700mhz band?

with the TV bands being "converted" to stricktly digital, will the slack space be used like MS and Dell want them to?

if common TV bands had so much wasted , why wasnt the FCC trying to get the manufacturers to economize the bandwidth to begin with? Money? or lack of creative engineers?

Technology.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

NTSC broadcast TV actually uses 4-plus MHz of spectrum space in order to deliver a picture signal with the frame rate, number of lines, color and sharpness that one sees in an NTSC broadcast receiver. I consider it fortunate that a color NTSC signal does not need closer to 3 times the spectrum space that a black-and-white one would, and a black-and-white NTSC signal needs about 3 MHz just to get horizontal resolution in the 400 line range. 200 vertical bright lines and 200 vertical dark lines with a

15.734 KHz horizontal scan rate means the video signal has amplitude modulated at 3.147 MHz, more considering that the visible portion of a horizontal line is less than 1/15,734 second, and such resolution requires that much bandwidth even with single-sideband modulation, no color information, and no sound information.

(I am not sure what the horizontal resolution is supposed to be, though I consider displaying 200 bright and 200 dark vertical lines barely a tall order).

An NTSC channel is 6 MHz. A majority of that is actually being used and has to be used in order to deliver the picture quality that is expected of an NTSC broadcast. The sound carrier uses a small bit of the channel bandwidth also, and has to not interfere with picture signal information.

20% less bandwidth could be used if the frame rate was 24 per second (of movies) instead of 30, but that would require technology not available in any practical way in the vacuum tube days in order to avoid flicker without resorting to long persistence phosphoprs that would blur moving images.

The NTSC standard allots 4.2 MHz for video information, and I suspect they make full use of it. 750 KHz of the 6 MHz channel is wasted in lower sideband of the video signal, and I suspect that is because of limitations in practical SSB technology when we first got broadcast TV. The lowest 250 KHz of the channel is supposed to lack signals to avoid interference to an adjacent channel, and the next 250 KHz is fringe of video lower sideband which may have weak signal. So the lowest 1.25 MHz of the 6 MHz channel is guard band and waste. On the top end, the audio carrier is centered .3 MHz above the high end of the standard range of video information, at 5.75 MHz above the low end of the 6 MHz channel. Above that is .25 MHz of combined guard band and audio carrier info. The audio signal is an FM one, so has both upper and lower sidebands.

Freeing up NTSC spectrum space pretty much requires abolishing NTSC broadcast TV.

- Don KLipstein ( snipped-for-privacy@misty.com)

The system was designed to provide approximately "square" (i.e., same horizontal and vertical) resolution capability. There are about 484 active lines per frame, which, since it IS an interlaced format and thus subject to a "Kell factor" loss of vertical resolution, winds up delivering a vertical resolution of about

0.7 (the assumed Kell factor) x 484 = ~340 lines

(This is an approximation, of course. The actual delivered vertical resolution depends on a number of factors, but this is what the system was designed to provide.)

Since TV uses a 4:3 aspect ratio, the luma (Y) channel bandwidth was chosen to given a horizontal resolution equal to about:

1.33 x 340 lines = 450 lines

...or, for the purposes of this discussion, 225 cycles (line pairs) per horizontal scan line.

The horizontal blanking time in the U.S. scanning standard leaves about 53 microseconds active time per line, so the required bandwidth for 225 cycles/line is

225/(53 microseconds) = 4.245 MHz

...so, not surprisingly, we're right back to the 4+ MHz range normally quoted for the Y signal bandwidth.

Not really wasted; TV video is broadcast using vestigial-sideband, but full-carrier, AM. Even today, suppressed-carrier AM (either DSB or SSB) isn't a good choice for sending any sort of signal where you really care about preserving the frequency/phase info in the original signal. True SSBSC was certainly available when the TV system was being designed - it just wasn't the right tool for the job.

Bob M.

Money. This allocation dates back to the early 1960's, (when your TV was built in the Midwest, not the Far East). (As I remember, 1963 was the year when all new TV sets had to have both a VHF and UHF tuner). And the regulations were based on the performance of tuners of that era.

Using the same 40-45 MHz IF strip as the VHF tuner, the UHF band had crummy image rejection. So to prevent interference, they didn't allocate those channels in the same area. Digging into some old FCC regulations, they required stations not be allocated near each for reasons of 1&2) beat and intermod, (video transmissions different in frequency by the standard IF), 3) adjacent channels, 4) local oscillator reradiation,

5&6) (superheterodyne) images for the video and sound carriers.

An NTSC signal is too damn easy to interfere with. Signals cause problems when even -40 db below the desired signal. As tuners got better, they did allocate land mobile services in the lower channels up to 512 MHz, in areas where those channels weren't used.

Also, before the days of cheap distribution (satellite, fiber), there were not expected to be very many stations in any one area.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

IIRC it's about 6Mhz wide.. (But don't quote me on that,just the figure that's stuck in my head for TV signals.)

U.S. TV broadcast channels are 6 MHz wide - but that includes the video AND audio signals, the vestigial lower sideband below the video carrier, and "guard band" space. Don was correct when he said that the video information ALONE occupies a bit over 4 MHz of that 6 MHz channel.

Bob M.

The 6 MHz is the allocated channel width, including guard bands and the FM sound channel.

(Oops! hit send too quick...)

Thanks guys,That makes sense..

4Mhz for video,6Mhz total.

I thought he was talking about the bandwidth of tv signals in general (which obviously was a product of the signal being sent, and once the standard was in place the sheer penetration of the standard postponed any change to something new).

The UHF tv channels were allocated before 1960, something like the late forties or early fifties. The rule requiring UHF tuners to be mandatory in tv sets came later, precisely because there was little use of the large segment of the spectrum allocated to TV. Few broadcasters wanted to use UHF when the viewership was limited by who had suitable receivers, and few viewers wanted to pay extra for UHF reception when there wasn't anything to view on the UHF band. A chicken and the egg situation. So your FCC mandated that UHF tuners be included in all or most tv sets after a certain date, which made it much more appealing for broadcasters to make use of the UHF channels.

The thing about the UHF allocation, and even the VHF tv channels, is that they were allocated pretty early, technology wise. Not a lot was going on up there just after WWII, though the war had obviously proved that such frequencies were useful, and pushed the technology to make the higher frequencies useable.

But apart from the need for 6MHz or so per channel, that there wasn't much need at the time for VHF and especially UHF likely made it easy to allocate all those frequencies to TV. It did not seem overly generous at the time, since the frequencies were otherwise unused.

Note that there were limitations on the use of VHF channels, you couldn't use adjacent channels in any given market, because they'd interfere with each other. I suspect only in some very large municipalities was lack of channels a real issue. I know even today, there are only a handful of local stations here, even though there are still 70 or so channels allocated to over the air broadcast.

It's only with time that the frequencies became valuable. Hence over the years, the UHF channels got whittled down.

Technology has advanced so now there is not only a means of making better use of the spectrum, but there are lots and lots of uses for the spectrum that wasn't even thought of in science fiction at the time the TV channels were allocated. So there is demand for the tv channels, and a means of letting some loose.

ANd the new technology has become inexpensive enough that forcing the viewers to switch over to the new scheme is deemed viable.

Michael

Plus the reference color burst signal at 3.58 MHz plus the audio information at 4.5 MHz.

Jim

-- "If you think you can, or think you can't, you're right." --Henry Ford