DIGITAL DVB-S AMATEUR TELEVISION
Introduction
DVB-S is the original
Digital Video Broadcasting
forward error coding
and modulation standard for
satellite television
created in 1994 in its first release with development from 1993, to 1997. The first
application was commercially available in France via Canal+, enabling digitally
broadcast, satellite-delivered television to the public.
It is used via satellites serving every continent of the world. DVB-S is used in
both
MCPC
and
SCPC modes
for
broadcast network feeds,
as well as for
direct broadcast satellite
services like
Sky Digital
(UK) via
Astra
in Europe,
Dish Network
and
GlobecastGlobecast
in the
U.S.
and
Bell TV
in
Canada.
While the actual DVB-S standard only specifies physical link characteristics and
framing, the overlaid
transport stream
delivered by DVB-S is mandated as
MPEG-2,
known as
MPEG-TS.
More details are available at the
www.Wikipedia.org
web sites by clicking on the underlined topics above.
Amateur digital television started somewhere around 2000 mainly in Europe with on
air signals not appearing until around 2002 when some digital board sets became
available. Since then amateur digital TV repeaters in Europe have been increasing
in popularity but sadly the interest seems to be lacking in the USA. In January
of 2004 the ATCO Group (Amateur Television in Central Ohio) in Columbus, Ohio installed
a DVB-S digital output to their repeater which has been in service 24-7 since then.
As of July 2009, the ATCO Group is still the only one in the USA with a digital
ATV (Amateur Television) repeater output.
The ATCO repeater digital output uses DVB-S modulation which we believe is the best
choice for amateur television. The following discussion details more fully why we
feel it is best along with operational experiences to back it up. I know of no other
group, USA or Europe, that justifies it with “in service” data. Therefore, we are
able to back up our statements with results and not just theoretical details.
DATV advantages over analog.
- Picture quality is near perfect.
Strong and weak signals are all “P5” which is a snow free signal. Historically,
analog amateur television signal strengths are indicated by the “P” unit system
where P0 is a barely detectable signal and P5 is snow free. The strengths increase
in 6dB steps from P0 to P5 so P5 is 6 x 5 = 30dB stronger than P0. That’s for ANALOG.
A digital signal that produces a blank receiver screen with a “P0” signal will produce
a “P5” or snow free picture if it’s only 1-2dB stronger. Therefore, in the analog
world, if the signal strength increased 1-2 dB greater than P0, the viewer would
see a barely discernable picture; in the digital world he’d see a snow free picture.
- Noise and Multipath cancellation possible.
The DVB-S QPSK modulation scheme uses forward error correction to cancel the effects
of atmospheric/man made noise and multipath (ghosting). The noise is handled by
Viterbi and multipath is handled by Reed-Solomon software algorithms which are highly
complex effective ways of handling the data streams but beyond the scope of this
discussion. Since the DVB-S modulation scheme is intended mainly for satellite to
ground communication, multipath is minimal so correction requirements are also minimal
and simple but adequate for ATV applications.
As mentioned above, the Viterbi coding algorithm reduces noise due to atmospheric
and man made influences but is minimal. Here also, Hams are willing to tolerate
some noise disturbances in the picture. However, it doesn’t show up as the typical
noise flashes in the picture as seen on an analog screen. Instead it will appear
as either a momentarily frozen picture or as momentary checkered squares scattered
through the picture. So, as you can imagine, it would be intolerable for a commercial
broadcast signal but quite acceptable for Hams!
Can occupy less bandwidth.
A commercial 8VSB digital broadcast signal occupies a fixed 6MHz bandwidth and is
not subject to modification. The DVB-S signal bandwidth however, can be tailored
to meet the users’ requirements. Therefore it can be made wider or significantly
narrower than 6MHz with corresponding tradeoffs. If a narrower bandwidth is needed,
video quality will suffer and fast motion may pixelate. By “pixelate” we mean that
checkered squares will appear in the picture where the data cannot be refreshed
accurately. For most Ham applications, we are not showing video of race cars and
the person “on camera” is usually not moving rapidly, so again, this is normally
not a problem. We have found that a Forward Error Correction (FEC) value of about
3/4 with a 3.125 megSymbol rate is adequate for normal motion with 2 video streams
in a 4MHz channel.
Less transmit power required than analog for same range.
Because the digital signal contains more data than an equal analog signal, less
power is needed to transmit an error free signal. Also, the signal envelope contains
more peak power spread out more evenly across the occupied bandwidth allowing more
information within the carrier envelope. An analog signal has most of the power
closest to the signal center carrier but the digital signal is spread out more evenly
across the spectrum. As a result, the digital signal looks squarer as viewed on
a spectrum analyzer seen above. As a rough rule of thumb, the digital signal transmit
power can be as low as 1/10th of the power of an analog signal for the same received
signal quality. Example: The ATCO digital QPSK 2.5 watt 1245MHz signal is received
about the same as its 30 watt 1260MHz analog signal. (Both signals use identical
antennas at the same elevation 10 feet apart).
