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LECTURE № 4. Methods and devices of modulation in radio and TV broadcasting

LECTURE № 4. Methods and devices of modulation in radio and TV broadcasting. 1. Modulation in analog radio and television broadcasting systems    - AM and its variants     - Angle modulation (frequency and phase) Открыть словарную статью

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LECTURE № 4. Methods and devices of modulation in radio and TV broadcasting

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  1. LECTURE № 4. Methods and devices of modulation in radio and TV broadcasting • 1. Modulation in analog radio and television broadcasting systems   - AM and its variants    - Angle modulation (frequency and phase) Открыть словарную статью • 2. Modulation in digital broadcasting systems     - amplitude, frequency, phase shift keying     - Quadrature modulation     - OFDM (Orthogonal Frequency Division Multiplexing)

  2. Modulation principle Change parameters of a carrier Information signal: Ac(t) fc(t) (t) Ac(t) : amplitude modulation AM ASK fc(t) : frequency modulation FM FSK (t) : phase modulation PM PSK Ac(t) and (t)  QAM (Digital) Analog Digital

  3. Analog Radio & TV systems Transport of analog information: i.e. base-band analog modulated CW carrier or pulse modulation Digital Radio & TV systems Transport of digital information: i.e. digital modulated CW carrier

  4. Amplitude modulated signals for m <1 and m> 1 AMPLITUDE MODULATION normal modulation Overmodulation  (distortion of the envelope)  Power in modulation mode for period HF carrier: if Average power for period of modulation : = AM modulation is not profitable, because peak power , average power only Advantage of AM - ease of modulation and demodulation (detection). It is very important for broadcasting (not expencive) Spectrum of AM signal in single-tone modulation have three frequency:

  5. Variants of AM.        Balanced, single sideband, with a partially suppressed carrier DSB-модуляция (double-sideband)

  6. Angel modulation (frequency – FM, phase - PM) Complete angle m - modulation index (unitless) FM PM (radians) sin(α+β)=sinαcosβ + sinβcosα m<1 m>1 Bessel function the first kind n-order

  7. Most important advantage of frequency modulation is the ability of FM receivers to suppress noise signals, i.e., possibility of a substantial (up to 20 ... 30 dB) to improve the relationship of the signal to noise power at the output frequency detector (Ps / Pn)outwith respect to at the receiver input (Ps / Pn)input. (Ps/Pn)out The noise suppression at the output of the FD (Ps/Pn)in Atexcess(Ps/Pn)inputofathresholdlevel - outputvoltageofthefrequencydetectorproportionalto square modulation caoeffitientm2.Consequently, a doubling of m increases the ratio (Ps/Pn)output in 4 times (6dB), i.e. reception quality increases with increasing frequency modulation index. When relations (Pc/Pn) input above the threshold level of the signal at the output of the demodulator is determined only by the value of the deviation and does not depend on the level of the input signal. It is believed that the receiver is captured by a signal - there is noise reduction.

  8. Frequency modulators

  9. 2. Modulation in digital broadcasting • Digital signals can carry both digital and analog information. Analog information is converted into digital based on the Nyquist theorem using vocoders, delta modulation, pulse code modulation, etc. (for the speech signal with Fmax = 3,4 kHz, f T ≥ 2F max = 6,8 kHz → f T = 8kHz. Given the sampling rate of the amplitude fTA = 8kHz, → fT∑ = fT · fTA = 64kHz → Vmax = 64 kb /c → after informative compression date rate will be V = 32 ... 1.2 kbits / c) Unlike analog used all three types of modulation AM, FM, PM, and them combinations. The type of modulation depends on the quality of reception, characterized by the probability of error

  10. Noise immunity of the reception of binar digital signals

  11. Bandwidth and information capacity Shannon limit for information capacity I = information capacity (bit/second) B = system bandwidth (Hertz) S/N = signal-to-noise power ratio (dimensionless)

  12. Binary data sequence signaling 0 1 1 0 1 0 0 0 1 1 Unipolar (On-off) A Tb t Polar (NRZ) Return-to-zero (RZ) Bipolar

  13. Digital modulation: • Amplitude Shift Keying (ASK) • Frequency-shift keying (FSK) • Phase-Shift Keying (PSK) • M-ary encoding • OFDM & COFDM • Amplitude Shift Keying (ASK) • Frequency-shift keying (FSK) • Phase-Shift Keying (PSK)

  14. Multi-position signals are signals, saving the frequency spectrum. Considered signals ASK, FSK and PSK are simple in the sense that they have two positions "0" and "1", each of which carries one bit information. Thus, the maximum data transmission speed in the frequency band can be 1bit/s, i.e., 2bit/s at 1 Hz bandwidth.    To increase the speed in a limited frequency band are used multi-position signals, in which a single pulse carries two, four or more bits of information. For this binary information symbols of duration Ts at the entrance combine in a blocks of M consecutive symbols. Each block is assigned its own parcel (channel signal) of duration M Ts. The width of the spectrum of multi-position signal with double sideband narrowing in L times compared with the signal with binary modulation L= log2 M, where M = 2, 4, 8, 16, ... .

