Winter 2010 Issue
United States broadcast digital television started in the early 1990s and the official transition to all digital took place in June 2009. Amateur digital television started somewhere around 2000 mainly in Europe with on-air signals not appearing until around 2002 when some digital-board sets became available. Since then amateur digital TV repeaters in Europe have been increasing in popularity, but sadly the interest seems to be lacking in the USA.
In January 2004 the ATCO Group (Amateur Television in Central Ohio) in Columbus, Ohio installed a DVB-S digital output to its repeater, which has been in service 24/7 since then. As of July 2009, the ATCO Group is still the only one in the USA with a digital ATV (Amateur Television) repeater output.
The ATCO repeater digital output uses DVB-S
modulation which we believe is the best choice for amateur television. The
following discussion details more fully why we feel it is best, along with
operational experiences to back it up. I know of no other group, in the
USA or Europe, that justifies it with “in service” data. Therefore, we are
able to back up our statements with results and not just theoretical
Picture quality is near perfect. Strong and weak signals are all “P5,” which is a snow-free signal. Historically, analog amateur television signal strengths are indicated by the “P” unit system where P0 is a barely detectable signal and P5 is snow-free. The strengths increase in 6-dB steps from P0 to P5, so P5 is 6 ¥ 5 = 30 dB stronger than P0. That’s for analog. A digital signal that produces a blank receiver screen with a P0 signal will produce a P5, or snow- free, picture if it’s only 1–2 dB stronger. Therefore, in the analog world, if the signal strength was 1–2 dB greater than P0, the viewer would see a barely discernable picture; in the digital world the viewer would see a snow-free picture. (See figure 1.)
Noise and multipath cancellation possible. The
DVB-S QPSK modulation scheme uses FEC (forward error correction) to cancel
the effects of atmospheric/manmade noise and multipath (ghosting). The
noise is handled by the Viterbi software algorithm and multipath is
handled by the Reed-Solomon software algorithm, which are highly complex
effective ways of handling the data streams but beyond the scope of this
discussion. Since the DVB-S modulation scheme is intended mainly for
satellite-to-ground communication, multipath is minimal, so correction
requirements are also minimal and simple but adequate for ATV
Can occupy less bandwidth. A commercial 8VSB digital broadcast signal occupies a fixed 6-MHz bandwidth and is not subject to modification. The DVB-S signal bandwidth, however, can be tailored to meet the users’ requirements. Therefore, it can be made wider or significantly narrower than 6 MHz with corresponding trade-offs. If a narrower bandwidth is needed, video quality will suffer and fast motion may pixelate. By “pixelate” we mean that checkered squares will appear in the picture where the data cannot be refreshed accurately. For most ham applications, we are not showing video of car races and the person “on camera” is usually not moving rapidly, so again, this normally is not a problem. We have found that a forward error correction value of about three-quarters with a 3.125-meg Symbol rate is adequate for normal motion with two video streams in a 4-MHz channel. (See figure 2.)
Less transmit power required than analog for same range. Because the digital signal contains more data than an equivalent analog signal, less power is needed to transmit an error-free signal. Also, the signal envelope contains more peak power spread out more evenly across the occupied bandwidth allowing more information within the carrier envelope. An analog signal has most of the power closest to the signal center carrier, but the digital signal is spread out more evenly across the spectrum. As a result, the digital signal looks more square as viewed on a spectrum analyzer as seen in figure 2. As a rough rule of thumb, the digital signal transmit power can be as low as one-tenth of the power of an analog signal for the same received signal quality. Example: The ATCO digital QPSK 2.5-watt 1245-MHz signal is received about the same as its 30-watt 1260-MHz analog signal. (Both signals use identical antennas at the same elevation 10 feet apart).
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