DVB-T2W
The DVB terrestrial television transmission solution for
Australia, Canada-Mexico and New Zealand
Abstract
DVB-T2 is in
process of being developed as a terrestrial transmission system for the
UK. However, as a DVB standard T2 has flaws and is limited in
bandwidth. DVB-T2's maximal transmission bandwidth is 10 MHz, a mode
that could not be used in Canada or anywhere where there are 6 MHz
television channels.
With an effective bandwidth increase
to 12 MHz, 16 MHz, 18 MHz or 24 MHz a better television transmission
system can be created. The bandwidth increases must naturally be in
multiples of either 6 MHz or 8 MHz. The 6 MHz or 8 MHz multiples are
needed to be backwardly compatible with NTSC and PAL & SECAM
bandwidth allocations.
- This
proposal does ignore the 7 MHz allocation used for PAL (in the VHF
band) in many parts of the world.
- DVB-T is
an adequate replacement transmission system for 7 MHz VHF allocations
formerly used by PAL, as it has a 7 MHz mode.
- Many
nations are abandoning 7 MHz VHF TV transmission, so RF output for 7
MHz TV globally will not be increasing.
The proposed wideband "DVB-T2W" system could be
implemented in Australia,
Canada-Mexico and New Zealand where it makes the most economic sense.
DVB-T2W is not the same thing as DVB-T2 -- it
is a more advanced
transmission format.
Primary design
goals (not in any specific order)
- Increase the number of channels in a Channel Group so one
transmitter can serve an entire region on behalf of multiple
broadcasters.
- Reduce RF channel planning complexity for regulators and
broadcasters.
- Reduce RF transmitter output power by a factor of (1/3 or 1/4) vs
DVB-T or DVB-T2.
- Reduce electromagnetic compatibility issues by avoiding some of
DVB-T2's extremely high datarate transmission modes.
- Increase Doppler Immunity vs DVB-T2, but make it no worse than
DVB-T high rate modes.
- Allow for open source CODECs to be used.
- Current DVB transmission systems do not support wavelet encoding
in the Video CODECs. There must be support for wavelet encoding of
reference images in the Video transmission subsystem so as to make the
signal more robust.
- Currently DVB and ATSC are shakeled with some CODEC components
that have intellectual property issues. This must end.
- Turbocodes, and Punctured Turbocodes must be permitted for use in
the Error Correction subsystem, as propagation conditions may evolve
that will not work with the current DVB-T2 error correction subsystem.
- Allow mobile devices to still have access to TV service via older
DVB (or ATSC) terrestrial transmitters.
- Allow for Digital Audio Broadcasting, at least nominally as DVB-T
currently does.
All of these
design goals can be met with increased bandwidth, and limited
modifications to the DVB-T2 transmission system. The only possible
major change that new video codecs will be required and the error
correction system must have an alternate to the current Low Density
Parity Check Codes + BCH currently implemented.
History matters : the role of
NTSC, PAL-7 and PAL-8
PAL broadcast systems
This table
illustrates the differences between the various PAL broadcast systems.
This proposal does not seek to replace PAL-Nc, PAL-N or PAL-M. These
rarer forms of PAL have been ommited from the table.
Signal
Paramiter
|
PAL
B |
PAL
G, H |
PAL
I |
PAL
D |
| Transmission
Band |
VHF |
UHF |
UHF/VHF |
VHF |
| Lines
/ Fields |
625/50 |
625/50 |
625/50 |
625/50 |
| Video
Bandwidth |
5.0 MHz |
5.0 MHz |
5.5 MHz |
6.0 MHz |
| Sound
Carrier |
5.5 MHz |
5.5 MHz |
6.0 MHz |
6.5 MHz |
| Channel
Bandwidth |
7.0
MHz |
8.0
MHz |
8.0
MHz |
8.0
MHz |
| Active
lines |
576i |
576i |
576i |
576i |
Implimention note
- Australia uses PAL-B on VHF and UHF
- NZ uses PAL-G on UHF, but PAL-B on VHF
- Canada uses NTSC
In the end these
systems were decommissioned and replaced with DVB-T (Australia, NZ) and
ATSC (Canada). However, this HDTV transmission system adoption is not
without its flaws and its problems.
DVB-T and DVB-T2 History
Towards
the
end
of
1991,
broadcasters,
equipment manufacturers and regulatory
bodies in Europe came together to discuss the formation of a group that
would oversee the introduction of digital TV. That group, which became
known as the European Launching Group (ELG), realized that a
consensus-based framework, through which all of the key stakeholders
could agree on the appropriate technologies to be used, would benefit
everybody involved.
