DIGITAL ATV
today and tomorrow
Prof. Uwe Kraus, DJ8DW
news043
First part of a series in the magazine TV-AMATEUR of AGAF (German ATV Club), written by Prof. Uwe Kraus, DJ8DW, AGAF-Member #50. Translated by Klaus DL4KCK.
1. Introduction
A lot of radio amateurs are used to digital data transmission with PC and radio since a long time, think of RTTY and Packet Radio. The two level impulsive data signals from a digital machine are modulated onto the radio carrier by amplitude or frequency or phase or a combination of these. The result is a carrier with few discrete states. In the receiver the demodulated signal has discrete states accordingly. Because of necessary bandwidth reductions in the radio channel the signal does not change rapidly from one state to the next but in a slower continouos way. The discrete nearly constant parts of the signal are representing the valid information - the transitory parts carry no information. Data reconstruction is made by sampling the valid signal parts and testing if the level is above or below one or more thresholds. Here does not count how far from the threshold the signal is. Interference (noise peaks) just above or below the threshold are suppressed, the data signal is regenerated completely.
Sampling and thresholding are criteria of the robustness in
digital transmissions. On the other hand with analog transmission
interference in the channel cannot be removed as the receiver is unable to distinguish
between wanted signal and noise. The advantages of digital transmission can be used for
pure analog signals (video and sound). At the transmitter end there is an A/D conversion
and then a digital modulation on the rf carrier; at the receiver end after reconstruction
of the digital signal a D/A conversion and reproduction on screen and loudspeaker.
Digital transmission technology on voice signals has been state of the art with mobile communication for a long time; DAB (digital audio broadcasting) is being tested by public radio. With television there are digital transmissions on satellite and cable worldwide, and digital modes of terrestrial broadcast that will remove the present analog technology is on test now.
After successful propagation of Packet Radio amoung
amateurs the Digital Amateur Television (DATV) is the next big challenge. This mode is of
corse interesting to all who practised analog ATV until now, but essential too for all who
are experienced in PR, with PC and multimedia systems and who want to involve in this
modern and futureful
technology.
DATV shows a wide field of activity for the experimenting OM. These series of articles will try to give an introduction to the new technology and then get to concrete circuit proposals. The author has been working on DATV with some fellows for more than three years now; in fall 1995 first still pictures were transmitted on the 70 cm band with 1,5 Mbit/s over 50 km distance. Some weeks ago we succeded in transmitting digital test signals with 2 Mbit/s on the 70 cm band with 15 Watt rf power into a 15 dB antenna over a 100 km distance. The quality of the regenerated data signals gives us hope to be able to transmit digital video signals too.
* On september 9th 1998 Prof. Uwe Kraus, DJ8DW, and his team succeeded in firstly transmitting moving color pictures with sound via digital amateur television link over a distance of 100 km with 2 Mhz bandwidth on 434 Mhz. The transmitter at the Bergische Universitaet in Wuppertal (near Cologne) sent 44 seconds of a car race from Video-CD in MPEG-1 using GMSK modulation via directional antennas to Someren in the Netherlands. There at the home qth of DJ8DW the signal was received clearly in spite of rain on the way and was saved on harddisk (about 10 MByte); software decoding of MPEG-1 video and sound is possible under Windows 95/98. *
2. Overview on articles
3. analog ATV transmission
On the 70 cm band vestigial side band AM is used with an rf bandwidth of app. 7 MHz, so the band is filled up nearly. As some other modes with equal rights are located there, this huge bandwidth mode has no future any more. On 23 cm and up there is used frequency modulation with a channel bandwidth of about 20 MHz with similar considerations valid like on 70 cm.
The advantage of analog ATV is based on familiar and reliable technology and a simple start. The disadvantage lies in the big bandwidth, the high rf signal-to-noise ratio needed, the signal is susceptible to interference with low rf levels, and the distorted signals cannot be regenerated. At multi hop services (repeaters) the noise is multiplied.
4. DATV
Before involvement in any new technology it seems advisable to be clear about the goals and about the possible difficulties to overcome. Targets for DATV:
New key technologies
- data reduction on video and sound signals
Digitizing of video and sound signals gives a data stream that needs considerable more
channel bandwidth on direct transmission than analog signals. Modern data reduction
technologies make it possible to reduce digital data to such an amount that the needed
bandwidth is far narrower than the original analog signal bandwidth with similar or even
better picture quality. Very high reduction factors give a loss in quality of course.
Worldwide standards for moving pictures are MPEG-1 and MPEG-2. MPEG-1 gives the well known
Video-CD quality with a 1,5 Mbit/s data stream that is really sufficient for DATV, at the
start at least. MPEG-2 is used at higher quality television broadcast facilities. There
are hardware modules for realtime data reduction in MPEG-1 which are put into the parallel
port of a PC and are able to process PAL video signals from a usual analog source. In the
future it would be desirable to have a solution without a PC.
- error correction coding
If the threshold in the digital decoder of the receiver is crossed irregularly by a noise
peak we get a wrong decision. The amount of wrong decisions compared with the sum of all
decisions, the error rate, depends upon the signal-to-noise ratio and affects the received
signal quality. Digital technology provides error correction coding that is not possible
with analog technology. To the wanted signal data stream are added some more correction
data derived from the wanted data. This increases the overall data rate and the required
rf bandwidth but it makes possible that the receiver can detect and correct errors for the
most part. We are distinguishing between block coding and convolutional coding, often both
measures are used together.
- digital modulation on the rf carrier
There are different procedures to be explained later and examined on usefulness for DATV.
- channel correction
Multipath reception causes distortion by reflections from mountains and buildings and
leads to overlays of timely different signals at the receiving antenna. With analog
transmission ghosting (double images) would appear, with digital transmission the received
sum signal would be rendered useless without correction efforts. The channel correction
circuit contains a digital filter that optimizes its transfer characteristic automatically
and cancels the signal distortions. In many cases "trainings sequences" are
added to the transmitted signal which are known to the receiver. Comparing the received
distorted sequence to the nominal sequence the receiver is able to adjust the correction
filter in the best possible way and
to react on the varying conditions.
(end of part 1; translation by Klaus, DL4KCK@t-online.de)