Double-sideband suppressed-carrier transmission


Double-sideband suppressed-carrier transmission

Double-sideband suppressed-carrier transmission (DSB-SC): transmission in which (a) frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the carrier level is reduced to the lowest practical level, ideally completely suppressed.

In the double-sideband suppressed-carrier transmission (DSB-SC) modulation, unlike AM, the wave carrier is not transmitted; thus, a great percentage of power that is dedicated to it is distributed between the sidebands, which implies an increase of the cover in DSB-SC, compared to AM, for the same power used.

DSB-SC transmission is a special case of Double-sideband reduced carrier transmission.

This is used for RDS (Radio Data System) because it is difficult to decouple.

Contents

Spectrum

This is basically an amplitude modulation wave without the carrier therefore reducing power wastage, giving it a 100% efficiency rate. This is an increase compared to normal AM transmission (DSB), which has a maximum efficiency of 33.33%, since efficiency is defined as:

E = (Sideband Power)/(Sideband Power + Carrier Power).

Spectrum plot of an DSB-SC signal: Spectrum DSBSC.svg

Generation

DSBSC is generated by a mixer. This consists of an audio source combined with the frequency carrier.


  \underbrace{V_m \cos \left( \omega_m t \right)}_{\mbox{Audio}} \times 
  \underbrace{V_c \cos \left( \omega_c t \right)}_{\mbox{Carrier}} = 
  \frac{V_m V_c}{2} \left[ 
    \underbrace{\cos\left(\left( \omega_c + \omega_m \right)t\right)}_{\mbox{USB}} +
    \underbrace{\cos\left(\left( \omega_c - \omega_m \right)t\right)}_{\mbox{LSB}}
  \right]

DSBSC Modulation.svg

Demodulation

For demodulation the audio frequency and the carrier frequency must be exact otherwise we get distortion. DSB-SC can be demodulated if modulation index is less than unity Demodulation distortion diagram.jpg

How it works

This is best shown graphically. Below, is a message signal that one may wish to modulate onto a carrier, consisting of a couple of sinusoidal components.

DSBSC Message Signal.png

The equation for this message signal is s(t) = \frac{1}{2}\cos\left(2\pi 800 t\right) - \frac{1}{2}\cos\left( 2\pi 1200 t\right).

The carrier, in this case, is a plain 5 kHz (c(t) = \cos\left( 2\pi 5000 t \right)) sinusoid—pictured below.

DSBSC Carrier Signal.png

The modulation is performed by multiplication in the time domain, which yields a 5 kHz carrier signal, whose amplitude varies in the same manner as the message signal.

DSBSC Modulated Output.png

x(t) = \underbrace{\cos\left( 2\pi 5000 t \right)}_\mbox{Carrier} \times \underbrace{\left[\frac{1}{2}\cos\left(2\pi 800 t\right) - \frac{1}{2}\cos\left( 2\pi 1200 t\right)\right]}_\mbox{Message Signal}

The name "suppressed carrier" comes about because the carrier signal component is suppressed—it does not appear (theoretically) in the output signal. This is apparent when the spectra of the output signal is viewed:

DSBSC Spectrum.png

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Double-sideband reduced-carrier transmission — (DSB RC): transmission in which (a) the frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier and (b) the carrier level is reduced for transmission at a fixed level below that which is provided to the… …   Wikipedia

  • Single-sideband suppressed-carrier transmission — Single sideband suppressed carrier (SSB SC) is a telecommunication technique, which belongs to the Amplitude modulation class.The information represented by the modulating signal is contained in both the upper and the lower sidebands. Since each… …   Wikipedia

  • Reduced-carrier transmission — is an amplitude modulation (AM) transmission in which the carrier wave level is reduced to reduce wasted electrical power. Suppressed carrier transmission is a special case in which the carrier level is reduced below that required for… …   Wikipedia

  • Sideband — The power of an AM signal plotted against frequency. Key: fc is the carrier frequency, fm is the maximum modulation frequency In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing …   Wikipedia

  • Single-sideband modulation — (SSB) is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth. It is closely related to vestigial sideband modulation (VSB) (see below).Amplitude modulation produces a modulated output signal that has… …   Wikipedia

  • Modulation — For musical change of key, see Modulation (music). For other uses, see Modulation (disambiguation). Passband modulation v · d · e …   Wikipedia

  • List of electronics topics — Alphabetization has been neglected in some parts of this article (the b section in particular). You can help by editing it. This is a list of communications, computers, electronic circuits, fiberoptics, microelectronics, medical electronics,… …   Wikipedia

  • DSB — may refer to: Contents 1 Science, technology and devices 2 Arts 3 Politics 4 Institutions, companies, products and trademarks …   Wikipedia

  • HamSphere — bajo Windows 7. HamSphere es un software basado en la radiafición que permite a radioaficionados licenciados oficialmente y a cualquiera interesado en la radioafición a comunicarse con otros utilizando una ionosfera virtual en la red, esto es, a… …   Wikipedia Español

  • Amplitude modulation — Passband modulation v · d · e Analog modulation AM · …   Wikipedia