Contents
1) What is MPEG ?

2) Modulation Schemes ?

3) What is DVB ?

4) Simple Block Diagram of digital receiver.

5) What equipment do I need ? & What's "Conditional Access" about ?



...............................................................
1) MPEG stands for Moving (or Motion) Picture Experts Group & is an organisation
of interested parties.It's run in similar manner to JPEG (Joint Picture Experts
Group) -JPEG being for still images;there is also a standard known as M-JPEG
(Moving JPEG) but this is intended more for the needs of the security industry.

How about the fact that discservers, and corresponding high-end editing suites
are only just moving from Motion Jpeg to MPEG (4:2:2). As an intra coding only
system it has been and still is widely used in these types of video
applications.

MPEG describes a form of compression for digital data where the data represents
moving images of a TV like-nature.The standard also allows for audio datastreams
sync'd with the video.MPEG1 is common on IBM PC's (& other platforms) using
*.mpg files. Xing, Mediamatics & other companies supply software players for
these and all but the cheapest PC VGA cards seem to have some hardware support
for MPEG1 files;normally you need a Pentium PC to have much chance of playing
mpeg1 files at reasonable speeds (25 frames per second or more).Anyway,mpeg1
isn't used for satellite TV;the industry needed a faster,more flexible &
efficient method. For broadcast use,less tendency to pixellation or "blockiness"
was desired with fewer "artefacts" -technical/marketing term for unwanted
material on the screen (that's a bit like calling a software bug an "anomaly"
!!).Now what the satellite industry wanted was to squeeze more channels into the
bandwidth taken up by a satellite transponder. Analogue satellite TV uses around
27-36 MHz of bandwidth for its FM video + audio FM subcarriers;this is for each
channel. So the operators want to put 5,10 or more separate channels, via a
digital datastream into a similar bandwidth.This allows many more channels or
needs fewer satellites to transmit a given number of channels. To give
flexibility,the actual compression ratio can be varied between "Studio Quality"
& "Video recorder quality". Studio needs 12 MBits/second data,broadcast needs 8
MBits, VHS needs 2 MBits/s.I don't want this faq to become too technical but
read DVBFAQ.TXT from Markus Kuhn for technical info. Just remember that the
compression ratio can be varied to cope with the needs of the supplier of the
video information. The digital data from several channels can be multiplexed
into an MPEG Transport Stream,along with various (compressed) audio channels
(which can include digital surround sound & multiple languages). Incidentally
MPEG1.5 is a hybrid (falling between 1 & 2) but any mpeg2 receiver SHOULD be
backwards compatible with older MPEG revisions. Actually MPEG1.5 isn't a real
(ratified) standard but covers several proprietary systems which tend to improve
on MPEG1 . Examples include "System2000"from NTL & "Orbit" from Scientific
Atlanta.

1 interesting point is that you can't compress "noise" with current MPEG
schemes.Imagine watching a film via digital (mpeg) satellite TV where,within the
plot of the film,the camera zooms in to show a TV screen switched on but with no
antenna connected. You expect to see nothing but noise ("snow)" on the
picture.This signal is entirely random & so can't be compressed -there's no
repeating pattern/redundancy in the signal. Apparently a future MPEG version
(mpeg3 or 4 ???) will have some kind of algorithm built in to get around this
problem. It looks like the next "official" (ratified) standard will be MPEG4.

MPeg2 is also used for DVD (digital video disk or digital versatile disc) &
other digital video delivery systems including cable,fibre & digital terrestrial
TV.

2) Modulation
----------
The real world is analogue so we have to find a way of transmitting our mpeg2
transport stream as information on a (non-digital) carrier wave.A traditional
analogue satellite transmission varies a carrier FREQUENCY in sympathy with the
video signal -"frequency modulation" or FM. Similar to this ,one can vary the
PHASE instead of the frequency - "Phase Modulation".Now we could code our
digital signal (which consists of simply binary or "ones & zeros") directly as
phase modulation in which case 0 degrees (our carrier reference frequency) could
represent a binary 0,whilst 180 degrees phase shift=binary 1;there would be a
practical difficulty in keeping track & always changing 180 degrees as there
will be natural phase variations over the transmission path. To solve this we
can instead make the phase changes cumulative -i.e. make the phase changes refer
to the previously signalled state rather than 0/180 degrees absolute. This is
known as DPSK -differential phase shift keying. Now those of you who know of
schemes used for digital transmission, in modems for example,will know that DPSK
is somewhat inefficient. There are various schemes that allow the data rate to
be doubled, quadrupled (or more) whilst maintaining the original signalling
rate.Thus Quaternary (or Quadrature) phase shift keying uses a 2-bit symbol
(instead of previously described 1-bit) based on 4 possible phases. At the same
time,0 degrees is avoided to prevent long periods of unmodulated carrier which
could cause problems in part of the circuitry -too complex to discuss here. So
we use typically 45,135,225 & 315 degrees.We now have a greater data rate in
bits per second than our actual baud (signalling) rate. This can ,in fact be
further extended by using a constellation of 8 or 16 phases & beyond -although
tolerance to noise (required signal to noise ratio for a suitably low bit error
rate) & to interference increases as the data rate rises. [This is one reason
why computer modems have trouble at high data rates on a poor line & your 56K
modem ends up communicating at 31,200 or less]. Other digital transmission media
can still use mpeg2 but change from qpsk to a different modulation
scheme.Normally this is QAM for cable (although qpsk can be used for the return
path back to the operator) whilst digital terrestrial TV will use CODFM -coded
orthoganal digital frequency modulation -with either 2048 or 8192 individual
carriers each seperately modulated. The choice of type of modulation is made
based on the sort of problems most prevalent for the medium,e.g.terrestrial is
more subject to multipath interference ("ghosting" in analogue TV) & CODFM is
fairly resistant to this. For a good,more technical description of qpsk etc. try
the following URL :- http://www.coolstf.com/mpeg

