Amplitude modulation with suppressed carrier
The amplitude modulation with suppressed carrier , abbreviated DSBSC for English double side band suppressed carrier , is a special form of amplitude modulation . The degree of modulation m cannot be defined in this case.
Comparison with amplitude modulation
In the case of amplitude modulation with a carrier, two additional frequency bands are created in addition to the original carrier signal: the sidebands, each of which contains the spectrum of the useful signal. The upper side band is called USB (upper side band) and the lower one is called LSB (lower side band).
The adjacent figures are shown in the time domain. For comparison, the amplitude modulation with carrier (DSBFC) is shown first, and then without carrier in the second figure. The signal to be modulated is shown in red, the modulated carrier signal in dark blue. The degree of modulation effects a variation of the amplitude, and the signal to be modulated is before modulation with a time constant component, the DC component , is provided. This DC component then represents the component of the carrier signal after the modulation in the modulated signal.
Carrier-free amplitude modulation (DSBSC)
With amplitude modulation without carrier (DSBSC), the carrier signal is suppressed. This means that only useful signals without a direct component, such as B. audio signals are transmitted. As with carrier transmission, the same useful signal information is contained on both sidebands.
In the second figure, the envelope curve of the RF signal corresponds to the amplitude of the useful signal. In the area of the zero crossings of the useful signal, the two remaining side frequencies are in antiphase and cancel each other out because they have the same amplitude. At the zero crossing the phase of the sum signal jumps by 180 degrees.
The benefit of carrier suppression is that a larger proportion (up to two thirds) of the transmission power can be saved. A further increase in efficiency is achieved with single sideband modulation (SSB) by not having two, but only one sideband for transmission.
The carrier frequency is not subsequently removed with the help of a notch filter , but a symmetrical , multiplying mixer ( ring modulator , double push-pull modulator ) does not generate it at its output. There you only measure the two side ligaments, not the carrier.
However, the carrier is required for demodulation; in contrast to single sideband modulation, it must be available in the receiver in a synchronous demodulator with precise frequency and phase. This can be done, for example, by transmission on a separate channel or with the aid of a pilot tone , the requirements being considerably higher than with single sideband modulation. This effort is a disadvantage of this modulation method.
You can see from the picture that incoherent demodulation (envelope curve demodulation) is no longer possible. The DSBSC is demodulated with the mixer , whereby the phase-correct carrier signal generated locally in the receiver must be fed to the mixer.
The amplitude modulation with suppressed carrier is used, among other things, for processing the FM stereo sound system. In addition to the actual sum signal from the left and right channel in the low-pass range, a further difference signal from the left and right channel is double sideband modulated onto a subcarrier in which the subcarrier disappears. Only with this signal, consisting of the sum signal and the two sidebands of the difference signal, is the actual carrier FM-modulated and broadcast.
literature
- Karl Dirk Kammeyer: message transmission . Teubner, Stuttgart 1996, ISBN 3-519-16142-7 .