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.

DVB-T

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.

DVB-C

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!

DVB-S

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 noise bandwidth.

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!