3) DVB
---
Like MPEG groups there is a DVB group -Digital Video Broadcast, made up of
interested parties , sharing information & setting the standards.It's somewhat
like the VESA group for PC graphics. DVB was set up by the EBU (European
Broadcast Union) to set the standards for digital video transmission.They have
published these via ETSI (European Telecommunications Standards Institute) who
also set standards for devices such as GSM telephones. In fact there are several
DVB standards for different transmission media.Some of these are :
DVB-S Satellite

DVB-C Cable

DVB-T Terrestrial

DVB-SI Specification for Service Information

DVB-CI Common Interface for conditional access


They've settled on using a subset of MPEG2 for their compression of the video &
audio. I've pasted in below a definition of the requirements to be met to claim
that your IRD (Integrated Receiver Decoder i.e. satellite box) is DVB compatible
:-

--------------------------------------------------------------------
To be DVB compliant a Satellite or a Cable receiver must, according to DVB
Document A001-revision 1, at least fulfill the following key features:
* Systems

o MPEG-2 Transport Stream is used

o Service information is based on MPEG-2 Program Specific Information

o Scrambling is as defined by CA Technical Group

o Conditional Access uses the MPEG-2 CA_descriptor

* Video

o MPEG-2 Main Profile at Main Level is used (1.5-15 Mbits/s)

* The frame rate is 25 Hz

o Encoded pictures may have either 4:3, 16:9 or 2.21:1 aspect ratio (4:3 is the
normal TV format, 16:9 is the widescreen format and 2.21:1 is the cinemascope
format that is use in the movie theaters) o IRDs will support 4:3 and 16:9 and
optionally 2.21:1 aspect ratios

o IRDs must support the use of pan and scan vectors to allow a 4:3 monitor to
give a full-screen display of a 16:9 coded picture o IRDs must support a full
screen display of 720 x 576 pixels (and a nominal full-screen display of 704 x
576) o IRDs must provide appropriate up conversion to produce a full-screen
display of 544 x 576 and 480 x 576 and a nominal full-screen display of 352 x
576 and 352 x 288 pixels.

* Audio

o MPEG-2 Layer I and Layer II must be supported by the IRD

o The use of Layer II is recommended for the encoded bitstream

o IRDs must support single channel, dual channel, joint stereo and the
extraction of at least a stereo pair from MPEG-2 compatible multichannel audio

o IRDs must support sampling rates of 32 kHz, 44.1 kHz and 48 kHz

o The encoded bitstream will not use emphasis


-------------------------------------------------------------------
Note that American DSS,DirecTV etc. systems are NOT DVB-compliant & won't work
in Europe.I do know of an attempt by someone in the USA to modify a European
Nokia digital receiver to decode DigiiCypher2 transmissions -but,at the time of
writing,this has not been successful.

4) Block Diagram (simplified)
-------------
See separate picture-file
Some observations on the afforementioned diagram :
DAC = Digital-to-analogue converter
ADC = Analogue-to-digital converter
The video encoder typically contains 4 or more DAC's which have to run at video
rates & quality. This infers 8 bit video DAC's (not cheap) .3 are needed for
RGB;another is needed for composite video out (PAL or SECAM).Some use 10 bit
DAC's & the difference *may* be visible by viewing sharp transitions like black
to white -hint : have a look at the On Screen Display if you want to try to spot
this effect. I have not included the conditional access module for simplicity.
Complex IC's are needed for many of these blocks.
A qpsk demodulator/ADC/Viterbi decoder can easily cost around $15-$20 in
manufacturers volumes! The mpeg transport demultiplexer & decoders cost even
more!I haven't included the CPU & memory (usually around 1-3Meg. is needed &
some of this may be fast,expensive SRAM). Perhaps you can now see why the
digital receivers cost a lot more than the analogue ones!!! It's worth noting
that on Astra,a Network Information Table (NIT) is transmitted every 10 seconds
on every DVB/mpeg transponder. The information sent includes the
FEC,S/R,frequencies etc.

