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tavakoli
Joined: 31 Dec 2003
Posts: 69
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 12 Apr 2004 13:29 amplitude modulation..? |
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| what is the difference between amplitude modulation & dsb-sc modulation..?
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boot_strap
Joined: 02 Dec 2003
Posts: 20
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| 12 Apr 2004 13:32 Re: amplitude modulation..? |
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in amplitude modulation your modulated signal is : xc(t) = x(t)*A*cos(wt)
but dsb-sc is :
xc(t) = (1+k*x(t))*A*cos(wt)
for details go to haykins communication book..!
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artem
Joined: 22 May 2003
Posts: 1638
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Location: Turan
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| 12 Apr 2004 13:45 amplitude modulation..? |
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The difference is that AM modulation does contain the carrier frequency , but DSB does not .
Common part is that both modulation provides 2 bands = Fcarrier -+ Fmodulating
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nandopg
Joined: 26 Apr 2001
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| 12 Apr 2004 15:25 Re: amplitude modulation..? |
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Regarding an AM-DSB modulator fed by a sine wave modulating signal wm and carrier frequency wc:
In the frequency domain: the spectrum is made up by three lines: one centered in wc, one centered in wc+wm and other centered in wc-wm
In time domain: The amplitude of the carrier will be changing according to the instantaneous value of wm. If the amplitude of wm is equal to the amplitude of wm, the modulated signal's amplitude will be changing from 0 and 2 x amplitude of wm (or wc). In this case the AM coefficient index is 100%. If you disconnect the modulating signal, the output of the modulator will be the a sine wave in wm.
Regarding an AM-DSB-SC modulator fed by a sine wave modulating signal wm and carrier frequency wc:
In the frequency domain: the spectrum is made up two lines: one centered in wc+wm and other centered in wc-wm. The carier is suppressed.
In time domain the modulated signal will be changing from 0 to the amplitude of wc. There is no definition of modulation index. If you turn off the modulating frequency, the modulator's output will be zero.
Now the most important characteristic of the AM-DSB-SC, is that the phase of the modulated signal jumps 180 deg. each time the modulating signal inverts its polariity.
By the way due to this characteristic AM-DSB-SC are the kind of modulator used to generate digital modulation in BPSK and QPSK.
NandoPG
Last edited by nandopg on 24 Jun 2004 15:12; edited 2 times in total |
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awan
Joined: 11 Jun 2004
Posts: 81
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| 24 Jun 2004 13:32 Re: amplitude modulation..? |
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| in a.m the signal is modulated by amplitude
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v_naren
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| 24 Jun 2004 13:43 Re: amplitude modulation..? |
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another main point is that in full AM you waste power in the carrier signal frequency in addition to the side band informarion power
but in DSB-SC AM one does not waste any power as the carrier signal is not present...
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cedance
Joined: 24 Oct 2003
Posts: 704
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Location: Germany
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| 08 Mar 2005 20:18 Re: amplitude modulation..? |
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| awan wrote: |
| in a.m the signal is modulated by amplitude |
and what modulation is DSBSC? same amplitude. what were u trying to say?
/cedance
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v_naren
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| 08 Mar 2005 20:27 Re: amplitude modulation..? |
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| cedance wrote: |
| awan wrote: |
| in a.m the signal is modulated by amplitude |
and what modulation is DSBSC? same amplitude. what were u trying to say?
/cedance |
If I may answer this question
normal AM-FC (AM with full carrier)
=V_c cos({\omega_0}t) (1+m(t)) "3$v_{AM-FC}(t)=V_c cos({\omega_0}t) (1+m(t))")
where m(t) is a scaled version of the the modulating or baseband signal that is to be transmitted, such that
|<=1 "3$|m(t)|<=1")
Double Side-band Suppressed Carrier (DSB-SC)
=V_c cos({\omega_0}t) m(t) "3$V_{DSB-SC}(t)=V_c cos({\omega_0}t) m(t)")
where m(t) is the modulating or baseband signal that is to be transmitted.
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djalli
Joined: 10 Nov 2001
Posts: 887
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Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 09 Mar 2005 0:16 amplitude modulation..? |
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| Another differences are money, management and situation. Engineering is business to be honest.
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aman
Joined: 25 Oct 2004
Posts: 145
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| 11 Mar 2005 11:50 Re: amplitude modulation..? |
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pls consult the foolwing url for ur ans
http://www.eas.asu.edu/~midle/jdsp/commex/Lab1(Amplitude%20mod).htm
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truebs
Joined: 21 Jan 2005
Posts: 322
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Location: Asia
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| 11 Mar 2005 19:45 Re: amplitude modulation..? |
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Amplitude modulation can be done in a number of ways...one among them is DSB Sc , the difference lies in two points :
1. Number of Sideband
2. Carrier signal.
In AM there can be either one or two sideband but the carrier signal is always sent with the modulated signal.
