DIGITAL DVB-S AMATEUR TELEVISION
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 andGlobecastGlobecast 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.
- Noise reduction. 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 1268MHz signal is received about the same as its 30 watt 1258MHz 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?
- Most DATV is in Europe. 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!
- Ungraceful fade margins. 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
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.
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.
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).
Our digital (DVB-S) 1245MHz (now 1268 MHz) 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!