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As we cross the threshold into a digital-ready version of analog television, it is easy to forget that over a decade ago, Japanese industries (led by the Ministry of International Trade and Industry - MITI) were developing a wide aspect ratio television system called HDTV. Although the system obtained some measure of success, most Japanese consumers did not accept the format. Many found the aspect ratio to be awkward for normal television viewing. But there were other factors at work. The Japanese HDTV system was analog, and the same manufacturers involved in the project recognized that they had other product lines that they wished to develop that were mobile and would require band space. |
SONY, for example, had a data-discman in the planning stages. They argued that the airwaves should be reserved for mobile applications, and that all other content distribution should be done through cable or phone lines. Japan's decision to abandon HDTV delayed the introduction of the format by a decade. During this time, agencies like the FCC have not stood still. The new formats are emerging, are being tested, and are deliverable now. The objective is to move audiences to the new mediums, and then reassign the portions of the frequency spectrum that are currently commercial, to emerging formats such as the Wireless Application Protocol (WAP) or almost certainly, a superior version of WAP.
A bit of history
Marconi discovered radio. More to the point, he discovered that a signal of any amplitude, modulated at a given frequency, could be detected by a receiver tuned to the same frequency. Radio was born, if only as a method of communication that consisted of dots and dashes. Fleming greatly enhanced Marconi's system by applying a discovery that he had made about another great invention: Notably that the addition of an electrode (called a plate) to Edison's light bulb permitted AC signals to be converted to DC. Although not an amplifier that would increase the range of Marconi's signals, it improved legibility. Marconi was primarily interested in improving his signal-to-noise ratio.
Fleming did not claim to understand why his diode worked. Like De Forest, who added one more electrode (the grid) to create a universal amplifier, Fleming was more concerned with how useful his own stroke of inspiration was. Understanding the fundamentals of vacuum tube electronics came with Armstrong, who arguably made the greatest contribution to radio and television communications. His basic patents, and they number into the hundreds, introduced designs that made it possible for commercial radio to exist.
Super regeneration and more…
Before super regeneration, a local radio station was restricted to a small geographic audience, and the format had practical limits which made it a nighttime hobby for the few listeners that were starting to embrace the format. Fessenden, who is now regarded as being the first 'on the air' in 1906, broadcast his signal at night. During the daytime, even minimal sunspot activity washed out the AM signal. Armstrong discovered that it was possible to transmit very powerful signals using a method that he called super regeneration. Armstrong fed the output back into the transmitter's input as positive feedback. By so doing, he saturated the transmitter. He later went on to develop a system that is used in all analog receiving devices known as the superheterodyne effect.
Prior to superheterodyne, broadcast receivers had to be carefully tuned to each station in incremental steps, and good on-center tuning called for several attempts. The enthusiast had to tune each stage, but skilled operators enjoyed zeroing in on a given frequency, and it can't be disputed that a properly set up TRF (tuned radio frequency) receiver had a wider bandwidth potential than Armstrong's system. Still, it was inconvenient and TRF radios were prohibitively expensive.
The superheterodyne system that Armstrong created greatly simplified the construction of radios. Instead of several adjustment points, the radio enthusiast had one dial to turn. The radio itself consisted of a 'front end' that converted all of the frequencies being selected to one frequency (455 khz for AM). This 'intermediate' frequency could then be amplified using 455 khz as the baseline.
FM works in exactly the same way. The intermediate frequency is different (10.7 mhz), the bandspread assigned to any broadcaster was also vast, allowing for easy development of multiple sub carriers within an assigned frequency. Both formats can offer impressive strengths: AM is truly a DX (distance communication) format. So much so that AM stations must adjust their transmitters in the evening (they must reduce their signal to limit interference with distant stations that are broadcasting at the same frequency), and increase their signal again at sunrise. FM is limited to line of sight transmission. Unlike AM signals that bounce off the upper atmosphere back toward Earth, FM signals travel in a straight line out into space.
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Rebroadcast recievers such as the Scott 4310 were capable of retrieving signals from distances up to 100 miles away. High sensitivity such as this is beyond the capabilities of most FM radios in domestic use, which lack the sensitivity and the ability to discriminate between signals suficiently to supply a clean signal for rebroadcast. On the down side, both AM and FM have their failings. As I indicated in my opening paragraph, AM radio is plagued by any interference that saturates its bandspread. EMI from passing streetcars produces a loud buzzing noise that is an artifact of the electric motor that the streetcar utilizes.
Cheap personal computers that offer insufficient shielding make AM reception impossible with most if not all AM receivers. The FM band suffers mostly from physical reception barriers: Tall buildings and reception canyons can make reception on the FM band difficult and frustrating. Motorists stuck with FM radios that do not have a defeatable stereo feature can be left with a signal that is illegible.
When CBC Radio 1 switched off Toronto's 740 KHz on June 19, 1999, they soon revised their broadcast strategy on 99.1 MHz from a stereo signal to mono operation. They appreciated that their mobile audience found the station's poor stereo performance to be aggravating. Only occasionally does Radio 1 'throw the switch' to stereo on some music programs. Radio 1 programs sound surprisingly congruent with the best that the CBC had to offer on AM: Dial 740 at its best offered a signal that pushed the bandwidth available to them to the maximum, and for those who had an AM receiver capable of wideband reception (how short that list is...), the CBC signal offered superb quality.
Although no more than a curiosity in an age when component AM sound is limited to 3.5 KHz, it is instructive to remember that some AM tuners were capable of signal retrieval to beyond 12 KHz. Such wideband performance was obtained by carefully integrated notch filters that suppressed beat frequencies. Prior to the introduction of time-switching multiplex FM, stereo enthusiasts could tune to a left-channel signal on the AM band and a right-channel signal on the FM band. Depending on the quality of the tuner, the stereo image thus produced provided infinite channel separation. FM multiplex supplies at best 35 dB of channel separation.
Although there is steady pressure to move commercial broadcasters on to new transmission systems that take advantage of digital's strengths -very narrow channel allocations that will permit an explosion of new channels, the field is messy: There are existing conflicts that need to be resolved globally with respect to frequency allocation. North America is out of synchronization with Europe, and until the spectrum can be universally reassigned, there is a good chance that AM and FM radio will fall between the cracks. Both occupy a comparatively small portion of the spectrum, and old habits die hard. The same is not the case for TV. Analog televison occupies several portions of the spectrum, all valuable for other uses, and the individual channel assignments are huge.
Local cable TV suppliers place their upper channels on portions of the band that are under-utilized. The necessity to support a 60 year old standard creates a great deal of wasted space in the frequency spectrum: Adjacent channels in any reception area must be left unused. If your local 'off the air' selection of channels includes channel 2, there will be no station occupying channel 3, for example. This is a tremendous waste of valuable frequency space and not surprisingly, analog television is the main target of those who wish to reassign the spectrum.
Although there may be a long 'legacy' period that sees a grandfathering of AM and FM, it is only a matter of time before broadcasters move to Digital Radio and make available their historic channels for different uses. The AM band, seemingly less valuable for other uses than FM, can still see an intensification of other uses. As a collector of vintage tuners, I get no joy from the prospect of having a collection of handsome boxes that do nothing other than look pretty. I am comforted by the knowledge that the transition will take a while.
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