4. DBS-TV: Simplified Structure
4
• Video (V), audio (A), and data (D) bit streams are compressed
using MPEG-2 or MPEG-4 compression techniques and formed
into packets and frames.
• Forward error correction (FEC) and modulation method can be
changed on command by the control line.
• The data stream is encrypted to prevent unauthorized access.
• The modulated signal is sent to the transmitter at an IF
frequency, often 700MHz.
• The home receiver uses a low noise block converter to down
convert the received signals to IF and sends them to the satellite
receiver, which reverses the operations carried out in the
transmitter.
7. DBS-TV: Receiver
7
• Single channel Ku-band LNB block diagram for satellite TV
reception.
• Input is two circularly polarized signals in the 12.2–12.7 GHz
band.
• The polarizer plate converts the LHCP signal to a vertical E-field
linearly polarized wave and the RHCP signal to an H-field wave.
• The orthogonal probes in the circular waveguide extract the V and
H polarized waves and feed two LNAs.
• A command signal sent via the coaxial cable switches on either
LNA 1 or LNA 2.
• The RF block is an inductor to prevent the IF signal entering the
polarization line.
11. DBS-TV: Master Control Station
11
• The upper blocks in the diagram are at baseband. The frames that
are sent to the PSK modulator depend on the signal format.
• The buffer assembles eight or 32 MPEG packets to form short or
long frames and applies a double layer of FEC coding.
• The scrambler multiplies the encoded data stream by a pseudo
random sequence to provide energy dispersal.
• In this example, the 70MHz PSK modulator can operate in QPSK
or 8-PSK modes and is followed by a SRRC band pass filter.
• The SRRC filter must be a digital finite impulse response (FIR)
filter to match the bandwidth of the QPSK or 8-PSK signal. The
PSK signals are upconverted to 700MHz and sent to the
transmitting antenna.
