SBCA Digital Transmission 2 — Questions and Answers
Question 1: Which modulation technique is used in DVB-S2 to achieve higher spectral efficiency compared to DVB-S?
- BPSK
- QPSK only
- 8PSK and 16APSK (Correct answer)
- FSK
Correct answer: 8PSK and 16APSK
DVB-S2 supports higher-order modulations including 8PSK and 16APSK, enabling greater spectral efficiency than DVB-S which uses QPSK.
Question 2: What is the primary purpose of a Reed-Solomon outer code in a digital satellite transmission chain?
- Scrambling the data for security
- Correcting burst errors after Viterbi decoding (Correct answer)
- Compressing video content
- Multiplexing multiple streams
Correct answer: Correcting burst errors after Viterbi decoding
Reed-Solomon outer coding corrects burst errors that remain after the inner Viterbi decoder, providing a concatenated error correction scheme.
Question 3: In MPEG-2 transport stream, what is the fixed packet size?
- 128 bytes
- 188 bytes (Correct answer)
- 256 bytes
- 512 bytes
Correct answer: 188 bytes
MPEG-2 transport stream packets are fixed at 188 bytes, consisting of a 4-byte header and 184 bytes of payload.
Question 4: What does the term 'spectral efficiency' measure in satellite digital transmission?
- The number of satellites in orbit
- Bits transmitted per second per hertz of bandwidth (Correct answer)
- The signal-to-noise ratio at the receiver
- The amount of forward error correction overhead
Correct answer: Bits transmitted per second per hertz of bandwidth
Spectral efficiency is measured in bits/s/Hz and indicates how effectively a transmission system uses available bandwidth.
Question 5: Which DVB-S2 feature allows the uplink to adapt modulation and coding in real time based on link conditions?
- Constant Coding and Modulation (CCM)
- Adaptive Coding and Modulation (ACM) (Correct answer)
- Time Division Multiplexing (TDM)
- Statistical Multiplexing
Correct answer: Adaptive Coding and Modulation (ACM)
ACM (Adaptive Coding and Modulation) dynamically adjusts modulation order and FEC rate per terminal based on real-time link margin.
Question 6: What is the function of an energy dispersal scrambler in a satellite uplink chain?
- Encrypting content for conditional access
- Spreading signal energy to avoid spectral lines (Correct answer)
- Increasing the symbol rate
- Reducing uplink power requirements
Correct answer: Spreading signal energy to avoid spectral lines
Energy dispersal randomizes data patterns to prevent strong spectral lines that would cause interference and violate regulatory power spectral density limits.
Question 7: In DVB-S2 framing, what is a BBFRAME (Baseband Frame) used for?
- Carrying audio elementary streams only
- Encapsulating input stream data before LDPC/BCH encoding (Correct answer)
- Synchronizing the LNB local oscillator
- Defining transponder bandwidth allocation
Correct answer: Encapsulating input stream data before LDPC/BCH encoding
The BBFRAME is the baseband framing structure that encapsulates input stream data and a header before applying BCH and LDPC forward error correction.
SBCA Digital Transmission 2
Which modulation technique is used in DVB-S2 to achieve higher spectral efficiency compared to DVB-S?