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Dvb s2-vcm

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Dvb s2-vcm

  1. 1. FEATURE VCM Tuner VCM Signal Reception • allows more than one stream for each transponder • dynamic change of FEC coding rates • different content can bes assigend to different transmission modes • compatible tuner needs function to determine coding rates • - VCM signals only receivable with compatible tuners224 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 04-05/2012 — www.TELE-satellite.com www.TELE-satellite.com — 04-05/2012 — TELE-satellite International — The World‘s Largest Digital TV Trade Magazine 225
  2. 2. FEATURE VCM TunerHow a tuner for VCM operates- an indepth look into today‘s mostadvanced transmission technologyJacek Pawlowski The DVB-S2 standard, published in er, became feasible only recently after transport streams and in ACM we can2003, is a significant step forward from advances in IC technology had been even dynamically change the codingDVB-S. Its most obvious advantage is a made. LDPC uses the following inner rates depending on the feedback infor-better spectral efficiency as compared code rates: 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, mation from the reception system.to DVB-S. Within the same transmis- 3/4, 4/5, 5/6, 8/9, 9/10. While ACM is very useful in point tosion channel you can send up to 30% These numbers are quite familiar for point transmissions where there is onlymore data. Its performance is very satellite fans. The meaning of them is one recipient of the information, VCMclose to the Shannon limit – only 1 dB simple – this is the ratio of the useful can be quite useful for satellite TVbelow, so in fact one should not expect bits to the sum of useful bits and ad- transmission for a large audience. Theany further improvement in data rate ditional bits added for error correction. content provider may choose to trans-achievable within a given transmission As you can deduce, the most effective mit the very basic package of the SDchannel. However, there is another as- inner coding rate is 9/10 but at the TV channels with a strong error cor-pect of digital data transmissions – for- same time is most vulnerable to inter- rection – thus making it receivable alsoward error correction or FEC. ference. Inner coding rate 1/4 is very at the edges of the satellite footprint, Although the actual implementa- robust as for one useful bit there are while the premium HD channels maytion is very complex, the essence of three additional bits for error correc- be transmitted with weaker correction,FEC is simple. Having digital informa- tion but, of course, the efficiency of the so that their reception is guaranteedtion (e.g. video, audio, Internet data) transmission is very poor. only in the central part of the footprintthe encoder divides it into packets or In the old days of DVB-S we had to where most of the subscribers live. Toframes and adds extra bits calculated use one fixed inner coding rate for one achieve stronger or weaker error cor-in a special way from the useful digital transponder. One transponder trans- rection, the provider chooses bothcontent. Those extra bits are transmit- mitted only one transport stream. modulation type (e.g.: QPSK or 8PSK)ted together with the “useful” bits. If Every TV channel or the other service and inner coding rate.some of the bits, either the useful ones included in the transport stream was As already described in TELE-satel-or the additional ones, are distorted (1 transmitted with the same spectral effi- lite (see issue 12-01/2012), there arebecomes 0 or vice versa), the recep- ciency and interference robustness be- already transponders transmitting intion system which “knows” what the cause the same inner coding was used VCM and ACM format. However, thererelationship between the normal and for the whole transponder. are still quite few receivers or receiveradditional bits should be, can do so- DVB-S2 offers more flexibility in this cards available today that can receivephisticated calculations and restore the aspect. Except for CCM (Constant Cod- VCM signal. How does the tuner of suchoriginal form of useful bits. ing and Modulation), we have two more a receiver differ from its standard CCM DVB-S2 introduced new algorithms options: VCM (Variable Coding and brother? Before we discuss the receiverfor FEC: BCH and LDPC. BCH (Bose- Modulation) and ACM (Adaptive Coding front end suitable for VCM transmis-Chaudhuri-Hocquenghem) replaced and Modulation). VCM and ACM are the sions, let us focus first on the transmit-Reed Solomon coding used in DVB-S. methods to send more than one trans- ter part – DVB-S2 modulator. Figure 1LDPC (Low Density Parity Check) re- port stream within the same carrier explains the principle of operation of aplaced Viterbi inner coding. LDPC, a (same transponder). In VCM, we can DVB-S2 modulator with MIS (Multi In-method invented in 1960’s by Gallag- set different coding rates for different put Streams) feature. ■ Figure 1. DVB-S2 Modulator with MIS feature.226 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 04-05/2012 — www.TELE-satellite.com