A Memorandum of Understanding (MoU) was drawn up, setting out the basis
on which competitors in the marketplace would come together in a spirit
of trust and mutual respect. The MoU was signed in September 1993 by
all ELG participants, and the DVB Project was born. A key report from
the Working Group on Digital Television was also central to setting out
important concepts that would go on to shape the introduction of
digital TV in Europe and far beyond.
By any measure the DVB Project has been a success. More than 500
million devices around the world are receiving services that use DVB
standards, including at least 100 million satellite receivers and at
least 150 million DVB-T receivers. DVB-C is the most commonly used
system for digital cable TV. DVB-T has seen phenomenal growth in the
last few years with services on air across Europe and in parts of Asia,
Africa and Latin America and many more countries that are planning
deployment. The economies of scale engendered by such success mean that
the prices consumers have to pay for receivers are falling all the time.
In
March 2006 DVB decided to study options for an upgraded DVB-T standard.
In June 2006, a formal study group named TM-T2 (Technical Module on
Next Generation DVB-T) was established by the DVB Group to develop an
advanced modulation scheme that could be adopted by a second generation
digital terrestrial television standard, to be named DVB-T2.
According
to
the
"Commercial
Requirements"
and
"Call
For Technologies" issued in
April 2007, the first phase of DVB-T2 would be devoted to provide
optimum reception for stationary (fixed) and portable receivers (units
which can be nomadic, but not fully mobile) using existing
aerials, whereas a second and third phase would study methods to
deliver higher payloads (with new aerials) and the mobile reception
issue. The novel system should provide a minimum 30% increase in
payload, under similar channel conditions already used for DVB-T.
The
BBC,
ITV,
Channel
4
and
Five
agreed with the regulator OFCOM to convert
one UK multiplex (B, or PSB3) to DVB-T2 to increase capacity for HDTV
via DTT. They expected the first TV region to use the new standard
would be Granada in November 2009 (with existing switched over regions
being changed at the same time). It was expected that over time there
would be enough DVB-T2 receivers sold to switch all DTT transmissions
to DVB-T2, and H.264.
- OFCOM
published its final decision on April 3, 2008 for HDTV using DVB-T2 and
H.264 : BBC HD would have one HD slot after DSO at Granada. ITV and C4
had, as expected, applied to OFCOM for the 2 additional HD slots
available from 2009 to 2012.
- OFCOM indicated that it found an unused channel covering 3.7
million
households in London, which could be used to broadcast the DVB-T2 HD
multiplex from 2010, i.e., before DSO in London. OFCOM indicated that
they would look for more unused UHF channels in other parts of the UK,
that can be used for the DVB-T2 HD multiplex from 2010 until DSO.
The
current DVB-T2 specification
The
DVB-T2
draft
standard
was
ratified
by
the DVB Steering Board on June 26, 2008, and
published on the DVB homepage as DVB-T2 standard BlueBook. The T3
specification was
handed over to the European Telecommunications Standards Institute
(ETSI) by DVB.ORG on June 20, 2008. The ETSI process resulted in the
DVB-T2 standard being adopted on September 9, 2009. The ETSI process
had several phases, but the only changes were text clarifications.
Since the DVB-T2 physical layer specification was complete, and there
would be no further technical enhancements, receiver VLSI chip design
started with confidence in stability of specification. A draft PSI/SI
(program and system information) specification document was also agreed
with the DVB-TM-GBS group.
DVB-T
and
DVB-T2
Compared
Bold text indicates changes
added to
the DVB base system
System
|
DVB-T |
DVB-T2 |
| FEC |
Convolutional Coding + Reed
Solomon 1/2, 2/3, 3/4, 5/6, 7/8 |
LDPC + BCH 1/2, 3/5, 2/3,
3/4, 4/5, 5/6 |
| Modes |
QPSK, 16QAM, 64QAM |
QPSK, 16QAM, 64QAM, 256QAM |
| Guard Interval |
1/4, 1/8, 1/16, 1/32 |
1/4, 19/256, 1/8, 19/128,
1/16,
1/32,
1/128 |
| FFT size |
2k, 8k |
1k, 2k, 4k, 8k, 16k,
32k |
| Scattered Pilots |
8% of total |
1%, 2%, 4%,
8%
of
total |
| Continual Pilots |
2.6% of total |
0.35% of total |
Now that the DVB
system has been adequately presented, the main
proposal for its modifications can be made.
