Very Basic Advice Needed

[arg2013]

Hi everyone,

I am not a professional broadcaster. In fact, I am working on a simple radio demonstration for some people. I need to show them the typical sound of processed FM audio. I already have an Orban Optimod at my disposal.

I need to know the cheapest solution to de-emphasize the signal after the Optimod. I need to show them the difference between 50µs and 75µs.

Since I am not transmitting the signal over the airwaves, I don't need a $1,000+ transmitter.

So what would you all suggest as the chepeast way to do the final de-emphasis? Specific brands or models would be very helpful.

Thank you everyone.

 

[TomZ]

Stay tuned. Someone will have your answer.

 

[arg2013]

Stay tuned. Someone will have your answer.

I hope so. I just need the cheapest piece of equipment to do 50us and 75us de-emphasis and then output that signal to speakers.

Thanks.

 

[Lazy J]

You mentioned that you have an Orban Optimod at your disposal. Depending on which model of Optimod, you can just use the audio output and turn the pre-emphasis on and off in the software. If it's one of the older Optimods (8000a or 8100a), you'll have to come up with a different solution.

 

[arg2013]

You mentioned that you have an Orban Optimod at your disposal. Depending on which model of Optimod, you can just use the audio output and turn the pre-emphasis on and off in the software. If it's one of the older Optimods (8000a or 8100a), you'll have to come up with a different solution.

Yes my Optimod has pre-emphasis but I need a cheap piece of equipment to go after the Optimod to de-emphasize the signal.

I hope that makes sense.

Thanks.

 

[boiseengineer]

http://www.repeater-builder.com/tech-info/pdemph-post.html

 

[arg2013]

http://www.repeater-builder.com/tech-info/pdemph-post.html

Thanks but I know what pre-emphasis and de-emphasis are. I am looking for a piece of equipment to purchase to do the de-emphasis. Something not very expensive if possible.

 

[bilco]

Pad the output way down and run it into a Mr. Microphone or other cheap FM transmitter. One of the little devices used to to feed your CD pllayer or cell phone to your car FM stero should work just fine. I have one I bought at Fry's for about 9 bucks. There are several out there that take audio in and may have enough bandwidth to pass the composite signal. they should at least pass the L+R mono signal and that would work for your demonstration. Then just turn the pre-emphasis on and off in the Optimod.

 

[kenglish]

Years ago, I needed to build a "side-chain" that would bridge audio from the input feed of a compressor/limiter, add 75 uS PRE-emphasis, and use that "boosted" signal to control the proc. I just added an R-L (resistor and inductor) network to the output of a "Stick-On" amplifier.

Maybe an RDL STA-1 (Radio Design Labs) amplifier, and a (complementary to the above) 75 us R-C network, plus another 50 uS R-C network and a switch, would work here. The STA-1 would provide a balanced load to the Optimod, and the output of the network and switch could feed a good hi-fi amp and speakers/headphones.

 

[frankberry]

A de-emphasis network is simple ... if you know the input impedance of the following amplifier stage.
The basic de-emphasis consists of a resistor in series with the audio and a capacitor from the output of the resistor to ground.
This is an example: a 100k resistor and a 750pf capacitor will produce a 75us de-emphasis. Changing the capacitor to 500pf will produce a 50us de-emphasis.
Of course, you can change the series resistor to 10k and increase the value of the capacitors by a factor of ten.

The difference between 50us and 75us at 15kHz is about 3.5dB.

 

[OKCRadioGuy]

Thanks Frankberry for answering his real question. Nice to know how to REALLY build a de-emph circuit. Now older processors can be used for fake-out air monitors (for those with digital delay) and stuff like that

 

[RadioFan2J3]

I need to know the cheapest solution to de-emphasize the signal after the Optimod. I need to show them the difference between 50µs and 75µs.


So what would you all suggest as the chepeast way to do the final de-emphasis? Specific brands or models would be very helpful.

Thank you everyone.

Maybe I am missing something, but if you have the matching de-emphasis network after the Orban, then in theory the total system frequency response is flat again.

On the other hand, by using the wrong (well, opposite) de-emphasis network, you could demonstrate the effects of setting the Optimod with the wrong pre-emphasis for the region where the system is going to operate.

Perhaps another question would be related to the operation of the Optimod - does it have significantly different audio processing characteristics when switched between the two pre-emphasis settings?

 

[rfry]

Probably most analog FM stereo broadcast audio processors are optimized to provide an "FCC" stereo composite waveform compatible with the baseband spectrum and input requirements of most analog FM stereo exciters and receivers designed for licensed, analog stereo FM broadcast service.

Some of those processors do not output discrete left- and right-channel audio waveforms. In such cases, audio de-emphasis of their encoded, composite, FM stereo output waveforms in the baseband spectrum of about 50-15,000 Hz will not produce what you wish to demonstrate, in stereo.

Two choices remain.

1. Apply the composite output of the FM stereo processor to the input of an FM exciter compatible with that encoded waveform, and demodulate its FM stereo r-f output waveform using a good FM stereo receiver.

Beware of the limits for such unlicensed, radiated signals permitted by FCC Part 15.239.

2. Apply the compatible, analog composite output of such an FM stereo processor to an analog FM stereo baseband demodulator such as the Belar FMSA-1 -- link: http://www.belar.com/FM/fmsa1.htm

Sorry, but very basic questions don't always result in simple or inexpensive answers/solutions.

RF