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III. Gm-C Filtering - Epublications - Université de Limoges

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Figure 7. Time representations of I, Q and output signals using a 4PSK modulation<br />

More complex modulations using more bits per symbol are also used, such as<br />

256QAM or even 1024QAM. It is also possible to use combine both modulation types in a<br />

16APSK modulation.<br />

These modulations can be <strong>de</strong>scribed in a Quadrature-phase versus In-phase graph, as<br />

illustrated in Figure 8. These graphs are called constellations.<br />

Figure 8. 8PSK (a) , 16QAM (b) and 64QAM (c) constellations<br />

I.1.b.iii Analog and Digital Standards<br />

For compatibility and interoperability reasons, all TV transmissions <strong>de</strong>pend on<br />

standards. These standards aim at specifying how TV transmissions take place, and in<br />

particular they <strong>de</strong>fine operating frequencies, channel bandwidths, allowed emission power<br />

levels, modulation types, picture formats... Emitted signals are based on either an analog or a<br />

digital modulation.<br />

Analog signals mainly use three different standards: NTSC, PAL and SECAM. These<br />

standards, used since the beginning of color TV transmissions, are less and less broadcasted<br />

since the quality of the reception is strongly related to the reception power level.<br />

The past ten years have seen the progressive emergence of digital TV. One of its<br />

advantages is that the quality of the reception is not related to the power level: it only requires<br />

a threshold power level to operate in good conditions. Above the threshold power level, the<br />

quality of the reception felt by the end-user becomes in<strong>de</strong>pen<strong>de</strong>nt of the received power. This<br />

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