L.A. TV Stations Free Up Some Spectrum For Wireless Broadband

alphadogg (971356) writes An effort to free up some of the airwaves used by TV broadcasts and make them available for wireless broadband took a big step forward this week in the U.S. Two TV stations in Los Angeles, KLCS and KCET, have agreed to share a single frequency to deliver their programming freeing up a channel that can be auctioned off to wireless carriers next year. The change, which the Federal Communications Commission calls "repackaging," is possible because digital TV broadcasts don't need the full 6MHz of broadcast spectrum that was used for analog TV.

Comments

Seconded!

[mmell]

Analog gets a little twitchy, you see snow (or ghosts). Sound has a little static to it. Your brain actually does a fantastic job of filtering these things out (like most people, you see and hear exactly what you want to see and hear).

Digital gets a little twitchy, you see a still frame (or nothing). Sound becomes silent. It's hard for your brain to actually filter out a blank screen and no audio.

Re:Seconded!

[petermgreen]

The problem with that theory is what is known of as "impulse interference". When some large and sudden electrical event (distant lightening strikes, switching of large loads, that sort of thing) happens it can create electromagnetic radiation that is very limited in the time domain but very widespread in the frequency domain.

With an analog transmission you get a very brief flicker but stuff almost immediately returns to normal. With compressed and error-corrected digital transmissions either nothing happens at all or the error correction is overwhelmed and the system loses sync. Once it loses sync it takes substantial time for it to get back into sync during which you typically get a frozen picture and no sound.

Re:Of course they don't need the full spectrum

[afidel]

The problem is that a 6MHz channel only allows ~18Mbps of usable bandwidth using 8VSB (current ATSC standard OTA encoding) which isn't a lot if you're using MPEG2 for 1080i/720p @30fps, cutting it down to ~9Mbps means you're getting worse than DVD bandwidth for what's supposed to be an HD signal.

Re:Of course they don't need the full spectrum

[olsmeister]

They dropped the transmit power when they switched to digital. Theoretically, this was OK because digital receivers can obtain a usable signal from a much lower input power. However in reality, the effective footprint of OTA channels was definitely reduced. During the transition the maximum output power was reduced from 100kW to 45kW, even though digital transmission is only 30% more efficient.

They also use considerably higher frequencies.

[mmell]

VHF frequencies tended to flow around obstructions. UHF frequencies tended to be more "line of sight". Modern digital television is on even higher frequencies. Crunch the numbers - this means less coverage by FREE broadcast mechanisms and more incentive for consumers to pay for cable or satellite reception. "Broadcast" companies get a reliable revenue stream, cable and satellite companies get a de facto monopoly and the government gets another choke point on communication. Everyone wins!

Well, everyone except Joe Sixpack; but he's just an ignorant dolt anyhow (an insensitive clod?).

Re:Sharing channel == worse picture quality

[crow]

Possibly, but more likely they're dropping their subchannels that were ignored by everyone anyway.

Most broadcasters use their physical channel for one HD logical channel and several SD streams. For example, 4.1 might be HD CBS, 4.2 might be the same thing in SD, and 4.3 might be continuous weather. If they drop the SD channels, they can probably fit in both HD channels with little degradation.

Re:Of course they don't need the full spectrum

[Trip Ericson]

Analog power was measured as peak power while digital power is measured as average power. If you measured analog power in RMS, that 100 kW would become about 25 kW (less than the 45 kW ceiling). A similar difference arises between 316 kW becoming about 80 kW (less than the 160 kW ceiling). On UHF, the power difference is 5000 kW for analog versus about 1250 kW for digital, slightly more than the 1000 kW ceiling, but only by about 1 dB.

On top of that, about 50 dB SNR was needed for a clear picture with an analog signal, while a digital signal requires only 16 dB for a perfect decode. So the difference in required SNR is more than 30 dB, but the power change, even if it actually was 5000 kW to 1000 kW, is only 7 dB.

Modern UHF vs. classical UHF

[davidwr]

Modern digital television is on even higher frequencies.[than analog UHF]

Not true. Digital television frequencies are basically the same as the analog ones, with some channels either no longer used or no longer used except under special circumstances (e.g. grandfathered stations, low-power stations, etc.).

In practice, most US digital TV stations are UHF stations between channels 14 and 69.