DVB-T2W
Specification
Proposed
DVB-T2W
System
System
characteristics
|
|
|
|
Notes
|
|
|
|
|
|
Original TV
Bandwidth
|
8 MHz (PAL
G/H/I)
|
7 MHz (PAL B)
|
6 MHz
(System
M) |
PAL, NTSC
|
|
|
|
|
|
Proposed T2W
Bandwith (1)
|
24 MHz
|
21 MHz
|
24 MHz
|
(3 x 8, 3 x
7, 4 x
6)
|
Proposed T2W
Bandwith (2)
|
16 MHz
|
14 MHz
|
18 MHz
|
(2 x 8, 2 x
7, 3 x
6)
|
Proposed T2W
Bandwith (3)
|
= NA =
|
= NA =
|
12 MHz
|
(NA,
NA, 2 x 6)
|
|
|
|
|
|
T2W Relative to T2
|
|
|
|
|
"Carrier
Density"
|
~= DVB-T2 |
~= DVB-T2 |
~= DVB-T2 |
No change vs
T2
|
"Number
of Carriers"
|
~= DVB-T2
|
~= DVB-T2 |
~= DVB-T2
|
No change vs
T2 |
|
|
|
|
|
Constellation
deletions
|
256 QAM
|
256 QAM |
256 QAM |
Cable TV Only
|
Constellation
additions
|
32 QAM
|
32 QAM |
32 QAM |
EM
compatibility
|
|
|
|
|
|
DVB-T2W FFT
sizes vs T2
|
No change
|
No change |
No change
|
EM
compatibility
|
Guard
intervals vs T2
|
No change
|
No change |
No change
|
EM
compatibility
|
|
|
|
|
|
Continual
pilots %
|
0.35, 0.70,
1.4
|
0.35, 0.70,
1.4
|
0.35, 0.70,
1.4
|
Propagation
|
FEC added to
T2(W)
|
Turbo Codes
|
Turbo Codes
|
Turbo Codes |
(+Punctured)
Fallback
|
|
|
|
|
|
CODECs added
|
Dirac +
Theadora
|
Dirac +
Theadora
|
Dirac +
Theadora
|
Open Source
CODECs
|
CODECs
deleted
|
MPEG (2
& 4)
|
MPEG (2
& 4)
|
MPEG (2
& 4)
|
JPEG &
AAC Core IP issues
|
|
|
|
|
|
Bandwidth Density
|
|
|
|
|
"goal" T2W
(wide)
|
5.55 mbs / 1
MHz
|
5.55 mbs / 1
MHz
|
5.55 mbs / 1
MHz
|
12, 16 MHz
|
"goal" T2W
(widest)
|
5.65 mbs / 1
MHz
|
5.65 mbs / 1
MHz |
5.65 mbs / 1
MHz
|
18, 24 MHz
|
"Maximal" T2
(reference)
|
6.29 mbs / 1
MHz
|
6.29 mbs / 1
MHz
|
6.29 mbs / 1
MHz
|
@ 8 MHz
nominal
|
|
|
|
|
|
Single
Frequency Networks
|
Possible,
permitted
|
Possible,
permitted |
Possible,
permitted
|
~ (DVB-T
&
T2) nominal
|
|
|
|
|
|
Virtual
Channel Codegroups
|
Forced
|
Forced |
Forced
|
RF channel
issues
|
|
|
|
|
|
Doppler
Immunity
|
~DVB-T,
>DVB-T2
|
~DVB-T,
>DVB-T2 |
~DVB-T,
>DVB-T2
|
Generally no
worse
than DVB-T!
|
Due to the
changes in error correction systems, density of scatter pilots, density
of continual pilots and other factors it is unknown if DVB-T2 will have
the same Doppler Immunity (aka non affectation of reviver motion) that
DVB has. DVB-T2W should have Doppler Immunity equivalent to DVB under
the highest rate transmission conditions. At the current time there is
at least some latent evidence that DVB-T2 may not be as robust as DVB-T
in motor cars or airplanes.
In moving to a
higher order QAM constellation (higher data rate and mode) in hostile
RF / microwave QAM application environments, such as in broadcasting or
telecommunications, interference (via multipath) typically increases.
Reduced noise immunity due to constellation separation makes it
difficult to achieve theoretical performance thresholds. There are
several test parameter measurements which can help determine an optimal
QAM mode for a specific operating environment.
- Carrier/interference ratio
- Carrier-to-noise ratio
- Threshold-to-noise ratio
Rectangular
QAM
Rectangular QAM
constellations are, in general, sub-optimal in the sense that they do
not maximally space the constellation points for a given energy.
