Stereo Tool

I am really enjoying Stereo Tool 7.41 but I may have stumbled on an issue using AM asymmetric limiting and clipping in combination with pre-emphasis on the FM Transmitter output. In testing with 7.41 the 125% positive peaks appear to be clipped in the output stage of Stereo Tool seen in the levels blinking red. In 7.40 I was able to lower the output by clicking on the output level indicator and lower the slider output control for the post amplifier to create the positive peak headroom and could easily run 150%+ positive peaks without showing clipping in the output stage level indicator.

I am testing using the Stereo Tool AM Transmitter -North America - More Bass preset. I simply turn on AM asymmetric limiting and clipping along with FM Transmitter with pre-emphasis and pre-emphasis output checked and adjusted for 75 us. I am using the FM Output and not Normal. If I don't use the FM transmitter option I cannot get pre-emphasis output to work in 7.40 or 7.41 for AM.

As you probably know in the U.S. the standard for mono AM broadcasting is to utilize a 75 us pre-emphasis with a sharp cutoff past 10 kHz as channel spacing in the U.S. is 10 kHz. For reference on the NRSC standard see http://www.nrscstandards.org/SG.asp

A question I do have is if I modulate an AM transmitter using AM asymmetric limiting and clipping in combination with pre-emphasis on the FM Transmitter output in Stereo Tool is de-emphasis removed past the the final clipper stage or will I be generating more overshoots or other anomalies?

Carmine

Ow, I completely overlooked this situation! Yes, because some people didn't seem to understand that Post Amp (which affects audio but not pilot and rds) should not be used to reduce FM modulation, I decided to just make that setting do nothing on FM outputs. But - you're using it for AM.

If I understand you correctly, I should add 75 us pre-emphasis to the AM panel (that way you don't have to enable FM), right?

The 75 us pre-emphasis should stay there when you use FM output (it did in 7.40), I don't see how you could get any overshoots there?

Hi - Yes. It would be best to have a "stand alone" AM Transmitter section with selectable pre-emphasis similar to FM. It would be helpful to see the output signal just like the FM section as well.

I would assume most everyone would use 75us in the U.S. Having the ability to select the pre-emphasis curve (or even making it continuously adjustable) and to use corresponding de-emphasis would be helpful for testing as well. The NRSC link I included in my original post presents the technical reference and parameters if you will be modifying the AM portion of ST.

Allowing positive peaks to 200%, even though not legal in the U.S. for AM broadcast, is useful for experimentation and amateur radio use. The ability to flip the phase of the peaks to positive or negative (you have it labeled as peaks on top or bottom - a English translation interpretation) is very useful too. I would suggest you don't change those features.

How will the processed output be varied to feed the AM transmitter? Like it was in FM in 7.40 with the slider?

Also, tilt correction, and high frequency delay and shelf controls to minimize ringing and overshoot are especially needed in AM transmission to correct for transmitter frequency response inadequacies, especially for older AM transmitters to achieve maximum modulation. So it's no longer FM only correction.

I really appreciate your efforts on this. I think this will be helpful for those wishing to use Stereo Tool use as an AM broadcast processor.

Carmine

Until very recently I actually thought that almost nobody used Stereo Tool for AM. Mainly because in the area where I live, AM is as good as dead - and has been for decades. Here in the Netherlands there are hundreds of FM stations, and only 4 AM stations (2 Christian stations, 1 simulcast of an FM station that has bad coverage in a part of the country, and 1 public station that mainly broadcasts talk programs for minorities). I only heard recently that AM is still very much in use in the US. I want to dive into this some time soon - also to support the whole roll off behavior around 10.2 kHz.

Great! There are just under 5000 licensed AM broadcast stations operating in the U.S. believe it or not. Many of them are small, local, operations I think Stereo Tool can fill need for relatively inexpensive AM processing.

I look forward to testing any development you have on the AM side. As a bonus it can also be adopted for shortwave broadcasting that is still in use in certain areas like in South America. I would assume bandwidth limitations there would filter frequencies above 4.5 kHz, I believe that is the international standard.

The cool thing is that we (actually you) are dealing with only with modifying software and not hardware to achieve these results so things can be very flexible - like a audio processing swiss army knife!


Carmine

Hans,

I use stereotool also for AM pirate broadcasting! There are many stations active in the pirate scene on mediuwwave as well. And if you could read there is a big market in the USA. I think stereotool can get some market there. As the big commercial processors are quite expensive.

Regards,

Michel

I have used ST with good results in the 'states. The suggestions for mods are valid. There's a lot of potential here.