I have a modest collection of vintage FM radios/tuners. I home brewed a low power mono transmitter to play my mp3s on them. Up to now I've been just ecstatic with the fidelity of results. But I've run into ONE song that produces substantial distortion. This is a beautiful tune by Julie London called "Cry Me a River". Played directly into my HiFi system, the sound is fantastic. Played through my little transmitter, the S's and T's are distorted unless I turn the modulation level way down. (BTW, I tried one of those Ipod type car modulators with this song, and the distortion is even worse!)
The only theory I have is that the recording must have a lot of energy beyond 15 KHz on the S's and T's of Julie's voice. Perhaps this is blowing up the bandwidth beyond what the radio can handle? I have a single pole roll off at 15 KHz that counter acts the boost from the pre-emphasis circuit above 15 KHz, so above 15 KHz the response flattens out but does not fall. Maybe I need more substantial filtering above 15 KHz? Does anyone have a good brick wall filter design for this application? Or maybe a better theory of what's causing this distortion?
BTW, how do the commercial stereo kits filter the L+R audio source before adding in the pilot and subcarrier?
Bob D.
My guess, Bob D., is that the pre-emphasis itself is overmodulating. Once we operated a full power FM transmitter with no limiter just during a test phase and many albums caused over-modulation in the pre-emphasis region and it isn't a problem the ear can easily anticipate. The carrier swings way out of channel when those highs hit.
But the other thing happened too, in a different situation. A station that used its sub-carrier to put background music in stores was easily "popped" by signals above 15kHz colliding with the subcarrier, which I think was at 75kHz, but maybe lower.
The engineer kept trying to "brick wall filter" so the problem would go away, but certain records still broke through, so the station owner started yanking those irksome records out of circulation.
I think a good limiter is worth having.
Good call, Carl. My home brew set does not have a limiter either, maybe that is the problem. The lack of a 15 KHz brick wall filter may be a red herring, especially since this is a mono transmitter. (Although, technically, the mono signal should be brick walled at 15 KHz stereo or not.) I have an SStran AMT 3000 AM transmitter that uses a SSM2166 chip as a limiter. I wonder if this chip would make a good FM limiter? Has anyone out there tried this? I suppose it should be placed after the pre-emphasis so it can detect the boosted highs that seem to be causing the distortion. As luck would have it, I have a couple of spare DIP SSM2166's in my junk drawer. I'm open to suggestions on limiter circuits, any recommendations?
Bob D.
I had this same problem on a non-commercial educational FM station that I was in charge of. As I recall, the transmitter used a Harris TE-3 Exciter. Remarkably, it had no low pass filter at the input, although it had a 19 kHz notch filter to protect the pilot. I built a pair of closely matched 9-pole Bessel low pass filters with an Fc of 15 kHz... but that was not good enough! I wanted to use Bessel filters to maintain low overshoot. To get the attenuation I needed, I had to change to a Butterworth design. Many commercial exciters of that time used Elliptical filters, which have a much steeper roll off, but they overshoot like crazy.
Here's some information which may save you some time and money as you pursue this.
I did some testing of the SSM2166 IC and associated circuitry and found that it is not a hard limiter so it does not prevent overmodulation on either AM or FM. It does a good job of "leveling" the volume though.
The reason for the inability to prevent overmodulation is that the chip AGC responds to the RMS voltage of the audio signal and has delays associated with the AGC circuitry time constants. Also, by definition, RMS is a time average so delay is inherent in the theory. When a music or voice peak hits it takes the circuit several milliseconds to react and by then it is too late to reduce the gain to prevent overmodulation. A compressor which would prevent overmodulation would need to "look ahead" at the signal by delaying the audio a bit.
A hard limiter or clipper would not be appropriate since it would introduce audio harmonics and distortion.
Since you are running mono FM there is no pilot to corrupt with the higher audio frequencies and the problem may be in the receivers. Do they all do this? Good receivers will turn off their stereo circuitry for a mono signal and distortion would then most likely be due to deviation outside of the IF passband in the receiver. It could also be that overdriving the FM transmitter circuit is causing non-linearities at high levels.
Since this is for your own personal enjoyment the easiest solution could be to lower the audio into the transmitter so that even on the peaks it doesn't distort.
