What percentage of analogue power are your digital signals?

[BMR]

It's a frequent moan of mine that British digital signals are underpowered at only 20% of analogue power. For instance my transmitter used to broadcast analogue TV at 250kw but since switch over only broadcasts at 50kw.

What percentage do you use over there?

 

[tripinva]

Power levels are measured differently between analog and digital. Analog power is measured as "peak" power, while digital as "average" power. In reality, analog signals didn't often reach peak. A 5000 kW UHF signal would only actually achieve 5000 kW when transmitting a solid black image. Otherwise, its power on average would usually be closer to 2000 kW. The maximum allowed power of a UHF digital is 1000 kW (average), a difference of 3 dB, which should be negligible in most cases.

- Trip

 

[Bengalsfan]

Power levels are measured differently between analog and digital. Analog power is measured as "peak" power, while digital as "average" power. In reality, analog signals didn't often reach peak. A 5000 kW UHF signal would only actually achieve 5000 kW when transmitting a solid black image. Otherwise, its power on average would usually be closer to 2000 kW. The maximum allowed power of a UHF digital is 1000 kW (average), a difference of 3 dB, which should be negligible in most cases.

- Trip

A TV station I used to worked for broadcast peak power more than usual during a particular master control operator's shift. She was more concerned about talking on the phone verses what was going out on the air.

 

[FreddyE1977]

I had read a story written by a Master Control Tech who worked for ITV in Britain during the early days.
Their studios were located across from a block of flats where a particular young woman would have her
boyfriend in on Friday evenings and they'd make love on the floor in front of the telly.

The ITV techs liked to watch through the open window with binoculars. Since it was positive modulation
back then they'd crank up the peak power beyond max to make that room as bright as possible....often
blowing out the transmitter as a result! :

 

[BMR]

Power levels are measured differently between analog and digital. Analog power is measured as "peak" power, while digital as "average" power. In reality, analog signals didn't often reach peak. A 5000 kW UHF signal would only actually achieve 5000 kW when transmitting a solid black image. Otherwise, its power on average would usually be closer to 2000 kW. The maximum allowed power of a UHF digital is 1000 kW (average), a difference of 3 dB, which should be negligible in most cases.

- Trip

1000kw in my dreams!

The most powerful digital transmitter in the UK only broadcasts 200kw. Like I say my transmitter broadcasts 50kw and it is 20 odd miles away.

Which doesn't help me, but it gives me something else to moan about the next time the picture breaks up.

 

[dhett]

Power levels are measured differently between analog and digital. Analog power is measured as "peak" power, while digital as "average" power. In reality, analog signals didn't often reach peak. A 5000 kW UHF signal would only actually achieve 5000 kW when transmitting a solid black image. Otherwise, its power on average would usually be closer to 2000 kW. The maximum allowed power of a UHF digital is 1000 kW (average), a difference of 3 dB, which should be negligible in most cases.

- Trip

1000kw in my dreams!

The most powerful digital transmitter in the UK only broadcasts 200kw. Like I say my transmitter broadcasts 50kw and it is 20 odd miles away.

Which doesn't help me, but it gives me something else to moan about the next time the picture breaks up.

50 kW at 20 miles should be plenty of power. I get 15 kW LPTV stations at that range. Your breakups are caused by something else.

 

[w9wi]

I'm with Dave -- we had a 7,600 watt station 20 miles away which I could receive with good reliability. (they've since increased to 1000kw) I thought the COFDM system in use in the UK was supposed to provide more reliable reception than our 8VSB anyway! (and that's certainly been my experience with OB link transmissions which are COFDM over here)

To be a bit pedantic about Trip's reply, an analog 5000kw signal wouldn't quite achieve 5000kw while transmitting black either. It would only reach 5000kw on sync pulses. I don't remember the modulation percentages offhand but I want to say it would be 3750kw when transmitting black. (75% of sync peak) I'm pretty sure the proportions were the same for analog PAL color TV in the UK.

That doesn't change the validity of Trip's premise though. The power of a TV station is continuously changing. In analog, peaks of maximum power happened at predictable times, and the quoted power of the station was measured at these peaks. In digital, peaks are not predictable -- so the quoted power of a digital station is averaged over time. The average power of an analog station is roughly half the peak power. Your 250kw analog station was more like 125kw average power -- and at that point, the difference from 50kw average at your digital transmitter isn't really a big deal.

Consider also.. that a U.S. TV license conveys authority for ONE transmitter. If you want more transmitters to cover the desired market area, you have to take out additional licenses -- and hope someone else doesn't get the spectrum first. Regulations here encourage a station to use as much power as possible -- to cover their desired market area with as few transmitters as possible.

 

[BMR]

50 kW at 20 miles should be plenty of power. I get 15 kW LPTV stations at that range. Your breakups are caused by something else.

Sure. The *immediate* cause of my breakups are things like next doors flourescent lights, car ignitions that sort of thing. But if the digital powers were higher, then the signal would be more resilent to this sort of thing (remember British cities are very densely populated so they are a mass of electrical noise).

 

[kenglish]

What kind of antenna are you using, and where is it located (set top, indoor, attic, rooftop)?

 

[BMR]

What kind of antenna are you using, and where is it located (set top, indoor, attic, rooftop)?

Its an outdoor aerial serving 3 flats. I think a lot of the interference is coming inside the distribution system somewhere.

 

[dhett]

What kind of antenna are you using, and where is it located (set top, indoor, attic, rooftop)?

Its an outdoor aerial serving 3 flats. I think a lot of the interference is coming inside the distribution system somewhere.

That could be your issue right there. Split a signal among too many receivers and your signal degrades. I don't imagine there's a preamp at the aerial? There's also the distance between the aerial and your flat, plus the cabling could also be a problem.

So many variables that the originating signal strength may not be the source of the problem, or the solution.

 

[BMR]

A bit geeky this question, but do you stations use horiziontal or vertical polarisation (or does it vary?)

 

[tripinva]

The FCC requires that the power in the vertical plane not exceed the power in the horizontal plane. So it can be horizontal-only, elliptical, or circular, but not vertical or elliptical favoring the vertical over the horizontal.

- Trip

 

[cd637299]

FWIW, during the analogue days, full power stations that ran co-channel had to have a separation of 200 miles or so. When Miami's channel 6 signed on, they had to broadcast from a place far enough south of Miami so as to not interfere with ch 6 in Orlando or ch 5 in West Palm Beach, 80 miles north.

With digital, "ch 6" today in Miami is using ch 31 UHF, and now can broadcast from the same tower as the other main Miami stations.

Also today, I think the co-channel separation might only be 120 miles. We have another UHF 31 and a 35 in Fort Myers, a little over 120 miles I think, and I have *never* caught them, due to Miami having the 31 and 35 also, and seemingly never going off the air!

(Trip or w9wi can correct me on the spacings & such)

cd