  3. 3. ■ Figure 2. In the process of coding, BB frames ■ Fig. 3. Block diagram of VCM compatible are appended with error correction bits. DVB-S2 receiver front end. Three exemplary transport streams dummy BB frames if needed. Multi- A block diagram of the reception sys- bits. The next block takes care of er-(yellow, green and blue) at the input of plexed BB frames are then interleaved tem is shown in Figure 3. This is in fact ror correction and does descramblingthe modulator consist of rather short and modulated. a partial and simplified block diagram if necessary. Unscrambled and errortransport packets. The TS packets are As mentioned earlier, each transport of a VCM compatible DVB-S2 receiver. corrected BB frames are fed into theconverted to longer BB (Base Band) stream (converted to BB frames) can Signal from an LNB is converted in the TS processor which turns them intoframes. They are either 16 or 64 kilo- have its own modulation: QPSK, 8PSK, L-Band tuner to a pair of quadrature familiar transport stream packets. The able to assign more than one transport streams you use. And because suchbytes long. In fact, there are more op- 16APSK or 32APSK and inner coding signals: I and Q, which are then con- transport stream is then processed as stream to one transponder. From the CCM transponders would have to beerations done in this module (like mode ratio. verted to the digital domain in the dual in any DVB receiver – a desired TV’s or end user’s perspective, a VCM DVB-S2 separated in frequency spectrum, theadaptation, stream adaptation) but as Except for the BB header, DVB-S2 analog-to-digital converter. Such archi- radio channel’s TS packets are filtered transponder is like a few classical DVB- total bandwidth occupied by themthey are not so relevant for understand- also uses PL (Physical Layer) headers. tecture is typical for most of the con- out and converted to a HDMI signal or S2 transponders overlapped one over would be greater than the bandwidthing the principle of operation, there is The PL header is always transmitted temporary RF devices – not only satel- to analog video and audio in digital-to- another on the same frequency. occupied by one VCM transponder.no need to describe them here. Then, using a robust modulation and strong lite TV receivers. analog converters. If you would like to transmit the VCM That’s the basic advantage of VCM.BB frames are time-multiplexed on the FEC rate. This helps the receiver lock The DVB-S2 demodulator, after lock- As you can see, the changes in hard- signal as a couple of ordinary CCM sig- The sky seems to be the limit for thesame carrier. In the process of coding, on the signal, as well as it provides the ing and processing the PL header, ware architecture are not so fundamen- nals, you would need as many classi- excellence in satellite bandwidth utili-BB frames are appended with error cor- information necessary to appropriately learns what is inside the signal: how tal but except for introducing them, cal transponders as many transport zation.rection bits (see Figure 2). Depending demodulate the remainder of the PL many different transport streams, what also software must be re-written. Foron the user settings, LDPC coding rates frame. The first thing the receiver does modulation they use and what their example, today’s receivers assume that 1. TELE-satellite first introduced MIS transmission technology (based on VCM) in TELE-satellite 10-11/2011 in a testreport about the TBS6925 PC card manufacturedcan be different for different streams. when tuned to a given frequency with FEC settings are. At the output of the one transponder carries just one trans- by Tenow (http://www.tele-satellite.com/TELE-satellite-1111/eng/tenow.pdf)...When no data is present, the modulator a given symbol rate is looking for a PL DVB-S2 demodulator we get BB frames port stream with a given modulation 2. ... and then again in a more detailed reception report in TELE-satellite 12-01/2012pads incomplete BB frames or inserts header. along with additional error correction and FEC ratio. New receivers must be (http://www.tele-satellite.com/TELE-satellite-1201/eng/tenow-TBS6925.pdf)1 2228 TELE-satellite International — The World‘s Largest Digital TV Trade Magazine — 04-05/2012 — www.TELE-satellite.com www.TELE-satellite.com — 04-05/2012 — TELE-satellite International — The World‘s Largest Digital TV Trade Magazine 229

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