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HDT-02_H.264 High Definition Digital Camera Transmitter
MANUAL V9.9 20
Besides, in COFDM, the sub-carrier frequencies are chosen so that the sub-
carriers are orthogonal to each other, meaning that cross-talk, interference,
between the sub-channels is eliminated. The orthogonality allows high
spectral efficiency.
On the other hand, COFDM system is invariably used in conjunction with
channel coding (forward error correction). The error correction code used in
this equipment is Reed-Solomon coding, which is concatenated with LDPC,
and there is an additional interleaving between the two layers of coding. Error
correcting codes build redundancy into the transmitted data stream. This
redundancy allows bits that are in error or even missing to be corrected in
the receiver.
The European ETSI EN 300755 standard defines the following LDPC coding
rates: 1/2, 3/5, 2/3, 3/4, 4/5, 5/6. There is a compromise between the coding
rate (signal robustness) and the transmitted bit rate. If the coding rate is
higher the signal transmission is more robust (1/2 is the most robust) but the
bit rate that the system is able to transmit is lower.
Used modulation scheme of each COFDM sub-carrier, QPSK, 16QAM, 64QAM
and 256 QAM is also connected with signal robustness and transmitted bit
rate. QPSK is the most robust and 256QAM is able to transport a higher bit
rate.
Besides the system can define 4 guard intervals: 1/4, 1/8, 1/16 and 1/32.
The guard interval is used to reduce intersymbol interferences due to the
multipath propagation.
In addition, it also provides several bandwidths: 1.7, 5, 6, 7 and 8 MHz in
case there are needed for different applications.
To summarize, with all of these characteristics, the maximum bit rate
achieved is 46 Mbps.
DVB-T features
The RF stage of the HDT-02 Transmitter is the same as the DVB-T2 one. The
difference is found in the modulation part as it is commented below.
The HDT-02 digital camera Transmitter uses COFDM (Coded Orthogonal
Frequency Division Multiplexing) modulation system (2K mode).
The European ETSI EN 300744 standard defines the following convolutional
coding rates: 1/2, 2/3, 3/4, 5/6, 7/8.
Used modulation scheme of each COFDM sub-carrier, QPSK, 16QAM and
64QAM, is also connected with signal robustness and transmitted bit rate.
QPSK is the most robust and 64QAM is able to transport a higher bit rate.
Besides the system can define 4 guard intervals: 1/4, 1/8, 1/16 and 1/32.
In conclusion, with all of these characteristics, the maximum bit rate achieved
is 31.67 Mbps.