Radio station reception

I read a lot about the compression of a radio signal affecting overall quality and enjoyment of radio broadcasts, but dont have any idea if the station I listen to does this or not, or to what extent.
I listen to KMFA, a great classical station here in Austin Texas. I have a Magnum Dynalab MD100 tuner linked up to 2 Marantz SM17SA in BTL mode and Adcom GFP 750 pre amp powering Sonus Faber Concertino's and Kef RDM3's. The station sounds good to me compared to my CD's through a Sony SCD 777. I have attached a link with KMFA's technical specs... could someone interpret them for me.

By the way, I live around 5 miles from transmitter and seem to receive a clean signal with little effort.
Kind of neat Pete. Thanks for posting that one.

As to the "specs", they didn't really list too much. They are actually transmitting slightly over 25,000 watts with an effective radiated power ( ERP ) of appr 60,000 watts due to antenna system gain. If running wide open, their transmitting amp is rated for 30,000 watts, so they are running her at about 84% of "full steam". This small reduction in power output will give them longer tube life and increased sonics via lower distortion, etc... Like anything else, the more you push a design, the shorter the lifespan, the harder it works, the more "bad things" happen, etc... In this case, "bad things" are distortion, splatter, etc... The FCC is not real up on those things, so they probably do their best to stay within the parameters of legal operation.

The biggest thing that i noticed is that the signal is limited to 20 dB's of dynamic range. Above that point, the signal is cut in half of what it should really be. In other words, a recording that is based on an 80 dB average level could be reproduced linearly up to appr 100 dB's. If the recording hit a peak of an additional 10 dB's ( making a total of 110 dB's ), you would only hear 105 dB's of that ( at most ). This is due to the fact that they are running what is known as "compression". Keep in mind that this is not a "Compander" ( compressor and expander all in one ) but only compression. It will limit the high peaks but will NOT increase the very quiet lower levels up to a more usable average.

While bringing up the lower levels to a higher average can help those with a high noise level hear everything that is going on a little better, it also reduces dynamic range and overall fidelity. However, some folks that do a LOT of listening to a Classical station while driving around would probably prefer that they do this. On the other hand, listeners at home in a quiet environment and using a good tuner / antenna system would probably prefer them NOT to do that for obvious reasons. As such, they ARE manipulating the signal somewhat, but not nearly as bad as they could be.

Using the compression only method means that you can get very quiet passages that are very "black" in terms of background but a loud blast way above that may be somewhat clipped off to protect their broadcast amp from damage or severe distortion. This can also save your audio system at the same time. As a case in point, you might adjust your volume WAY up to hear a very quiet passage. If the recording changed tempo drastically and the whole band / orchestra went into full swing, the nearly instantaneous volume increase might pop your speakers, amp or both. They therefore put a "clamp" on how much the signal can vary in dynamic range before they begin to limit the output of the transmistter.

Most stations use a predetermined amount of compression and this seems reasonable ( kind of a guess here, as i'm not a broadcast engineer by any means ). Rock, Rap, Pop, Country, etc... would all tend to use quite a bit more compression along with expansion ( called a "compander" ). This gives you less dynamic range and more signal "squishing" to achieve a louder overall level. Most broadcast companies feel that a louder signal that can be easily heard is more likely to bring listeners in than a signal of very high fidelity that may not be nearly as loud. As such, the "bad" popular music recordings that we complain about on our stereo's actually sound somewhat worse on the radio ( in terms of ultimate hi-fi reproduction ).

One other interesting fact. They are using circular polarization. This gives pretty even coverage for all different types of antennas. In a car or if someone is using a whip like the Magnum / Fanfare design, they would be vertically polarized. A common wire dipole ( "T" ) or a TV style antenna ( Yagi or Log Periodic* ) would be horizontal. Transmitting a circular pattern puts signal into both vertical and horizontal planes equally well, making reasonable reception possible on both types of antennas. Obviously, if one could come up with an antenna that was circular in polarization and tuned reasonably well for the FM band, you would be able to obtain the best performance possible from this specific station. Quite honestly though, i don't know how many stations / transmitters make use of circular polarization. Some might transmit both a vertical and a horizontal signal simultaneously, but i don't know of a too many doing circular. Once again though, i am far from a broadcast engineer, so there may be quite a few out there doing something like this.

I hope this helps a little. I did the best i could with what i had to work with : ) Sean

* Log Periodic antenna was designed and patented by John Dunlavy of Dunlavy Speaker fame. He was an RF engineer long before digging into audio gear.
Thanks Sean
I have seen quite a few of your replys on Agon and you always seem to 'know your stuff'...
Thanks for all the detail... Based on what you say, it seems I am getting a pretty good signal from KMFA. I actually use one of the MD whip antennas you mention.

Thanks again for providing me with a primer on the subject.

Very informative as usual.

If you are interested in an antenna that will take advantage of circular polarization check here.
I forgot to mention something about their power rating. While they say that the amp is rated for 30 KW ( kilowatts ) of output, my experience is that commercial duty broadcast amps are VERY conservatively rated. As such, the 84% of rated power output that i quoted is actually a much higher percentage than what it is probably capable of.

I have seen commercial broadcast tubes maintain power output levels appr 4 times their rated output. That is, so long as the antenna was properly tuned and adequate "ram air" cooling was provided. Otherwise, the tubes go into "thermonuclear meltdown" pretty rapidly when pushed like that.

With that in mind, they might be able to crank up their power to 50 KW actual output and still stay within tolerance. Obviously, tube life would be shortened, but they would have doubled their effective power. While going from 25,000 watts up to 50,000 watts seems like a HUGE increase, it is really only a 3 dB jump in power. Kind of like going from 2 watts to 4 watts. Due to all of those zero's though, it just seems like a LOT more : ) Sean