However, they have the considerable advantage that they may be easily
transmitted as two pulse amplitude modulation (PAM) signals on
quadrature carriers, and can be easily demodulated. The non-square
constellations achieve marginally better bit-error rate (BER) but are
harder to modulate and demodulate.
The first
rectangular QAM constellation usually encountered is 16-QAM. In QAM
modulation systems, there is a Gray coded bit-assignment that has
evolved that is nearly standard to the format.
- The reason that 16-QAM is usually the first kind of QAM
modulation encountered by people is that 2-QAM and 4-QAM are really no
more than complex forms of binary phase-shift keying (BPSK) and
quadrature phase-shift keying (QPSK).
- The error-rate performance of 8-QAM is close to that of 16-QAM
(at best ~0.5 dB better), but 8-QAM's data rate is 75% (3/4ths) that of
16-QAM.
Non-rectangular
QAM -- a
transmission format that should be added to T2W
It most be noted that T2 permits the QAM constellation to be tilted up
to ~40ยบ. It is reasonable to preserve this tilting feature in T2W.
However, it is
the nature of QAM modulation that many different constellations can be
constructed. Circular QAM should be allowed at lower datarates as it
may provide better signal propagation versus tilted Rectangular QAM.
Circular QAM may be more electromagnetically benign versus Rectangular
QAM.
- The circular 8-QAM constellation is known to be the optimal 8-QAM
constellation in the sense of requiring the least mean power for a
given minimum Euclidean distance.
- The 16-QAM constellation is suboptimal although the optimal one
may be constructed along the same lines as the 8-QAM constellation.
- Circular QAM constellations are unique in that they are very
similar to PSK constellations, and PSK is fairly robust with reasonable
Doppler Immunity.
- It is consequently hard to establish expressions for the error
rates of non-rectangular QAM since it necessarily depends on the
constellation, so further research will be required.
- The bit-error rate with all QAM systems depends on the assignment
of bits to symbols, but constellation shaping can help in reducing
error rates.
Areas of
absolutely no change : The video resolution layers set up in ATSC that
are identical to DVB will not change. However, abandonment of
interlaced HDTV modes may come in the near future but for the sake of
the standard it is wise to permit these transmission formats.
Interoperability
Tuner
interoperability
- Yes, interoperability is a big deal. There are a lot of deployed
DVB-T tuners and in future DVB-T2 tuners that will have to scan for
channels in future -- and will come across T2W transmissions and have
problems.
- My view is that minor modifications of carrier spacings vs DVB-T
and DVB-T2 will make most tuners reject T2W signals as RFI. These
carrier distance modifications should not affect the final datarates,
but should confuse older receivers into thinking that they are seeing
RFI.
General
electromagnetic interoperability
- The goal of this system is to reduce transmitter power, the
number of transmitters used per populated region AND and receiver
complexity.
- However, DVB transmission waveforms wider tan 12 MHz are bound to
cause some problems no matter what. Care must be taken to limit
possible harm.
- Generally, except for low power "point to point" links in the
microwave band 256-QAM is to be discouraged.
- 256-QAM has its problems as a general purpose microwave band
telecommunications waveform. There is no existing evidence that
advocates using any more than 64-QAM for this wide a waveform is
feasible or safe.
- Large population centers will be exposed to this family of
waveforms. Also, small but dense population centers will be exposed to
these waveforms.
Technical
references
Transmission
systems
- DVB-T (the base
DVB system)
- DVB-T2 (the
upgraded DVB system, still narrowband)
- Quadrature_amplitude_modulation
(an important modulation scheme for high datarate DVB-T & T2)
- Turbo_codes
(Turbocodes are a class of high-performance forward error correction
(FEC) codes developed in 1993, which were the first practical codes to
closely approach the channel capacity. Turbocodes can reach a
theoretical maximum for the
channel noise at which reliable communication is still possible given a
code rate. For DVB-T2W, Punctured Turbo Codes should also be part of
the Turbo Codes option.)
CODECs (Next
Generation, with no intellectual property issues)
- Dirac
(codec) [latest
specification] -- a more flexible CODEC than MPEG (2 or 4)
with
wavelet compression for reference frames
- Theora -- a
more flexible CODEC than MPEG (2 or 4) is broadly comparable in design
and bitrate efficiency to MPEG-4 Part 2, early versions of Windows
Media Video, and RealVideo while lacking some of the features present
in some of these other codecs. It is comparable in open standards
philosophy to the BBC's Dirac CODEC.
Created by
Max Power
Created
14 August 2010
Last Modified
21 August 2010