It’s neat to be on the cutting edge (bragging rights).
Last but not least, it’s neat to be able to tell people that your signal is the
latest digital technology coming from a home built Amateur transmitter. A number
of club members have been acquired just because of that fact. Everyone likes to
be a leader, right?
DATV disadvantages
Up to this time, it’s clear that the European Hams are more creative in regard to
DATV. They pioneered it in the early stages starting at the turn of the 21st century.
I don’t know the real reason why but guess that they are still building their own
equipment whereas many Americans have given in to simply buying what they need and
simply “plug it in” to get them on the air. That’s not necessarily BAD but it DOES
limit DATV operation here in the USA.
- Transmit boards are expensive
Transmit boards available from European sources are NOT CHEAP and as we all know
USA Hams are not exactly that! The board sets usually will run over $1500 for a
2.5 watt signal! It is therefore clear to me that the Europeans who spent a few
years writing and perfecting the needed code want to be reimbursed for their effort.
I can’t blame them but it doesn’t sit well with our “cheap US Hams” so to this date…no
economical solution.
- Transmit boards are difficult to build.
Well, not really, but the hardware is the easy part as a number of manufacturers
have created individual IC’s at reasonable prices but writing the software
code for these is another matter. What we REALLY need is to have some experienced
Ham software engineers knowledgeable with digital TV sit down and help write some
useable code for a board set. Creating the hardware around the code is “a piece
of cake” but I don’t know of anyone willing to take time away from their “real job”
long enough to create useful software for the good of DATV.
- Modulators require interlaced video.
This is not a major drawback but one must be aware of it. To my knowledge, the software
written for all boards available now require full interlaced NTSC video for error
free MPEG-2 compression to take place. Almost all cameras output interlaced video
but ID generators DO NOT. The most common ID generator is the ElkTronics ID board
used to generate the station ID for most ATV repeaters but it does not have interlaced
video so as a result, the signal pixelates and freezes frequently making the signal
almost unusable. I know of no commercially available interlaced video ID boards.
Because of this the ATCO group custom made one from a Sandisk picture frame board
and loaded it with the needed video ID slides. Maybe future software designs will
overcome this problem.
- Transmit delay of 1 to 2 seconds.
There is about a 1 to 2 second latency delay during the MPEG-2 compression (transmitter)
and decompression (receiver). Most of the time it is of novel interest being able
to watch the analog transmission and then the digital transmission occur with a
1 to 2 second offset. However, if any DATV linking between repeaters is anticipated,
it may be very cumbersome when using full duplex for people at each end to wait
a couple of seconds before responding to a given comment. (Full duplex will create
a 2-4 second delay). However, that may be fun to watch also, so who knows, maybe
that’s more entertainment!
Analog has a graceful fade margin. That is, the picture is recognizable while noise
and signal fading increase and decrease from “snow free” down to within about 3dB
of disappearing altogether. Digital, however, is unforgiving as it stays absolutely
snow free down to within about 1-2dB of the threshold. Therefore the digital signal
will remain viewable longer, but when that “cliff effect” point is reached, the
signal is totally gone with no visible traces of it. The corresponding analog signal
may have excessive snow but viewable traces of the signal remain allowing antenna
optimization efforts. So analog has an advantage when receiving a weak DX signal
under rapid fading conditions.
Analog ATV Signal Reporting
DATV Signal Reporting
DVB-S - Why is it best fo D-ATV?
DVB-S Advantage Summary
- Used Free-to-air Receivers are readily available
- Receivers are “cheap” - $10 to $50 on EBay
- New receivers are $125 at local satellite stores
- High linearity amplifiers not required to transmit error free signal
- If amps not linear – excessive transmit signal spectral regrowth occurs but minimal
errors
- Inexpensive LDMOS “brick” amplifiers for transmit can be used and are easy to build
- Format multipath cancellation is adequate for Ham use
- Modulation method not subject to motion limits – tested OK for mobile
- Less bandwidth needed than others for acceptable picture
- Bandwidth modifiable for motion/resolution tradeoff selections
- Multiple video channels within single carrier possible
- Seems best for Ham space shuttle D-ATV communication
DVB-S Details
Modulation Method
QPSK (frequency modulation) is used exclusively here. QPSK (Quadrature Phase Shift
Keying) basically means that the signal is phase (FM) modulated in 4 quadrants of
360 degrees to essentially contain at least 4 times the data as a simple FM signal.