  15. I/Q Modulation Modulated signal can to present by sum of two quadrature component QPSK: quadrature phase shift keying The two carrier frequencies are the same, but their phase is offset by 90 degrees (that is, they are “in quadrature”) • Quadrature means the signal shifts among phase states that are separated by 90 degrees • The signal shifts in increments of 90 degrees from 45° to 135°, -45° (315°), or -135° (225°) • Data into the modulator is separated into two channels called I and Q • Two bits are transmitted simultaneously, one per channel “I-Q” format—Polar to rectangular conversion Polar display—Magnitude and phase represented together QPSK Symbol Mapping QPSK Constellation

  16. I/Q Modulator • A single carrier generated by a local oscillator (L.O.) circuit is split into two paths. • One path is delayed by an amount of time equal to 1/4 of the carrier’s cycle time, or 90 degrees. • The two carriers are amplitude modulated—one by the I signal, the other by the Q signal. • The two modulated carriers are added together in a summing circuit. • The output is a digitally modulated signal whose amplitude and phase are determined by the amplitudes of the two modulating signals The same symbol transmitted multiple times mixes randomly each time with noise in the transmission path. As a result, each received symbol’s plotted position on the constellation is slightly different. In this example, all of the received signals’ phases and amplitudes are able to be interpreted as “11”. What About Impairments? This is what is transmitted—the RF signal’s instantaneous amplitude and phase represent the symbol “11” This is what is received after noise randomly mixes with the transmitted signal somewhere in the transmission path. Because the received RF signal’s phase and amplitude didn’t change very much from what was actually transmitted, the data receiver interprets the signal as “11”

  17. Modulation Error Ratio (MER) QPSK typically requires a minimum MER of 10~13 dB A large “cloud” of symbol points means low MER—this is not good! A small “cloud” of symbol points means high MER—this is good! MER = 10log(average symbol power / average error power) 16-QAM typically requires a minimum MER of 17~20 dB 16-QAM Noise appears random CW produces “donut” shape

  18. OFDM (OrthogonalFrequencyDivisionMultiplexing) The new method of modulation, offered comparatively recently, is OFDM (Orthogonal Frequency Division Multiplexing). Like quadrature modulation, this method utillizes the ortogonal subcarriers, but unlike quadrature modulation of frequency these carriers are not identical, they are located in some range, taken for communication of data by modulation. Modulation of OFDM implies, that all of range of every channel of digital TV broadcasting (in Europe are 8 Mhz) is broken on the great number of the ortogonal subcarriers (рис.3.20). Frequencies of these subcarriers un (t) = cos[2π(f0 +n/Ts)t], where f0 is beginning of interval a frequency, n is a number of subcarrier, being in a range from 0 to (N-1). Thereby number of all subcarriers is N. Ts is duration of interval of transmission of single subcarrier, which equal duration of serial binar (N-bit) substream. Thus the serial stream of data is broken up on N of parallel substreams, each of which modulated with much less speed. Frequency shifting of these subcarriers 1/Ts is equal . At Δf=8 MHz and Ts=1мс have N=8000. Applying the rules of trigonometry, these oscillations indeed are ortogonal, that their everage mutual power is equal zero. That, as well as in the case of quadrature modulation, possibility of their division means on a reception even at the partial ceiling of their side-bands. The principle of OFDM is shown in fig.3.21. Each serial sabstream of basic data is transformes in a parallel form. For example, if number of subcerriers 8000, duration of information symbol, modulating every subcarrier, is increasing in 8000 times. Consequently, duration of one transmission symbol is increasing in 8000 times. Possibility to insert yet a large enough protective interval between the transmission of nearby substreams allows additional decrease (diminish) distortions, arising up at the multibeam reception of digital TV. Exactly these property of OFDM was stipulated by their use in the European standard DBV-T. Fig. 21

  19. Till recently to realize the OFDM modulator (see fig. 21) at such number of subcarriers it was practically impossible. But developments of algorithms and integrated circuits of fast FT allowed to decide this problem (see fig.22). As all of calculations are made in a digital form, DAC appears on an output. Demodulating can be built on the base of direct FT. Naturally, that an ADC must be on an input of OFDM modulator. Modulation of OFDM found application in the systems of the digital broadcasting and in the system of the digital TV broadcasting of DVB-T standard, developed the group of the European countries. The diagram of digital part transmitter of DVB-T standard is shown in fig.23. In this picture, except OFDM and quadraturemodulators, there are the devices of the preliminary error codding of digital transport MPEG-2 picture-sound substream which cosists of a 188 byte (1 synchro byte +187 byte of information). Fig.22 Fig.23

  20. DISTRIBUTING OF DVB STANDARDS AREAS

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