ATV - Amateur Radio Fast Scan TV
Forward by N4UJW
Way back in the late 40s or early 50s
before color TV when I was just a young barefooted boy trying to have
fun all the time, my Dad and Mom took us visiting out in the country
to a relative that had invited us to a nice country style home cooked
meal one weekend.
I had seen black and white TV before on a few
occasions, but never anything like this one. It had a bluish tint on top
of the screen like the sky, and a light pink like tint in the middle
and green tint near the bottom....WOW! COLOR TV!
Many years later real color TV was on the retail market and everyone that could afford them had one. My Dad finally was able to get our family one....and I was even more amazed at the real thing! That NBC peacock was really something!
Now with the advancement of television
technology and with many ham radio operators actually transmitting live
real time fast scan "color" TV over the ham bands, we present
the articles below to introduce you to Amateur Radio Television, ATV.
Article written by Mike Collis, WA6SVT
Section 1: What is ATV?
Amateur Television (ATV) is divided into two primary types: Slow Scan - a system used on the HF bands occupying the audio bandwidth of an amateur station to transmit a few still pictures per minute to another station usually over long distances and Fast Scan - a system of sending broadcast quality full motion pictures over shorter distances on the UHF and microwave bands.
In this presentation we will examine the Fast Scan version of ATV.
Back in the late 1940's hams in many parts of the country helped develop commercial television. The old Amateur 5 meter band was used for this mission. They were very helpful evaluating
reception of different system types and many engineers were also hams using their vast technical knowledge for television development. The hams - being hams - decided to build their own
stations. In the early days it was home brew or converted war surplus UHF equipment.
By the 1960s home brew and converted UHF two-way radios were used.
By the 1970s technology changes were afoot with modulator and downconverter kits and completed boards followed a few years later by a complete ATV station in a box were available from PC Electronics and other manufacturers. By the mid 1970s Metrovision in Washington DC was the group that had built and licensed the first ATV repeater in America. By 1979 WA6SVT had built the first wide coverage repeater in California on top of Mt Wilson.
Over the years a group called Amateur TV Network (ATN) was formed to support the repeater and many more repeaters soon followed.
ATN now has six state chapters across the country.
Today it is easier than ever to get on the air with ATV for less than $700 for all new equipment and less than $100 for the builder. Check current prices. The oldest and most widely used mode of ATV is AM and a related modulation - Vestigial Sideband (VSB). A cable ready TV set can directly pick up ATV on the 420 MHz band. A downconverter is needed for the higher bands. Your camcorder can be used for your ATV camera. All that is needed is a transmitter and antenna and you are on the air!
FM ATV is one of the fastest growing modes of ATV. FM ATV uses 4 MHz deviation (the terrestrial commercial TV standard used for studio to transmitter links and ENG) in the 0.9, 1.2, 2.4 GHz and higher bands. A few ATVers use the satellite (TVRO) standard of 11 MHz on the 3.3 GHz and higher microwave bands. FM ATV using converted (Part 15 FCC) TV room to room links - such as the WAVECOM units - is available from ATV vendors. FM ATV is the preferred mode in Europe on 1.2 and 2.4 GHz bands.
Digital ATV is just starting out by converting analog video to MPEG-2 bit stream with QPSK, 8-VSB, and DVB modes of digital modulation. Most of the research to date is done in Germany by the DATV group using standard definition DTV on 434 MHz using 2 MHz of occupied bandwidth and HDTV on 1.2 GHz using 6 or 7 MHz of bandwidth. In this country ATN has started experiments using the methods above and using internet pipelines to link distant ATV
repeaters (see http://www.atn-tv.org) and look under ATN on the internet for more details). The HSMM group is experimenting with multimedia formats including ATV using 802.11b and WiFi part 15 equipment occupying 22 MHz in the 2.4 GHz band.
Section 2: Operation
ATV is unique in that it enables a ham to show and tell another ham in real time his shack, latest project, field day, home video of the family's vacation, and other events. ATV for public service allows pictures in real time to be sent to emergency operation centers to report storms and damage assessment.
Most ATVers use a 2 meter calling and coordination frequency to set up ATV contacts. 144.34 MHz is popular in the Midwest and some areas of the East Coast. 146.43 MHz is popular in the west.
Most ATV repeaters have a 2 meter receiver on site to mix in the calling channel audio with the TV audio. On the 420 MHz band polarization is usually vertical with areas that use 434 MHz and horizontal in areas that use 439.25 MHz and areas with inband 421.25 MHz out and 439.25 MHz in repeaters. Most cross-band repeaters use vertical polarization on both bands.
Lighting is important for good ATV pictures. More detail is available in "Advancing the ATV Art Workshop" produced by ATN.
A camcorder, CCTV camera and most analog output computer cameras work well for ATV. Antennas should be above the tree line for good DX on simplex and operation to far off ATV repeaters.