Whereas DSB has two sidebands and the carrier is suppressed.
What this implies is there would be no carrier signal with the modulated signal,this would have several implications on Bandwdth.
DSB Sc is considered superior to AM due to its lesser BW requirement and better S/N ratio (for proper design ).
Refer to Analog and Digital Communication Techniques by Couch for more details.
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electron_boy
Joined: 14 Nov 2004
Posts: 193
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Location: Chennai, India
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| 12 Mar 2005 1:33 Re: amplitude modulation..? |
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| amplitude modulation includes the transmission of the carrier. if the carrier is removed then it is DSB SC
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CAP
Joined: 16 Jun 2003
Posts: 41
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| 12 Mar 2005 14:15 Re: amplitude modulation..? |
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| electron_boy wrote: |
| amplitude modulation includes the transmission of the carrier. if the carrier is removed then it is DSB SC |
why ? DSB SC have the carrier to. anothe way its not a SC
DSB have too sites of signal spektrum.
SSB have only one.
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electron_boy
Joined: 14 Nov 2004
Posts: 193
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Location: Chennai, India
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| 13 Mar 2005 8:01 Re: amplitude modulation..? |
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| CAP wrote: |
| electron_boy wrote: |
| amplitude modulation includes the transmission of the carrier. if the carrier is removed then it is DSB SC |
why ? DSB SC have the carrier to. anothe way its not a SC
DSB have too sites of signal spektrum.
SSB have only one. |
then what the term 'suppressed carrier' mean??
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v_naren
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| 13 Mar 2005 8:25 Re: amplitude modulation..? |
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| electron_boy wrote: |
| CAP wrote: |
| electron_boy wrote: |
| amplitude modulation includes the transmission of the carrier. if the carrier is removed then it is DSB SC |
why ? DSB SC have the carrier to. anothe way its not a SC
DSB have too sites of signal spektrum.
SSB have only one. |
then what the term 'suppressed carrier' mean?? |
electron_boy is correct...but sometimes the carrier is not completely removed. Its power is reduced relative to the sidebands. This is to facilitate "synchronous demodulation" in RXCVRs where a phase locked loop mechanism is used to "lock" and generate an in-phase LO signal and then this LO and the DSB-SC AM signal are both fed into a mixer to downconvert and hence perform demodulation to recover the transmitted baseband signal.
http://www.ece.eps.hw.ac.uk/Modules/B33cl2/cl-8-03.pdf
Section 8.6
In particular 8.6.2 to 8.6.3
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checkmate
Joined: 25 Feb 2004
Posts: 489
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| 13 Mar 2005 8:26 Re: amplitude modulation..? |
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| It really just means "do not include carrier frequency component". Coherent demodulation would then have to be done using other synchronization techniques such as PLLs.
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CAP
Joined: 16 Jun 2003
Posts: 41
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| 15 Mar 2005 15:50 amplitude modulation..? |
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often the term 'suppressed carrier' mean, that there are NO diskrete component at the F=SC in signal PSD.
it's there, but its spread about hole usefull bandwidth by phase/frequence/amplitude modulation.
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TnT
Joined: 12 Oct 2004
Posts: 17
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| 19 Mar 2005 11:45 Re: amplitude modulation..? |
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What about the S/N ratio of AM and DSB modulation ?
This is the difference. In analog modulation, people allways want to increase S/N ratio
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pankaj waghmare
Joined: 18 Mar 2005
Posts: 45
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| 29 Mar 2005 6:37 Re: amplitude modulation..? |
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in dsb-sc,limitations of am is removed.
it saves transmitted power and supress the carrier
for detail plz refer g.kennedy`s book
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ppenday
Joined: 17 Jan 2005
Posts: 70
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| 29 Mar 2005 6:39 Re: amplitude modulation..? |
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| am contains carrier and ds-sc does not. you can find much about this in communication systems by simon haykin.
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savadi
Joined: 14 Sep 2004
Posts: 27
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| 19 Apr 2005 17:02 amplitude modulation..? |
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| DSB-Sc is one kind of AM . In conventional AM the carrier is not suppresed but in DSB-SC the carrier is suppresed. Conventional AM has a simple demodulator unlike DSB-Sc.
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caep_daniel
Joined: 04 Sep 2003
Posts: 44
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| 19 Apr 2005 17:32 amplitude modulation..? |
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| that's too easy.no problem to analysis so in detail.
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as_518
Joined: 10 Apr 2005
Posts: 80
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| 20 Apr 2005 17:05 Re: amplitude modulation..? |
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| It should be noted that both dsbtc and dsbsc are AM modulation techniques. In DSBTC modulation a nonlinear element (like diode) is used for modulation while in DSBSC a balnced modulator is used. In demodulation an envolpe detector (a diode followed by LPF) could be used for DSPTC while for DSBSCa coherent detection must be used.
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