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TV Frequencies...

From an amateur radio operators perspective.

What is it?
A noise free way to transmit and receive television.
Digital TV smarts process the RF signals to retrieve that picture perfect image and sound information. The introduction of commercial "Digital TV" to Australia will provide high quality sound and image to anyone that can receive. Perfect copies (well almost 99.x% clone of the original source) of audio-visual data only limited by the quality of the receivers equipment, stored on hard disk as a Personal Video Recorder or viewed on a DiGiTaL TV screen.

Amateur Television, will experiment with these "new" digital modes and conceivably network the city and country or just the metropolitan amateur radio communities together via the many digital means and modes on offer. As the analogue TV services are switched off, DiGiTaL services will make audio-visual communication "a perfect copy, to everyone everywhere" possible and permit yet to be imagined experiments plausible. What devilish schemes of mind and mater await digital communications?

How does it work?
Microcomputers inside your set top box or DiGiTaL TV receiver obtain TV pictures and sounds like an Internet file download, except any errors are ignored, as the next signal frame has the updated information. Because it is digital, line of sight is not a major requirement although should still be sought after, even if the picture and sound information can be reclaimed from very low signal levels.

At near no quality loss (always perfect picture?) the trade off with digital is it will either see a signal or no signal at all. There is nothing in between *, unlike analogue which will display noise as a signal e.g. TV snow... * Digital does have between states but its effectively flipping between the two states trying to figure out the best signal to use and looking very poor indeed.

Experiments with Slow Scan TV (a form of digital TV with personal computer and sound card) has shown very good picture quality on the HF [1-30Mhz] bands using analogue audio frequencies [100-2800hz] between stations on opposite sides of the planet at relatively low power levels [<100watts]. At VHF, UHF and above frequencies [30Mhz-daylight] it is hard not to get a good picture. You still have signal echoes (ghosts) but these are filtered out by the microcomputers and electro-mechanical filters.

Digital TV just took that extra step with the invention of microcomputers and encoded the series of frequencies that made up the picture and sound to 1's and 0's. Then put that signal on the RF carrier. Digital TV stills sends out and receives a series of analogue frequencies or RF carriers, but they represent simply 1's or 0's (two states). It is the electronics / computer inside the digital TV receiver that manipulate and calculate what those received analogue frequencies should be and what they should represent.

DTV picture and sound information is now stored as 1's and 0's "in" the RF signal, not as a series of complex frequencies and levels, superimposed "on" to the RF signal. This is a very important re-invention of signal encoding which is only possible with the invention of microcomputer processing and their relative speed of calculations, discrete components could not achieve the same efficiency of signal processing. You will not be able to interpret the TV information on the RF signals without some microprocessor (computer) to decode. All an analogue receiver will hear is a series of tones or lines on the screen. It is these tones that the DTV receiver will hear and decode to make the moving pictures with sound. But it needs to be on the correct starting frequency for each TV network channel else it will report "no signal". Although the DTV receivers can find these start frequencies automatically they can get it wrong due to many factors, signal noise, wake up programmed for wrong area (part of world), broken or shorted antenna / cable, internal electronics fail due to heat or static electricity / lightning strike.

Ghosting still occurs to, which is when echoes of the primary RF signal are received a fraction of a second later, thus interfering with the complexity of the primary signal information. These signal echoes can still occur in digital transmissions but the electronics can apply filters, which effectively reduce the RF signal level to recover the picture and sound information from the strongest signal. This is where the DTV smarts can get it wrong "auto-tune" works fine in an ideal location with direct line of sight to the TV transmitter.

Reality however is different and not as easy to overcome, the complex and changing structures of inner-city living cause signals to bounce off many things, confusing the DTV smarts trying to figure out what signal is what. This filtering system can result in no decodable digital signal or "no signal" even though you receive other stations OK. Manual intervention is the only remedy but you need Channel Frequency numbers for your area to over-ride the auto-detected ones.

Aircraft flying over an analogue TV network caused that "picture flutter / fade", with Digital TV it could be a series of momentary freeze, unfreeze while it bounces the TV signal below its flight path, but you will never really know what caused it. Other artefacts you may experience are "block" images on screen, fine lines, speckles, deformed / stretched / shrunken images all caused by poor signal or signal interference from other Radio Frequency sources (e.g. electric hair dryer, drill, cordless phone, hand-held games/devices) but usually it is a multi-path TV signals washing around your antenna like waves on a lake. So when a plane, truck, tram, crane etc. blocks or passes by it can cause the signals to bounce away or to your antenna. Hopefully the effects are brief, but inner-city dwellers may experience lots of these events each day from many different sources.

The quick fix is rotate the antenna a few degrees to find a stronger signal and check all stations again to see if the DTV smarts can figure out a decodable signal.


How does it affect me?
Perfect picture with DVD sound quality via either paid or free to view over the air services. The worse case would be you are swapping ghosting for jumpy "start / stop" pictures with "out of sync" sound, but only until sync / picture is re-established which may take seconds to never.

At some time soon after total digital conversion TV companies will push to expand their ever decreasing viewer audience to wider areas or more accurately try to access other types of TV receivers. Its already started with communication companies offering pay per view TV shows on your mobile phone, direct links to the Internet with 1000's of audio visual services to view. Why stay with a dozen or so local TV stations pushing free to view of what they want, when you can purchase and upload to view what you want, when you want and you control when to stop, start or repeat ...

Your personal mobile communication device and your in-home entertainment system interlinked, sharing material you have acquired and can be connected to many more sources of material via the Internet. The TV station will just be another one of the many sources of material clamouring for your attention and patronage...

For "Interactive Digital TV" the receiver (you) needs some other way to communicate with the sender. In Australia, this is expected to be via the telephone network, either mobile or land line or via the limited cable TV network (Internet data). The limitation will be the speed of the "back" connection and the amount of channels back to the sender. In Australia the telephone network has the edge while the limited cable TV comes a distance second, the mobile phone network has potential to be more interactive as this technology is overlapping into DTV as "multimedia" and "Internet" access. The mobile phone now having video send and receive, data storage, Internet access and unique user ID for each handset has all the "interactive" DTV requirements... A portable communication device for information storage and retrieval with interactive real time visual display and voice channels. This device being separate from the media being accessed / viewed (Internet / television) has a good security fail-safe potential, it can not be "spoofed" or intercepted easily...


73's for now
Gary, VK3KHB




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