Neil
The FM broadcast system used in the U.S. involves almost 17 dB of pre-emphasis between 400 Hz and 15 kHz. That is a very steep HF boost! Without any AGC or dynamic high frequency limiting, it is very easy to overmodulate. If you are trying to even approach the subjective loudness of a commercial station, you will be over-deviating like crazy at higher frequencies, which is probably why you hear distortion. A good FM limiter and AGC chain would make a big difference.
Mp3 files are a funny animal. I have noticed this problem as well and what the others have stated above is right on target.
My contribution to this thread is this, every computer is not created equally. To be more exact not every sound card is created equally.
Depending on the software one chooses to use affects how digital materials are reproduced on any given computer. Also sound levels are not always the same from one machine to the other.
What sounds good on one computer may be less or more than ideal for others.
Recently on a blog entry I spoke high praises for "Stereo Tool," a Winamp plugin by Hans van Zutphen. There are other good mentions of this software elsewhere here on the site, including discussion of an added feature built into "Stereo Tools," a complete FM transmitter processor.
I am using "Stereo Tools" to compress/limit/equalize my stream, AM and FM transmitters.
It is not a toy and is very well designed to provide professional results.
I use good Old analog for processing.
I too had the S's scratch the Audio at One Time.
To Fix this all i had to do was turn down the 15k to 0
(scale of +12 to -12) on a Graphic Equalizer and That Solved it.
Hello to The Crow,
I have had the same problem at times with the Ramsey Fm 25B, it has always had a bit too much treble for my tastes.
Luck for me my son brought over a Teac EQA-20 which was at one time a part of a larger stereo system. It works well since the Ramsey has a hyper active pre-emphasis section, i too have eliminated the 16 k range.
Now all is well with most tracks played over the air.
It's funny this came up as i had a chat with a friend online about the same problem. If i were to do it differently, i would probably switch out the capacitors for pre-emphasis to variable capacitors. Still might do that anyway.
Wow! This "Stereo Tools" software sounds like the perfect solution. I've already dedicated an old laptop computer to my transmitter. Right now I'm interested in the "limiter" and the "low pass filter" features. Down the road I might try the stereo encoder. I listen to mp3s mostly, but I also listen to "http://zoomerradio.ca/listen-live". Do I want the "stand alone" or the "Win Amp plug-in" version of the software? (Looks like there is a nice "Stereo Tools" forum, but you have to buy one of the versions before you can post.)
Bob D.
We use the Winamp version of Stereo Tools because Winamp is the playlist we use for streaming and broadcasting. Until you mentioned it I didn't realize there is a stand-alone version.
I am so impressed by the way it makes us sound that I find listening to our signal an actual pleasure, both from the computer and over AM and FM.
You can at least read the Stereo Tools Forum as a non-member.
In my Opinion Stereo Tools made my audio sound just like a full power FM. Which I didn't really like.
I guess I just got used to no compression or limiting lol!
In my opinion an FM sounds better with no compression or hard limiting. Maybe some Light Limiting. really all I have going through my Chain is the Built in AGC that Zara Has and a Graphic Equalizer. Resulting in a sound that I think Surpasses all the locals in my Area.
I'll have to play with Stereo Tools to see if I can get this same result.
(I Have Two FM Stations, I'm Testing It On The Talk Station, Really just interested in the RDS)
The Crow has made an important distinction, and I am able to take both sides of the argument because of different situations from past experience.
In the 1970s I was programming for the only mono FM station in the city, simply owing to budget constraints, but the small group of engineers and I agreed that the wide-open unfiltered frequency response compared to a stereo station is something very worth experiencing. For classical music, jazz and voice, the hi fi frequency response with only limiting to protect from over-modulation, the station became reasonably popular.
In today's situation, mostly talk, I have created an equalization curve based on emphasizing clarity and intelligibility, and compress so as to bring out quiet background or low recording levels, and hard limiting to allow getting right up to the top edge of danger.
In my opinion many of the FM stations today are set up to be loud, but I am guessing that many of the people who do the adjustment may not understand music and speech and they often get a fatiguing sound that is annoying to listen to for very long.
It's like being a chef at a fancy restaurant. There are spices and mixtures.
I am wondering if any of you have tried Stereo Tool as an AM processor? Either for mono or stereo AM? Will it mix stereo to mono? And can the processing be adjusted to work for AM? Thanks for your opinion.