DIGITAL AMATEUR TELEVISION “SIMPLIFIED”?
The following material has been compiled and edited with the aid of the descriptions at the
Dutch ATV web site along with my verbiage to hopefully make it easier for the beginner. The
complete unedited text may be seen at the Dutch Amateur TV web site http://d-atv.com. WA8RMC.
A New Standard For Amateur Television
Digital Amateur Television, formally known as D-ATV, is growing in popularity. D-ATV is
based on the Digital Video Broadcast Satellite standard, developed by the DVB organization.
The Dutch team has designed a digital modulator according to the DVB-S standard for amateur
and professional purposes. The design and construction of a digital ATV transmitter is far
too complex for most of us but many of us will now start exploring this new area of our hobby.
More and more ready built D-ATV transmitters will be sold in the near future including the
Dutch design. Therefore it is important to be aware of the advantages, disadvantages and
inherent problems with this new digital modulation scheme.
The following should give you more insight with a more or less theoretical description
of the different modulation techniques and the inherent problems. Some knowledge must
be present among us when we use digital video transmissions in the future. Furthermore
we will discuss some of the different professional standards that currently exist for
digital video broadcasting (DVB-S, DVB-C and DVB-T) and the possibility for using
these standards for amateur purposes.
The DVB-T standard was developed for terrestrial digital television communication
and is presently in use by US broadcast television stations for digital and HDTV
known best as 8VSB modulation (8 quadrant Vestigial Sideband). The aim is to overcome
the destructive effects of multipath reflections from objects in the signal path such
as buildings and towers, which produce “ghosts” in the picture of analog transmissions.
In digital television transmission, the data rates are very high so multipath reflections
will normally be even higher resulting in a partly distorted received signal. Also the
multipath reflections cause Inter Symbol Interference because reflections of the
received signal interfere with the direct received path. Nevertheless, higher bit
rates (symbol rate) produce higher negative effects so, to overcome these disturbances
for DVB-T the effective bit rate is spread out over a large amount of digitally
modulated carriers. The larger the amount of carriers produces lower effective
bit rates for each carrier. The lower the effective bit rate per carrier, the
lower the negative effects of multipath reflections which is the basic idea
behind DVB-T. This produces a very complex signal so it is the hardest to
reproduce and most expensive because it requires very high speed parts.
It’s necessary for broadcast TV but not practical for amateur purposes.
The DVB-C “standard” was developed for cable digital television transmission using
QAM modulation. A cable environment is a relatively protected environment with respect
to distortion and signal path attenuation so a higher signal to noise ratio can be
achieved. Also, because there is no negative effect from multipath, it is able to
implement higher order modulation schemes. DVB-C generally requires higher signal
to noise ratios at the receiver side due to the higher order modulation schemes.
Because of minimal error correction, it is more susceptible to multipath reflections.
This is one reason why DVB-C is not preferred for Digital Amateur Television. It’s
too bad because it’s the easiest and cheapest method. Also various cable companies
that use 8VSB or QPSK implement DVB-C in a variety of forms so no one uses a common
standard. No surprise here because each cable company wants you to use THEIR set top
box and not the competition. Therefore a cable box for one cable company will not
necessarily work for another. We wouldn’t want to become a part of this fiasco!
The DVB-S standard is developed for satellite digital television transmission using QPSK
modulation (Quadrature Phase Shift Keying), which is a type of FM modulation. Why do we
recommend D-ATV use the commercial DVB-S standard? A DVB-S digital system has some
distinct advantages and also some disadvantages.
One of the main advantages of a digital ATV system is the fact that picture quality
is improved above that of most analog systems. We do not encounter the negative
effects of noise. We do not encounter video group delay problems; an item on which
much attention has been paid by lots of amateurs and audio quality is improved.
With digital ATV we get high quality audio channels and these high quality audio
channels don't disturb picture quality!