Low loss feedline should be used. A low noise preamp is a good idea if you use a cable ready TV or an older downconverter.
At least 10 watts is needed for good ATV distance and 100 watts or more for long haul DX work.
ATVQ magazine (http://www.hampubs.com) is a good resource for information on what is happening in your area on ATV, projects you can build, ATV group information and advertising for the latest ATV gadgets for sale by reputable ATV vendors and manufacturers.
Section 3: Receivers
The simplest ATV receiver for AM or VSB is the standard TV set using a 6 MHz wide channel. A cable ready TV can receive the 420 MHz band ATV signals - just add an antenna (and preamp for even better performance) and you are ready to receive ATV! For a non-
cable ready TV add a downconverter and for the higher bands a downconverter is needed for all TV sets.
FM ATV needs a TV with A/V inputs or a video monitor, both requiring a full FM TV receiver. Low cost Part 15 domestic units work well on 2.4 GHz and imported Part 15 type units work well for 1.2 GHz or 2.4 GHz bands. A satellite receiver can work on 0.9 and 1.2 GHz bands for FM TV but are set up for wideband FMTV and need a preamp and filter for better operation. They work well for Wideband ATV with a downconverter on the 3.3 GHz band and above.
Section 4: Transmitters
It used to be said that AM TV on the 420 MHz band was the easiest way to get on ATV and that is still probably true but the Part 15 FM TV units are also simple to use on 2.4 GHz. Most ATVers use off the shelf transmitters or a transmitter with a built in downconverter. Transmitters use crystal control or PLL to set frequency and AM modulate the carrier directly with video. Audio is modulated on a 4.5 MHz subcarrier and mixed in at the video modulator. The transmitter is double sideband occupying 9 MHz. The easiest way to build a VSB ATV transmitter is to either add an external RF 6 MHz wide bandpass filter to your existing AM transmitter or use a CATV Modulator.
CATV modulators are rack mountable and are much more sophisticated. They modulate a 45.75 MHz IF with video then filtered though a VSB 5 MHz wide IF filter. The audio is
modulated on a 41.25 MHz carrier at 25 KHz deviation. Usually the aural carrier is phase locked to the visual carrier maintaining a precise 4.5 MHz difference. The aural and visual carriers are mixed to the final output frequency and amplified. Most CATV modulators can produce an output to 550 MHz making them suitable for the 420 MHz band. The modulator output is in the 10 to 20 m/w level requiring amplification with a class AB RF power module.
The easiest FM ATV transmitter is a Part 15 TV unit on 2.4 GHz. The frequency chip can be changed to put all four channels into the ham band on coordinated ATV frequencies. Amplifiers are available from ATV vendors. Imported Part 15 type TV units for 1.2 GHz band are available from ATV vendors.
Section 5: Antennas
The antenna system and its placement is one of the most important items in designing any ham station. In ATV we need more signal as compared with voice modes due to our larger bandwidth. Base stations should use a directional 13 dbd or better gain antenna to get as much signal as possible and to reduce co-channel QRM and multipath. The polarization is dependent on what is used in your area. Stacking yagis or using larger microwave dish
antennas will give better DX on ATV.
The best location for your antenna is above the roof line and trees. Stay away from RG-58, RG-8 and other HF-VHF feedlines.
They have too much loss at UHF and even more on microwave.
The same goes for the PL-259 connector. Use type N or other quality connectors. LMR-400, 9913 and heliax are preferred feedlines for ATV. Try to keep losses under 3 dB. A Waveguide is used for the 5 and 10 GHz bands. DX can reach 50 to 100 miles with good antenna systems and several hundred miles with tropo ducting. KH6HME's ATV transmission from Hawaii was received by ATV stations 2500 miles away in California in full color with tropo ducting.
Section 6: Repeaters
ATV repeaters are fast becoming popular for ATV activity.
Today many hams are finding themselves in antenna restricted communities reducing simplex ATV to about 10 miles but an ATV repeater on a high tower or mountain top allows longer distant ATV contacts. Many ATV groups and individuals have built ATV repeaters. ATN has a linked network of interstate repeaters allowing ATV contacts over hundreds of miles.
Editors note: Here is an excellent list of ATV related web sites on the web.
The two types of repeaters are:
Inband where both the input and output are in the same band (popular in the Midwest since existing ATV simplex stations do not require additional equipment to use the repeater) and
Cross band repeaters have the input and output in different bands allowing the sending station to see his own picture, make adjustments to his station and hear distant stations talk back to
him over the repeater via the ATV 2 meter calling channel audio mixed at the repeater. A separate antenna and downconverter or transmitter is needed compared to simplex operation.
The Microwave Experimental Television Society (METS) uses a wideband FM input on 10.4 GHz using Gunplexers to transmit and slightly modified domestic C band satellite receivers to receive their 3.4 GHz wideband FM TV repeater output.
