DTV Modulator
Cable, satellite, and terrestrial video service providers use digital television (DTV) modulators to transform video programs (including video, audio, and ancillary data) into a format that can be broadcast over mediums such as coaxial cables or air waves. Modulators receive data as a single- or multi-program transport stream (SPTS or MPTS), transmitted via the digital video broadcasting (DVB) asynchronous serial interface (ASI) at 270 Mbps. The transport stream (TS) includes audio, video, and ancillary data inputs that have been encoded with compression schemes such as Moving Picture Experts Group (MPEG)-2.
DTV modulators perform forward error correction (FEC) encoding and map the binary data into a modulation scheme suitable for broadcast. Different parts of the world have employed different modulation standards for the transition to DTV. Table 1 summarizes the modulation schemes employed by geography and mode of transmission. Examples of modulation schemes used for DTV broadcasting are quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), coded orthogonal frequency division multiplexing (COFDM), and vestigial sideband (VSB).
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Table 1. Modulation Schemes by Geography and Mode of Transmission
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| Location |
Cable |
Satellite |
Terrestrial |
| North America |
ITU-T/J.83B (64/256QAM) |
DVB-S, DSS (QPSK), DVB-S2 |
ATSC (8VSB) |
| Europe |
DVB-C (64QAM) |
DVB-S (QPSK), DVB-S2 |
DVB-T, DVB-H (COFDM) |
| China |
DVB-C, DTV-C (64QAM) |
DVB-S (QPSK) |
TDS-OFDM |
| Japan |
ITU-T/J.83A (64QAM) |
ISDB-S (8PSK) |
ISDB-T |
Irrespective of the different DTV standards across geographies, all modulators have to provide some form of FEC encoding and then map the data into a modulation scheme suitable for broadcast. Figures 1, 2, 3, and 4 show various functional blocks of the different modulation schemes based on the broadcast medium.
In terrestrial broadcast, the COFDM modulation is used in Europe. The data in COFDM is distributed over many frequency carriers that are orthogonal to each other. The COFDM modulation has a baseband interface to accept MPEG-TS. It employs Reed Solomon coding and Viterbi techniques to provide FEC and performs interleaving to reduce the burst type errors. The data is then QAM mapped to the signal and upconverted. Refer to Figure 1 for a DVB-T block diagram. The advanced television systems committee (ATSC) DTV broadcast standard is employed in North America for the terrestrial broadcast and uses 8-VSB for signal modulation.
Figure 1. Digital Terrestrial (DVB-T) Modulator
Notes:
- TPS: Transmission parameter signaling
- IFFT: Inverse fast Fourier transform
DVB-S is the satellite broadcast standard that has the baseband interface to accept single MPEG-TS or multiple MPEG-TS. FEC in DVB-S is the same as DVB-T. DVB-S2, the next-generation satellite broadcast standard, employs outer block-level coding, low-density parity check coding (LDPC), and bit interleaving for error correction. The DVB-S standard supports only QPSK modulation. However, DVB-S2 additionally supports 8PSK, 16APSK, and 32APSK modulation schemes.
Figure 2. Digital Satellite (DVB-S2) Modulator
Notes:
- APSK: Amplitude/phase-shift keying
- PSK: Phase-shift keying
The cable modulation schemes across all geographies are similar and employ QAM techniques for modulating the signals. For details on functional blocks and their functionalities with respect to cable modulation, refer to the QAM modulator (PDF) white paper or the Digital Cable QAM page.
Figure 3. J.83 Digital Cable Modulator
China released the Digital Multimedia Broadcast-Terrestrial/Handheld (DMB-TH also referred to as DTMB) standard in 2006. The official name for the standard is Framing Structure, Channel Coding, and Modulation for Digital Television Terrestrial Broadcasting.
Figure 4. DMB-TH Modulator
Notes:
- PN: Pseudo-random noise
- DUC: Digital upconverter
Examples of Altera® intellectual property (IP) from the IP MegaStoreTM used in DTV Modulators include:
- Complete J.83 A/B/C multi-channel core
- DVB-T and DVB-S2 modulator
- DMB-TH modulator core
- Reed-Solomon decoder
- Viterbi decoder
- De-interleaver
- Numerically controlled oscillator (NCO)
- Finite impulse response (FIR) filter
- 8b/10b encoder
- DVB-ASI reference design
Feature-Rich Programmable Solutions for DTV Modulators
The feature-rich architectures of Stratix® III FPGAs provide an excellent solution for the DTV modulator's video delivery needs. These device families give you flexibility, performance, integration, and design resources that are not available in any other solution. The Stratix III devices include up to 768 embedded 18 x 18 multipliers and 14 Mbits of TriMatrix memory.
Altera’s low-cost Cyclone® III FPGAs are built on a 65-nm, low-power process technology. The Cyclone III family is composed of eight devices ranging from 5K to 120K logic elements (LEs) and up to 534 user I/O pins. Cyclone III FPGAs offer up to 4 Mbits of embedded memory, 288 embedded 18 x 18 multipliers, and dedicated external memory interface circuitry, making them ideal for implementing applications such as modulators in digital broadcasting applications.
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