Signal strenght needed to decode AM digital signal

[Len14043]

This is somewhat unscientific, but I am trying to determine the power levels needed to decode AM digital signals. Using my Radiosophy for indoor reception and my JVC for automobile reception, I have documented how the digital signal performs under 3 conditions: indoors, outdoors in urban environments, and ideal conditions (on the freeway away from power lines.) At my location, WLW has a signal strenght (SS) of 250 mv/m and WCKY has a SS of 50 mv/m. This translates into a digital SS of 2.5 mv/m for WLW and 0.5 mv/m for WCKY. The digital signal for WLW is robust, while futzing with the antenna is necessary for WCKY to decode. Based on this observation, a digital SS of 1.0 mv/m (i.e. 0.5 times a safety factor of 2) is needed to decode the digital signal under the most severe conditions.

On my car radio, the digital signals start failing in urban environments when the analog SS is about 5 mv/m. That translates into a digital SS of 0.05 mv/m. Again using a safety factor of 2, a digital SS of 0.1 mv/m appears to be required to decode a digital signal in noisy urban environments.

Under ideal conditions (on the freeway away from powerlines), the digital signal fails where the analog signal is about 0.5 mv/m, which translates into a digital SS of 0.005 mv/m. Hence, a digital SS of 0.01 mv/m appears to be capable of decoding a digital signal under ideal conditions.

To sum things up, a SS 1.0 mv/m is needed to reliably decode a signal under the most severe conditions encountered in a home, 0.1 mv/m is adequate in moderate conditions encountered outdoors in urban environments, and 0.01 mv/m will decode digital signals under ideal conditions.

Again, this is unscientific but hopefully gives us some insight into the power requirements for AM digital signals.

 

[Play Freebird]

Your findings make perfect sense; the ability to decode digital signals depends greatly on ambient noise and interference, not just raw field strength.

I recently ran some tests on the FM side of my Accurian HD receiver to measure its digital "sensitivity". To determine the decoding threshold, I used an external antenna (Stereo Probe 9 Yagi, about 8 meters AGL) as signal source and put a variable 75 ohm pad in line, which could be switched in 1 dB steps. Starting with no attenuation, I increased the loss until the receiver blended from digital to analog, backed it off 1 dB, then substituted a calibrated spectrum analyzer for the receiver and read the analog carrier level in dBm.

Some stations held in there better than others. Most of the local stations started blending around -77 dBm, but a non-comm FM at the low end of the dial was still OK at -84. The worst performance was -69, but when examining this signal on the spectrum analyzer, I noticed the digital sidebands were about 8 dB lower than normal. It turns out this station (98.1 WOGL) uses "space combining" with the IBOC transmitter feeding an aux antenna side-mounted at a lower elevation on the tower, so in certain directions, their digital coverage would suffer.

In general, I could insert 25 to 30 dB of attenuation in line with the yagi and still get reliable digital decoding on a typical local station. Yet, when I connected the supplied dipole antenna directly to the receiver, digital reception was hit-and-miss. The likely reason: the external antenna provides several dB of gain over the dipole and is away from noise sources, such as computers, monitors, and the receiver itself.

 

[audiophile.]

Some of you may have missed this chart:

http://picshome.com/download.php?id=DD05232E1

With all do respect, no one but us geeks is going to use attenuator (or even an outside antenna for that matter) to listen to FM. My tests done as were as close to average radio user as possible. I use the Accuran supplied "rat tail" antenna and strung it up vertical in a window and the loop antenna for AM. This was done on the first floor of a house on level terrain.

Also I think you should qoute it in terms of the analog signal strength, not the digital signal strength.

 

[Len14043]

Some of you may have missed this chart:

http://picshome.com/download.php?id=DD05232E1

Also I think you should qoute it in terms of the analog signal strength, not the digital signal strength.

Your study was obviously more in depth than mine. The reason the digital signal was expressed in terms of the digital signal strenght (SS) was to get an idea as to the amount of power necessary to propagate an all-digital signal under varying conditions. For a 50000 watt analog station, the digital power is 500 watts. In an area where the analog signal is 2.0 mv/m, the digital would be 0.02 mv/m. Both of our observations support that fact that robust digital coverage is possible with a fraction of the power used for analog stations. As your chart indicates, several stations in your area support "OK" digital performance with an analog SS of about 4.0 mv/m. Seeing that the digital SS is 1/100 of the analog SS, that translates into a digital SS of 0.04 mv/m. How do you think an analog signal would sound with a SS of 0.04 mv/m? I suspect it would be nearly inaudible. That leads me to believe the all-digital method would be superior to analog with all things being equal, but additional testing would be useful.

 

[SUPERCASTER]

Some of you may have missed this chart:

http://picshome.com/download.php?id=DD05232E1

With all do respect, no one but us geeks is going to use attenuator (or even an outside antenna for that matter) to listen to FM. My tests done as were as close to average radio user as possible. I use the Accuran supplied "rat tail" antenna and strung it up vertical in a window and the loop antenna for AM. This was done on the first floor of a house on level terrain.

Also I think you should qoute it in terms of the analog signal strength, not the digital signal strength.

Your link (above) did not work for me.