Ham radio television
In amateur radio television (ATV), as in general television, moving images and an associated sound are transmitted. ATV is an operating mode of the amateur radio service and is possible in frequency bands over 2 m. According to German law, the content of the broadcasts must relate to the topics of amateur radio and should have a technical-experimental character. In the 2000s, the analog operating modes were increasingly replaced by digital processes (GMSK and QPSK modulation), collectively referred to as digital amateur radio television (DATV).
Analog amateur radio television
ATV operation is possible from 430 MHz upwards on almost all amateur radio bands , e.g. B. on the 70 cm , 23 cm , 13 cm and 3 cm tapes. The transmitted images can come from a video camera, from a video recorder, from a computer with moving image graphics or from a test image generator, for example with text overlay. This video signal and the sound from a microphone or video recorder are fed to an ATV transmitter.
The ATV transmitter combines the video and audio signals and converts both into the frequency range in which the radio transmission is to take place. This process, known as modulation, is divided into, for example, amplitude modulation (AM) and frequency modulation (FM). With frequency modulation, the frequency value of the transmission frequency for picture and sound is changed together. This is the method used by analog television satellites. Advantage: simple circuit technology at the transmitter, analog satellite TV receivers can be used for reception. In addition, a high image quality, great robustness of the transmission against external signals (e.g. due to multipath propagation). Disadvantage: large bandwidth of approx. 20 MHz. FM-ATV operates on the 23 cm band and higher bands. With AM, the signal is modulated directly onto the carrier of the radio signal. Advantage: less bandwidth. Disadvantage: higher susceptibility to failure.
Digital amateur radio television
Digital amateur radio television (DATV) is to replace analog television transmission methods in the medium term, as is currently the case in television broadcasting. The main advantages of digital technology can be seen in a comparatively small transmission bandwidth (from 2 MHz) as well as a greater range in open terrain and better image quality with the same transmission power. The GMSK modulation, which has been tried and tested in mobile communications, is used in the 1st generation devices, while in a further developed variant (3rd generation) the QPSK modulation with higher image quality, which has proven itself in digital satellite TV (DVB-S), can be activated. In addition, new, highly integrated MPEG2 coders and decoder modules now also enable radio amateurs to send and receive digital live TV. Disadvantages: Reception problems in the mountains due to reflections, often no reception despite good field strength; if the FEC (error correction) is too small, it is highly susceptible to interference from external signals; Great technical and financial effort with dependency on initially only one dominant manufacturer of the transmission components / software. From 2012, however, two do-it-yourself projects in the USA and Great Britain (here "DigiLite") led to significantly cheaper transmission modules through the use of modern PC capabilities. In March 2014, a temporary DATV transmission was activated by the International Space Station ISS with "Ham-Video" (broadcast on 2395 MHz in DVB-S with a symbol rate of 2.0 Ms / s). A chain of DATV receiving stations with trackable directional antennas was brought together via Internet video links in the Goonhilly ground station in southern England in order to transmit a continuous live image of the astronaut to connected school classes for 10 to 20 minutes if they asked their questions as part of the lang answered pre-planned "ARISS" school contacts. At the beginning of 2019, the "Qatar-OSCAR-100" amateur radio transponder developed by the German AMSAT-DL was activated on the geostationary TV satellite Es'hail-2, which was financed by Qatar, at 26 degrees east. In addition to the narrow band section for CW and voice radio, it contains a broadband section for DATV tests with different channel bandwidths between 2 MHz and 80 kHz. The DATV broadcasting stations, which extend from Brazil to Thailand, discuss technology and limited airtime on the Internet chat operated by the British BATC.
Narrow band amateur radio television
Narrowband amateur radio television (SATV) is the variant of AM-ATV that requires significantly less bandwidth compared to the modulation parameters commonly used in commercial applications. The sound carrier in FM can be modulated directly onto the video carrier. Due to the low video bandwidth below 1 MHz, the transmission quality with SATV is worse than with ATV, but the range is significantly greater with the same performance due to the possible smaller receiver bandwidth.
There is also the extremely narrow-band variant “ Narrow Bandwidth Television ” (NBTV). In a modern PC-based form with 2.6 images per second, this only occupies the bandwidth of an SSB signal (2.5 kHz) and can therefore also be used on the shortwave amateur bands. The fast NBTV transmissions with 12.5 images per second in the "NBTVA standard" (32 vertical lines, scanning from bottom right to top left without interlacing, aspect ratio 3: 2 at 12.5 images per second) occupy around 20 kHz bandwidth. using the mechanical Nipkow disk scanning modeled on the TV pioneer John Logie Baird , or increasingly also electronically or by means of a PC .
Slow Scan TV
A special process called Slow Scan Television (SSTV) was developed for the transmission of still images on shortwave . With this method, 120 lines are originally sent in 8 seconds (black and white), but in further developments also up to 496 lines in 406 seconds (P7, color), which are temporarily stored in a computer. It is advantageous that
- short-term disturbances only falsify a few pixels
- the required bandwidth is only around 2.5 kHz.
Here, too, there are now narrow-band digital variants with error protection such as WinDRM and DigTRX, but these require powerful computers. A similar process is used to send the images to Earth that satellites have taken from the surface of other planets such as Mars.
ATV relay
ATV relay stations are operated in areas with high ATV activity, such as in metropolitan areas . Such a relay receives ATV broadcasts on the so-called input frequency. The relay then converts the received signal to one or more output frequencies in another or the same amateur radio band and re-emits them there. The type of modulation (AM, FM or DVB) may also be changed. Since the relays are often located in high locations, radio amateurs with low-power transmitters or in unfavorable locations can still be received by many other stations.
Many radio amateurs operating ATV have joined forces in Germany to form the Arbeitsgemeinschaft Amateurfunkfernsehen (AGAF) eV. The quarterly club magazine "TV-AMATEUR" provides many practical tips and suggestions for building transmitters, converters and accessories yourself, as well as converting "normal" devices for ATV operation. ATV-specific devices are rarely manufactured commercially.