Encoding
As in most other standards, MPEG2 is used here also for data encoding. Forward error
correction is employed using Viterbi and Reed-Soloman coding to correct for noise
and multipath effects. The degree of correction is selectable as needs dictate making
DVB-S very desirable because it allows the user to change it for various conditions.
Linearity Requirements
Linear amplifiers in the transmit chain can become VERY expensive. Therefore it
is important for Ham use to employ the transmission method most tolerant of non-linearities.
DVB-S is it! Since the modulation method is frequency modulation, it is inherently
insensitive to non-linearities. This is not entirely so but it is found that an
amplifier can be close to its 1dB compression point before the error correction
approaches its limit. This is HUGE as it opens up the transmitter design choice
tremendously. Simple LDMOS “brick” amplifiers like the Mitsubishi RA18H1213G unit
are ideal for use on the 1240-1300MHz band to get a 10 watt (average) digital signal
from as little as a 50 milliwatt source. That brick has a bias input allowing for
adjustment of FM or linear operation making it easy to see what the limit is for
a given configuration. Now, non-linearities DO cause other problems though. Each
time the signal passes through an amplifier stage, it creates spectral regrowth
in the output waveform proportional to the degree of non-linearity. These are sidebands
above and below the main envelope at a reduced amplitude level. Therefore, although
the signal has minimum errors, the overall bandwidth will be wider. This may be
a problem in some cases where the allocated channel is defined or where it just
makes sense to minimize spectrum interference. The bottom line is to choose the
highest linearity amp affordable then use a good interdigital type of steep skirted
bandpass filter to remove the remaining sidebands. See the spectrum analyzer comparisons
below.
Power Level Measurements
At this point it is worth noting that output power level measurements using a standard
“Bird” wattmeter are NOT accurate. The output spectrum envelope is somewhat rectangular
instead of sinusoidal so average power measurements do not apply. Because of this
rectangular waveshape, most of the power is at peak values longer making measurements
with a “Bird” or bolometer wattmeter read higher than they actually are. I personally
feel that the only meaningful value is the actual peak reading obtained reliably
with a spectrum analyzer. If you use a” Bird”, I’d divide its reading by at least
3 to get the actual transmit power. Also, when designing an amplifier chain, I’d
make sure the amplifier input will handle 10 times more input power than the peak
value of the digital signal. (100 mw peak DATV signal to a 100 mw rated amplifier
to prevent excessive power dissipation).
Operational Results
Our digital (DVB-S) 1245MHz signal has been operational since January 2004 with
a 2.5 watt signal receivable within about a 20 mile range. Recently we added a power
amp to boost the output to about 10 watts average extending the range to roughly
40 miles. There are about (15) ATCO club members with digital receive capability
using surplus “Free To Air” digital receivers obtained on EBay for about $50 each.
Some additional people have obtained various receivers from EBay and elsewhere for
$10 to $75. All have worked ok and have had no trouble locking onto our DVB-S signal
using only a minimal loop yagi antenna mounted less than 30 feet in the air. I personally
have a 20 element loop yagi mounted 30 feet up my tower permanently pointed to the
repeater 15 miles away connected to 75 feet of 7/8” Heliax. In the shack I have
a 2 way splitter with my analog receiver connected to one port and the digital receiver
connected to the other. Tests have proven there is 10dB of excess signal needed
for simultaneous P5 picture reception on both receivers.
The transmitter DVB-S board set of choice is the Netherlands D-ATV boards. We use
(2) NPEG-2 encoder boards connected to an I/Q baseband board and then to a 1.8 milliwatt
modulator/exciter board providing us with two channels of video. The 1.8 milliwatt
signal is fed to a Kuhne Electronics ultra linear amplifier (we were told high linearity
amps were a MUST at the time) costing about $500 alone. The Kuhne 2.5 watt output
was connected directly to the antenna until recently when an LDMOS “brick” amp was
added to produce a 10 watt (average by Bird) signal. That output is fed to a custom
made interdigital bandpass filter with steep skirts with a bandpass of 5 MHz. Using
3.125 megSymbol rate with a 3/4 FEC, our overall signal is about 4 MHz wide excluding
the spectral regrowth sidebands. This correlates closely with the formula, signal
bandwidth = 1.3 x symbol rate. With the filter in place the resulting signal on
a spectrum analyzer looks very clean with the regrowth signal down more than 50dB
from peak carrier.
We have tested mobile operation with great success. The DVB-S modulation scheme
is supposed to be reasonably insensitive to motion and we proved that it indeed
is! The vehicle in motion was accelerated to over 50 MPH with no loss of signal.
In fact, the normal signal flutter and fading was very surprisingly non-existent.
While traveling under a bridge underpass, the signal was maintained with only a
momentary picture freeze unnoticeable if not looking for it. The normal analog mobile
ATV signal flutter which was very annoying was virtually gone with digital!
