World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0019363877
Reproduction Date:

Title: Dsbsc  
Author: World Heritage Encyclopedia
Language: English
Subject: Reduced-carrier transmission
Publisher: World Heritage Encyclopedia


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

In the DSB-SC modulation, unlike in AM, the wave carrier is not transmitted; thus, much of the power 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. It is used for radio data systems.


DSB-SC is basically an amplitude modulation wave without the carrier, therefore reducing power waste, giving it a 50% efficiency. This is an increase compared to normal AM transmission (DSB), which has a maximum efficiency of 33.333%, since 2/3 of the Power is in the Carrier which carries NO intelligence, and each Sideband carries the SAME Information. Single_Side_Band (SSB) Suppressed_Carrier is 100% efficient.

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


DSB-SC is generated by a mixer. This consists of a message signal 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}}

File:DSBSC Modulation.svg


For demodulation, the audio frequency and the carrier frequency must be exact. Otherwise, distortion will occur. DSB-SC can be demodulated if modulation index is less than unity.

File:Demodulation distortion diagram.png

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.

File: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.

File: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.

File: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 in the output signal. This is apparent when the spectrum of the output signal is viewed:

File:DSBSC Spectrum.png


 This article incorporates MIL-STD-188).

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.