymmetric; they download at a rate of 56 Kbps and upload at 33.6 Kbps.
• A constellation diagram shows us the amplitude and phase of a signal element, particularly when we are using two carriers (one in-phase and one quadrature).
• Analog-to-analog conversion is the representation of analog information by an analog signal. Conversion is needed if the medium is bandpass in nature or if only a bandpass bandwidth is available to us.
• In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of the carrier remain the same; only the amplitude changes to follow variations in the information.
•  In PM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. The peak amplitude and phase of the carrier signal remain constant, but as the amplitude of the information signal changes, the frequency of the carrier changes correspondingly.
• In PM transmission, the phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. The peak amplitude and frequency of the carrier signal remain constant, but as the amplitude of the information signal changes, the phase of the carrier changes correspondingly. 
• In AM radio, the bandwidth of the modulated signal must be twice the bandwidth of the modulating signal. 
• In FM radio, the bandwidth of the modulated signal must be 10 times the bandwidth of the modulating signal.

Types of digital-to-analog conversion

Digital-to-analog modulation can be accomplished using the following

• Amplitude shift keying (ASK) - the amplitude of the carrier signal varies.
• Frequency shift keying (FSK) - the frequency of the carrier signal varies.
• Phase shift keying (PSK) - the phase of the carrier signal varies.
• Quadrature amplitude modulation (QAM) - both the phase and amplitude of the carrier signal vary.  It combines ASK and PSK.

PSK and QAM modulation have two advantages over ASK:

• They are not as susceptible to noise. 
• Each signal change can represent more than one bit. 

Analog-to-analog conversion can be accomplished in three ways:

• amplitude modulation (AM) 
• frequency modulation (FM)
• phase modulation (PM)

The Total Bandwidth Required:

• The total bandwidth required for AM can be determined from the bandwidth of the audio signal: B_AM = 2B.
• The total bandwidth required for FM can be determined from the bandwidth of the audio signal: B_FM = 2(1 + β)B.
• The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal: B_PM = 2(1 + β)B.

AM band allocation

FM band allocation

Note: You can proceed to take the multiple choice exam regarding this topic. Analog Transmission - Set 1 MCQs

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