Winter 2010 Issue
Digital Amateur Television – An Introduction
By Thomas Dean, KB1JIJ
Tom Dean, KB1JIJ, is currently a junior at the United States Military Academy at West Point. Tom has been licensed since 2002. His operating interests include HF contesting and digital modes. He is active in the Academy’s amateur radio club, W2KGY, where he is involved in building the Academy’s first CubeSat. Tom is a student of electrical engineering. His academic interests include signal processing, software defined radio, and complex variable methods in partial differential equations.
In his column, Tom plans to explore non-traditional methods and applications of Amateur Television. He also wants to help show that building an amateur television station is not quite as difficult as it might appear. It has been a long-standing goal of his to have an easily-obtainable digital amateur radio setup that would operate in manner comparable to current analog methods in amateur television. —N6CL
Amateur Television has been on the UHF bands for quite some time. In the past, it has been standard practice to match the standard for commercial broadcasting of television on the amateur bands, making operating ATV easily accessible to the typical ham. The recent mandate from the FCC that all terrestrial broadcasts must be digital raises the possibility of making digital amateur television, or D-ATV, common practice. There are many advantages to transmitting digital signals. The use of compression allows for the ability to send higher resolution videos in the same bandwidth. Additionally, forward error correction schemes allow for weaker signals to be received with less error. Unfortunately, transmitting the digital signal is a slightly more difficult task than standard ATV.
This issue’s column provides an overview of
digital television emission standards, and presents several theoretical
approaches that could be used to create a digital television station.
There has been no widespread use of D-ATV other than experimentation, but
hopefully in the near future this will become as common as ATV is today.
There are several possible ways to encode and
modulate a digital television signal. Cable-based systems tend to rely on
a modulation technique known as 64-level Quadrature Amplitude Modulation
(64-QAM). Alternatively, terrestrial broadcast stations rely on a
different modulation method that is similar to that used by analog
television, 8-level Vestigial Sideband (or 8-VSB). This type of emission
is more suited for terrestrial channels, and is therefore used in the
Advanced Television Systems Committee (ATSC) standard which was adopted by
the FCC. The standard also incorporates a forward-error-correction scheme
which is optimized for a terrestrial environment. If a D-ATV station uses
a commonly accepted standard, the demodulation of the signal becomes much
like the demodulation of an ATV signal; the signal must be down-converted
to a standard broadcast TV frequency and can then be displayed on an
off-the-shelf digital television. For this reason, I would recommend the
use of ATSC for any D-ATV projects.
A video signal must go through a series of steps before it becomes an ATSC signal. The signal must first be digitized and compressed into an MPEG-2 stream. This stream can contain both the audio and video signal of the transmission. It is not uncommon to find cameras that will output an MPEG-2 stream, as this is the same format adapted by the HDV standard used in most camcorders. Additionally, there many commercial boards available that will digitize and compress an analog video signal.
The MPEG-2 stream is then taken through forward error correction. The data is typically first randomized1, and then sent to a Reed-Solomon encoder followed by an interleaver and a Trellis Encoder2. There are options within the standard to allow additional forward-error correction to be added, at the expense of video quality, to help in more adverse conditions. Synchronization signals are then inserted into this stream. This separates the signal into fields and segments. The purpose of the synchronization signals is to help the receiver lock onto the signal.
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