Another advantage is it does not extend the occupied bandwidth of our signal,
something that is the case with analog where we need some FM modulated audio
carriers above our video signal. Other main advantages are the fact that analog
ATV systems occupy a lot of bandwidth. A wide occupied bandwidth means several
disadvantages among which are less room for others to communicate and a higher
The first item is clear. We want to be as efficient as possible. If this can be
done without throwing away any quality then this is good. If we can improve quality
with less occupied bandwidth then we have even more benefit! The second item is also
very interesting. The higher the bandwidth the higher the received noise level will be
at the receiver because noise is integrated over bandwidth. Some digital modulation
schemes are able to demodulate at lower threshold levels than possible with analog FM
ATV systems. One of them is for example QPSK. With QPSK we are able to occupy less
bandwidth and also make use of lower thresholds. This means that we can get more out
of such a system with less power, better quality and less bandwidth!
A satellite to earth system needs low threshold demodulation and a good signal to
noise ratio so only QPSK can be used. QPSK is a very robust modulation scheme because
it just has to make a decision in one of four quadrants. The low signal to noise ratio
on the other hand will be a source for bit errors, both burst errors as single bit errors.
To overcome this weakness, the DVB-S standard uses different layers of Forward Error
Correction (FEC) for a very robust protection against any kind of errors. The FEC
consists of a Reed Solomon coding that protects against burst errors and also
additional convolutional interleaving to spread out the impact of burst errors.
The convolutional encoding is better known among users of satellite television
and is recognizable in a satellite receiver setup menu under the menu item FEC
rate. The fact that satellite communication is basically line of sight communication
without obstacles in the transmission path tells us that less attention is paid in
this system on multipath effects. Therefore, the DVB-S standard will be moderate
when it comes to robustness against multipath reflections.
Mathematically all these carriers are orthogonally spaced from each other with an
Inverse Fast Fourier Transform (IFFT). It works like this: The incoming bitstream
is encoded with Forward Error Correction blocks like Reed Solomon and convolutional
interleaving and finally convolutional encoding. After the FEC the resulting bitstream
is mapped on all the constellations for the separate carriers. The resulting constellations
are the input for the IFFT processor block which performs the actual transformation
from frequency to time domain. After the IFFT a cyclic extension is performed on the
resulting OFDM symbol, which is used for the guard interval that gives additional
protection against multipath reflections. The resultant complex output of the IFFT
block can then be converted to RF with an I/Q modulator. There, did you get all of
that? If not, don’t be discouraged because you don’t have to understand it to apply it.
There is still a large disadvantage left. QPSK using OFDM modulation requires very
linear amplifiers surpassing the requirements of present SSB linear amplifiers. The
large amplitude swings of the carrier will introduce very high intermodulation levels
when the signal is non-linearly amplified. Although QPSK is quite robust and will still
work correctly with quite high spectral regrowth levels, there is also a need to
transmit a nicely shaped spectrum in order to be spectrally efficient. As stated
before, D-ATV generally will need less power compared to FM TV techniques but
this will not mean that the amplifiers need to be smaller! In fact, in order to
keep spectral regrowth levels low enough, power amplifiers will need to be biased
class A and the output drive levels need to be in the order of 7-10 dB below the
1dB compression point to keep spectral regrowth below -40 dBc. Commonly used class
AB power brick modules will not work.
In summary, DVB-S has high error protection, uses very robust QPSK for modulation
requiring low signal to noise ratios for proper demodulation and isn't the best
choice against multipath. However, the fact that lot of foreign experiments ended
with very positive results demonstrates that these negative effects are smaller
then expected. Besides, a lot of cheap commercial satellite set-top boxes exist
which is a major advantage so DVB-S is the best choice for D-ATV.
I hope I didn’t bore you with the technical talk. I tried to simplify the process
while giving an insight to the really complex nature of the overall process. No,
you don’t have to research the Inverse Fast Fourier Transform process to get a
grip on what’s going on but it’s kind of nice to be able to say, “Yes, I’ve
heard of that before. It’s needed to encode and decode the digital TV process”.
End of story!