5) Equipment Needed
----------------
First of all ,a universal lnb is recommended as digital receivers for Europe are
optimised for use with these.A universal lnb will have low phase noise (required
so as not to confuse the qpsk modulator) & 2 local oscillators,1 at 9.75 GHz & 1
at 10.6 GHz. The default is to enable the 9.75 GHz osc. whilst a 22KHz tone
generated by the receiver enables the 10.6 GHz oscillator. Any receiver made for
European digital reception may work up to a point **BUT** (big problem) many
receivers are sold for use on a particular operator's "bouquet" (multiplex) of
channels & often have internal software that prevents you receiving anything
else!!!! All Pace receivers up to late 1997 seem to suffer from this
and,according to a recent French magazine report,so do the Sagem boxes sold for
the French TPS (Television Par satellit) bouquet. You also need the relevant
Conditional Access Module (CAM) for any subscription channel along with
appropriate smartcard (which could include a pirate card -these started to
appear in summer 1997 although many were knocked out via ECM's from the
operators). There are several different conditional access schemes in use by the
different operators & each system needs the relevant CAM (as well as the
smartcard for subscription channels). IRDETO is the commonest in Europe.

***See diagram of a generic CA (conditional access) system as used in an IRD***
CARCVR2.bmp

Just to explain a some of the acronyms used in the diagram :-
ACS = Access Control System
ECM = Entitlement Control Message
EMM = Entitlement Management Message
CW = Control Word
MMI = Man-Machine Interface (smartcard reader in this case).
DVB-CI was a "cop-out" in that they could have specified that all DVB receivers
used the same form of conditional access. Instead they paralled the situation
that occurs in analogue satellite transmissions i.e. many different systems
co-exist. This kept the status quo with the operators (& Hollywood) but runs
totally against EU open market policy.Only the CAM interface is covered by the
DVB spec. -so they can use their own proprietary encryption systems. In Spain
the 3 digital satellite operators were each going to use a different encryption
method. However,the Spanish authorities stepped in & forced them all to agree to
use compatible systems. This promotes healthy competition ,allowing the Spanish
consumer to buy one digital receiver & choose to take 1 ,2 or 3 subscriptions
(involving 1,2 or 3 smartcards) but they will all work on the one receiver &
CAM.
If you want to complain,I suggest those in Europe write to their Euro MP
(officially known as "MEP") !

IRDETO - used by Kirch/DF1,Nethold/Multichoice,Telepiu, M-Net (C-band)
SECA - French.Used by Canal Plus/CSN (Canal Satellite Numerique) Also known as
"MediaGuard".Additionally used by the German Premiere bouquet.I believe SECA
(Societe Europeene de Controle d'Acces) is a partnership between Canal Plus &
Bertelsmann.
VIACCESS -Used by TPS (Television Par Satellit),French Bouquet which can be
found on Hotbird 2 (13 degrees east). Also AB-Sat on the same satellite.I
believe rights are owned by France Telecomm as it was developed in their CCETT
research centre.
CRYPTOWORKS -Used by RTL (Austria/Switzerland).Cryptoworks is a trademark of
Philips.
CONAX -Used by Canal Plus for Scandinavian packages.Conax is a Telenor company.
VIDEOGUARD (NDC) - To be used by Sky Digital in UK.Proprietary encryption system
with no CAM used -the CAM function is built into the motherboard and is
non-removeable.
The CAM modules use PCMCIA connections -a technology borrowed from laptop
computers. This should allow you to unplug one module & insert another to switch
from ,for example, IRDETO to Viaccess.However this isn't a simple 5-minute task
& the internal software isn't guaranteed to support the change!!Add to that the
difficulty in easily obtaining CAM modules other than the one supplied within
the IRD ,so this isn't going to be an easy option for many people. Of course,it
gets easier with receivers that have 2 CI CAM slots. There is a group known as
OKAPI trying to cut through the Conditional Access mess! OKAPI = Open Kernel for
Access to Protected Interoperable interactive services). They consider :
a)Simulcrypt -proprietary systems & common scrambling algorithm =
interoperabilty.
b) Multicrypt -proprietary systems,common scrambling algorithm & DVB CI =
openness & equitabilty
c) Equicrypt (from OKAPI) -TTP (Trusted Third Party),Public Key
Cryptography,common smartcard DVB CI = openness,equitability AND
interoperabilty.

Multicrypt and Simulcrypt
-------------------------
Multicrypt transmissions allow two different encryption systems to co-exist in
the same receiver.The MPEG transport stream is sent sequentially through
different modules that are inserted into the CI & each CAM will receive its
entitlement messages.
Simulcrypt,on the other hand,allows different decoders with different CAMs (not
necessarily CI compatible) to decrypt valid entitled channels (i.e. channels for
which a valid smartcard is present in the CAM).
This adds complexity to the service provider's equipment but allows the use of
existing receivers without modification.Any individual IRD selects the
entitlement information it requires whilst ignoring entitlement messages that
are destined for other IRDs that use a different decryption system.Therefore,a
CI compatible IRD is NOT required.However,global security is necessarily weaker.




Chris Muriel (chris.muriel@analog.com),G3ZDM, June 6th , 1998