ATV repeaters are located in a high centrally located area and use omnidirectional antennas. The repeater's transmitter is keyed up upon detection of horizontal sync on the repeater receiver. ID is usually done visually by momentary interruption of the received ATV signal by an ID screen or done via video overlay.
Some repeaters have two inputs: one is the old 420 MHz channel and the 2nd is a 2.4 GHz FM TV channel. See the ATV repeater frequency chart at bottom of page.
MPEG-2 Motion Picture Engineering Group's broadcast digital video standard
DVB European HDTV and DTV standard
QPSK Quadature Phase Shift Keying
8-VSB 8 Level digital Vestigial Sideband, the US HDTV and DTV standard
ATV 101 An Introduction to Fast Scan Amateur Television
Amplitude and Frequency Modulation
There are two basic types of RF as related to FSTV - AM and FM.
AM consists of a constant frequency carrier modulated by mixing audio (as in the standard AM radio band) or video (as in television) or, in a few cases, both. The amplitude of the result varies with the input signal.
This mixing results in three outputs: the carrier itself and two sidebands - upper and lower - which carry identical mirror images of the modulating signal.
For example, assume an AM station on 1000 KHz. When an audio frequency - say 1000 Hz - is mixed with the carrier we get the original 1000 KHz and 1000 KHz minus 1000 Hz (999 KHz) and 1000 KHz plus 1000 Hz (1001 KHz). In the case of AM radio the frequencies can be up to about 5000 Hz or more. See Drawing below.
In the case of television the modulating signal will be a band of frequencies up to about 4 MHz wide. Therefore, the video signal created could be up to 8 MHz wide - carrier minus 4 MHz and carrier plus 4 MHz. Color and audio can increase this up to 9 MHz wide. More on this later.
FM varies the carrier frequency itself and the amplitude remains the same. The modulating signal can be as high as 100 KHz since an FM radio channel is at max, 200 KHz wide - as opposed to 10 KHz for AM radio.
Types of Amplitude Modulation
There are several types of AM - Double Sideband with Carrier (commonly known as AM radio), Double Sideband with reduced or suppressed carrier (which is rarely used), Single Sideband with carrier (rarely used), Single Sideband with reduced - or usually suppressed - carrier (known as SSB), Vestigial Sideband with carrier (VSB - as is used in American TV), and Independent Sideband with carrier.
Almost all ham HF operation is SSB. The carrier and one sideband are suppressed.
Independent Sideband has different information on each sideband - unlike "regular" AM which has, as described above, the same information in mirror image on each sideband.
Television uses VSB. A full 9 MHz wide signal is created but all of the lower side band (LSB) except for 1.25 MHz below the video carrier is truncated. Thus, all that is left is a vestige ? hence the term "vestigial" - of the LSB. Some LSB must be left so that the TV receiver can lock on to the carrier. As been said above - the LSB information is a mirror image of the USB information so no information is lost. See drawing below.
Except for the two ATV channels at the band edges - 421.25 MHz and 1241.25 MHz where VSB is mandatory - VSB is not required for HAM TV. VSB filters are expensive.
Audio in TV and HAM TV
Standard American TV has separate video and audio transmitters 4.5 MHz apart with the audio transmitter "above" the video transmitter. HAM TV uses one of two methods for audio transmission: subcarrier audio or on-carrier audio.
On-carrier audio is probably the older of the two methods stemming from the use - early on - of converted UHF commercial two-way radios. The radios were modified to run wideband video and the video was AM'ed on to the carrier. The audio was left basically unchanged.
As equipment was designed and built specifically for HAM TV a separate 4.5 MHz FM subcarrier was used. The audio is carried on the FM subcarrier and then the resultant FM signal is AM'ed on to the video carrier. The TV receiver cannot tell the difference.
Most ATV repeaters now use a separate aural carrier 4.5 MHz above the visual carrier instead of a 4.5 MHz subcarrier for better audio quality.
Because DSB is used and we don't usually use the VSB filters the ATV signal is a little over 9 MHz wide. As in American TV the color information is generated in the video source itself so no external circuitry is required.
about ten years ago some new ATV transmitters were still available with
on-carrier sound but it is not used much out west.
P.C. Electronics The Leaders
in Amateur Television Equipment
An Introduction to Amateur Television - Part 1
QST April 1993, pp. 19-23 pdf file download from ARRL WEB SITE
An Introduction to Amateur Television - Part 2 ARRL WEB SITE
QST May 1993, pp. 43-47 pdf file download
The basic ATV Station
An Introduction to Amateur Television - Part 3 ARRL WEB SITE
QST June 1993, pp. 35-41 pdf file download
NOTE-- when pdf files download, scroll down for article.
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article by Bil Munsil, K1ATV - HAM TV in Mesa, AZ
Articles edited by N4UJW