Receiving satellite signal. Common pitfalls. Part 3, digital

Previous parts

First part. Geometrical.

Second part. Second pat.

Signal parameters

In the previous parts we described how satellite signals can be received and brought to a headend station.
80% of the work is done, if only the remaining 20% did not take 80% of the time.
So let’s consider that a signal is given to the input of a headend. The receiver (tuner) is tuned in to the desired frequency / polarization. Then the signal goes to the demodulator - a special chip that converts the radio pulses to the digital data for further processing. But not every such chip will cope with any type of a signal. A support of a complete set of the characteristics is required:

• SR - symbol rate, shows the number of modulation states (symbols) transmitted on the given transponder. Typical values are tens of millions of simbols per second.
• Modulation type - that is the type of conversion the bits into radio signal elements. Without going into details, we say that in satellite television, as a rule, PSK phase modulation is used, although it is possible to use amplitude or amplitude-phase modulation. For broadcasting in the DVB-S standard, quadrature phase modulation (QPSK) is usually used, for DVB-S2 - it is octal (8PSK). Each QPSK symbol encodes 2 bits of data, 8PSK encodes 3 bits.
• FEC - Forward Error Correction. The satellite is far away from the Earth, with the ionosphere, rains and the thunderstorms on the way. The signal can be delivered distorted, so only a fraction of the data is useful information, and all the rest is the service data for correcting the errors. For example, FEC 7/8 means that 1 bit of control data arrives at 7 bits of payload. Unfortunately, even the FEC does not always save. One of the qualitative indicators of the signal is the BER parameter (bit error rate), equal to the ratio of erroneously transmitted data to the total. At a value of 10^-4, artifacts and the picture scattering are already possible.

In our headend stations we use Sony demodulation chips with the following characteristics for satellite signal(terrestial and digital cable signals are also supported) : modulations QPSK, 8PSK, SR 1-45 Msymbol/s for DVB-S2 and 1-62 Msymbol/s for DVB-S, FEC from 1/2 to 9/10, which covers 99,99% of all satellite signal reception requirements.
It is easy to calculate the useful bit-rate of the transponder once all three parameters are known.

For example, the transponder 11727 L of the Eutelsat W4 satellite has a bitrate of 27500 * 3/4 * 3 = 61,875 megabits / s

Closed channels

If we receive a stream of digital data, we can choose specific TV channels from it. The only last detail remains. TV channels can be encrypted by the content provider to protect them from unauthorized access. To do this, systems of concealment (conditional access system - CAS) are used, which perform block data encryption. To encrypt the data, usually, the CSA algorithm is used to this day (it is not very complicated for decoding and hacking), but keys in different systems are transmitted in different ways. The BISS system means encoding with a "persistent" session key. It is used to transfer data to a specific circle of people to whom this key is communicated. If the key is known, then NetUP Streamer can decode the signal without additional devices. In other cases, special modules are required - CAM (Conditional Access Module), which are selected depending on the encryption system used.

These modules are installed in the CI-slots (common interface) of the headend. The module can be equipped with a smart card that carries the algorithm for processing keys, rights to a particular content and subscriber ID. In rare cases, this algorithm is embedded directly to the module. Sending encrypted channels to the CAM-module, the station expects their return in decrypted form for further broadcasting. The performance of CAM-modules is limited. Household modules usually open only 1 channel, professional ones can process 4, 8, 10, 12, 16 channels simultaneously. We did't meet more powerful ones. In practice, this means that even if there are 20 channels on one transponder, 2 modules will be needed to decode them and 2 slots will have to be used on the headend.
Also a few years ago, the market appeared modules of CI+ standard (conventional is CI), many satellite operators started to use them for individuals. These modules can not be used in professional headends, since they imply reverse encryption. That is, the TV channel from the module comes out encrypted and playback is possible only on devices that support this standard (TVs, subscriber receivers). A little offensive, but not terrible, because such modules are single-channel and their use in professional headends is economically inexpedient.