DARC
Berg. Universitšt Wuppertal
Arbeitsgemeinschaft Amateurfunkfernsehen e.V.

Digital  Amateur  TeleVision



 

First part of a series in the magazine TV-AMATEUR of AGAF (German ATV Club)
written by Prof. Uwe Kraus, DJ8DW, AGAF-Member #50

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

- analog ATV transmission
- DATV: targets and new key technologies
- digital modulation techniques: depiction and criteria for DATV

- processing of digital video data signals
- GMSK test device
- 70 cm transmitter and receiver: concept and circuit details
- first test transmissions
- suitability of UHF/SHF amateur bands
- further developments

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:
- distinctive smaller bandwidth
- wider range with the same rf power and the same picture quality
- robust on channel interference
- spectrum shaping for minimal interference to other modes
- combination with other digital modes ( i.e. high speed PR)
- combination with modern multimedia technologies

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)



Part 2 of Introduction to Digital ATV

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