Scheme of a DVB-C transmission system
With reference to the figure, a short description of the single processing blocks follows.
- Source coding and MPEG-2 multiplexing (MUX): video, audio, and data streams are multiplexed into an MPEG program stream (MPEG-PS). One or more MPEG-PSs are joined together into an MPEG transport stream (MPEG-TS). This is the basic digital stream which is being transmitted and received by home set top boxes (STB) or relevant integrable decoder (e.g.Conax) module. Allowed bitrates for the transported MPEG-2 depend on a number of modulation parameters: it can range from about 6 to about 64 Mbit/s (see the bottom figure for a complete listing).
- MUX adaptation and energy dispersal: the MPEG-TS is identified as a sequence of data packets, of fixed length (188 bytes). With a technique called energy dispersal, the byte sequence is decorrelated.
- External encoder: a first level of protection is applied to the transmitted data, using a nonbinary block code, a Reed-Solomon RS (204, 188) code, allowing the correction of up to a maximum of 8 wrong bytes for each 188-byte packet.
- External interleaver: convolutional interleaving is used to rearrange the transmitted data sequence, such way it becomes more rugged to long sequences of errors.
- Byte/m-tuple conversion: data bytes are encoded into bit m-tuples (m = 4, 5, 6, 7, or 8).
- Differential coding: In order to get a rotation-invariant constellation, this unit shall apply a differential encoding of the two Most Significant Bits (MSBs) of each symbol.
- QAM Mapper: the bit sequence is mapped into a base-band digital sequence of complex symbols. There are 5 allowed modulation modes: 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM.
- Base-band shaping: the QAM signal is filtered with a raised-cosine shaped filter, in order to remove mutual signal interference at the receiving side.
- DAC and front-end: the digital signal is transformed into an analog signal, with a digital-to-analog converter (DAC), and then modulated to radio frequency by the RF front-end.
Available bitrates for a DVB-C system (Mbit/s)
The receiving STB adopts techniques which are dual to those ones used in the transmission.
- Front-end and ADC: the analog RF signal is converted to base-band and transformed into a digital signal, using an analog-to-digital converter (ADC).
- QAM Demodulation
- Differential decoding
- Outer interleaving
- Outer decoding
- MUX adaptation
- MPEG-2 demultiplexing and source decoding
- Programmable Transport Stream