OTA ERP Levels (Analogue vs. DTV) - why so different?

About as general and non-specific as I can be (please don't flame me, transmitter techs!) an electron beam runs continuously in a klystron tube, but after it is blasted into the tube at one end the electron beam has to be caught, so the collector does that job at the other end. An IOT does that quite simply and effectively, but at a loss of great energy that is usually dispersed as heat. An MSDC is a special collector that not only catches the electron beam as usual but then acts as an "energy reclaimer" that allows recovery of much more of the electron beam's residual energy than with a simple IOT. An analogy would be that an IOT is just a big baseball catcher's mitt, while an MSDC has the catcher's mitt located at the end of several hallways that cause the baseball to lose speed as it ricochets off each wall and bounces a few times. WIth electrons, the heat involved in an MSDC is huge so that end of the klystron needs to have great cooling capacity, but the payoff is a higher rate of recaptured energy that can thus be reinvested in the entire process.

I would leave it to L'inquisiteur, Billsmith, and other transmitter techs to go into more detail if needed.

majortom said:

...looking at Stampeder's station status, seems i may be in the same boat with CFTO/CTV as most canadians from Toronto/Hamilton are with WNGS.
CFTO's ERP post transition is gonna be like 15 dB down from current ERP,
on the same channel, at the same height. didn't realize that till now.
be interesting if it works here.

10 * log (10800 / 325000) = -14.8 dB

Click to expand...

The comparison is not valid because the NTSC and ATSC signal are completely different. In NTSC, the maximum ERP corresponds to the sync tips (negative modulation). A 100 KW transmitter is only at full power during the sync tips while the maximum video is about 66 KW when the station is sending black. At maximum white, ERP will be around 5 KW. The sound is transmitted with a separate FM transmitter which operates with a consistent power output. Energy across the TV channel is concentrated around the visual carrier, colour sub-carrier and the aural carrier.

The ATSC signal spreads energy across the whole 6 MHz band regardless of the picture content. The signal level changes with each data symbol to a handful of specific power levels but when observed on a spectrum analyser the signal has almost vertical sides and a flat top.

In NTSC, every change of signal level corresponds to a change in picture brightness. Any electrical noise in the signal appears as a random change in brightness which we call 'snow'. In ATSC, signal noise causes data errors but because of the channel coding and error coding, the errors can be corrected or masked until the signal drops below a level called the threshold where the number of data errors exceeds the ability of the coding schemes to make corrections. The ability of the digital system to withstand data errors is called coding gain.

The ATSC system also has an equalizer built into the tuner which reduces the effects of multipath transmission (ghosts) resulting in a cleaner signal being fed to the digital detector. An NTSC ghost cancelling reference signal was developed and tested but never became a feature in analog sets.

In summary, ATSC signals can be transmitted at much lower numerical ERP levels and still provide the same coverage area as an NTSC station.

As a TV design engineer in my earlier life, I will add my input to this discussion.

Analogue TV is not that robust. In order to receive a perfect picture, any interfering signal has to be -53dB or lower on the wanted signal so as not to be visible. Put another way, the wanted signal has to be ~400 times stronger than any interference. This includes tuner or amplifier noise (snow) and co-channel interference from a distant TV transmitter. In the design lab, we used to assume a noise free picture on UHF required 750µV of signal at the TV antenna input. So, analogue transmitters have to be powerful to provide a good field strength in the service area.

Analogue TV transmitters often have a positive or negative frequency offset from the actual channel frequency. This is to minimise interference effects from another TV transmitter on the same channel, often for adverse propagation conditions when signals travel further than intended. The frequency offset is related to the horizontal frequency divided by a whole number up to 12, but more often in the order of 6 or 8. This value is added to or subtracted from the actual channel frequency. Then if there is co-channel interference from another TV transmitter, it appears as thin light and dark horizontal bars. If the co-channel interference increases, the intensity of the bars gets worse and can sometimes be sufficient to ruin reception. BTW, there is nothing worse than watching a zero beat between two transmitters. This appears as very broad light and dark diagonal bars - maybe only 2 to 4 over the screen - which rotate and increase or reduce in numbers as the frequency difference varies by a few Hz.

As mentioned in the first paragraph, 750µV is needed on a UHF channel on analogue TV for a noise free picture (53dB S/N ratio). By contrast, the FCC has determined that the drop dead S/N ratio on DTV is 16dB on UHF. This equates to about 7.5µV at 600 MHz, or a factor of 100 times or 40dB Voltage ratio between ATV and DTV. As we know, with DTV, there is a cliff effect where the signal is received well or not, so there needs to be a margin to allow for fades etc. Even if 75µV is assumed as a good strength for DTV, this is still 20dB or 10 times lower in Voltage ratio. Therefore, the transmitter power on DTV can theoretically be reduced to 1/10 or less of that of the analogue transmitter to achieve the same coverage on the same channel.

Many of the US DTV transmitters are putting out higher power than really necessary for service area coverage. The fact that less signal is required at a DTV receiver than on ATV, this means those of us favourably placed can pick up US DTV stations from 100 miles or more and be perfectly watchable. (The picture, that is, not the programme).