The document discusses Variable Coding and Modulation (VCM) in DVB-S2 satellite transmissions. VCM allows a single transponder to transmit multiple transport streams, with each stream using a different modulation type and forward error correction rate. This makes it possible to provide stronger error correction for basic channels while using weaker correction for premium channels. A VCM-compatible tuner can determine the coding rates being used and properly demodulate the different streams. This provides more flexible use of satellite bandwidth than conventional systems with a single coding scheme per transponder.

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
tuners
224 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. FEATURE VCM Tuner
How a tuner for VCM operates
- an indepth look into today‘s most
advanced transmission technology
Jacek Pawlowski
The DVB-S2 standard, published in er, became feasible only recently after transport streams and in ACM we can
2003, is a significant step forward from advances in IC technology had been even dynamically change the coding
DVB-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 to
sion channel you can send up to 30% These numbers are quite familiar for point transmissions where there is only
more data. Its performance is very satellite fans. The meaning of them is one recipient of the information, VCM
close to the Shannon limit – only 1 dB simple – this is the ratio of the useful can be quite useful for satellite TV
below, so in fact one should not expect bits to the sum of useful bits and ad- transmission for a large audience. The
any 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 SD
channel. 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 also
ward 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 may
tion 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 guaranteed
tion (e.g. video, audio, Internet data) transmission is very poor. only in the central part of the footprint
the encoder divides it into packets or In the old days of DVB-S we had to where most of the subscribers live. To
frames 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 both
content. 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 are
becomes 0 or vice versa), the recep- ciency and interference robustness be- already transponders transmitting in
tion system which “knows” what the cause the same inner coding was used VCM and ACM format. However, there
relationship between the normal and for the whole transponder. are still quite few receivers or receiver
additional bits should be, can do so- DVB-S2 offers more flexibility in this cards available today that can receive
phisticated calculations and restore the aspect. Except for CCM (Constant Cod- VCM signal. How does the tuner of such
original 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 receiver
for 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 1
LDPC (Low Density Parity Check) re- port stream within the same carrier explains the principle of operation of a
placed 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. ■ 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 descrambling
the modulator consist of rather short and modulated. a partial and simplified block diagram if necessary. Unscrambled and error
transport packets. The TS packets are As mentioned earlier, each transport of a VCM compatible DVB-S2 receiver. corrected BB frames are fed into the
converted to longer BB (Base Band) stream (converted to BB frames) can Signal from an LNB is converted in the TS processor which turns them into
frames. 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 such
bytes 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 be
erations 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, the
adaptation, 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 them
they 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 bandwidth
ing 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 the
same 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. For
on 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 manufactured
can 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 12-01/2012
pads 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 2
228 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