Double-sideband suppressed-carrier transmission
The amplitude modulation with suppressed carrier abbreviated DSBSC for English Double sideband suppressed carrier, is a special form of amplitude modulation. The modulation index m is not defined in this case.
Compared with amplitude modulation
In the amplitude modulation with the carrier formed in addition to the original carrier signal, two additional frequency bands: the sidebands, in each of which the spectrum of the useful signal is contained. The upper sideband USB ( Upper Side Band), and the lower designated analogous to LSB (lower side band).
In these illustrations, the amplitude modulation is shown with vehicle ( DSBFC ) in the time domain and to compare first. The signal to be modulated is shown in red, the modulated carrier signal in dark blue. The modulation depth results in a variation of the amplitude, and the signal to be modulated is provided in front of the modulation with a time constant component, the DC component. This DC component is then after the modulation in the modulated signal represents the proportion of the carrier signal
Amplitude modulation without carrier ( DSBSC )
In the amplitude modulation without carrier ( DSBSC ) signal, the carrier is suppressed. This is achieved by suppressing the DC component in the desired signal, and this results in a waveform in the time domain as in the second figure. Is visible in the time domain, that the low frequency signal having the same amplitude as the carrier signal of the carrier disappears in the region of zero crossings of the user signal. As with the transmission carrier is on both sidebands contain the same Nutzsignalinformation.
The benefit of the carrier suppression is that a greater proportion may be reduced ( to two thirds ) of the transmission power. A further increase in efficiency is achieved with the single-sideband (SSB ) by not two but only one sideband brings to emit.
The carrier frequency is not subsequently removed by means of a notch filter, but a symmetrical multiplying mixer ( ring modulator, a double -balanced modulator ) produces not this at its output. There one measures only the two sidebands, but not the support.
For demodulation of the carrier is needed, however, he must - in contrast to the single-sideband modulation - in the receiver are in a synchronous re frequency and phase exactly available. This can be done for example by transmitting on a separate channel or by means of a pilot tone, wherein the requirements are considerably higher than that of single-sideband modulation. This complexity is a disadvantage of this modulation method.
It can be seen on the screen that a non-coherent demodulation ( envelope detection ) is no longer possible. Demodulation of the DSBSC carried out with the mixer, wherein the locally generated in the receiver, and in-phase carrier signal of the mixer needs to be supplied.
The amplitude modulation with suppressed carrier is used, among other things, in the treatment of FM stereo sound system. In this case, a further difference signal of left and right channel is double-sideband modulated on a subcarrier in addition to the actual sum signal of left and right channels in the low pass region, wherein the sub-carrier disappears. Only with this signal consisting of the sum signal and the two side bands of the difference signal, then the actual support is FM - modulated and broadcast.