I need a new project and was thinking about dinking around with AM audio processing.
There seems to be very little information out there on how to properly implement asymmetrical modulation where you can get up to positive 125% modulation and no more than 95% negative. Reason for this because sudden modulation dropout or negative modulation beyond 100% causes nasty adjacent splatter and audible distortion. Positive modulation above 100% can increase loudness on the dial at an expense of light audible distortion but without the effects of splatter.
So, most broadcast stations on the dial do something around -95% modulation and +105-125% modulation. In other words the modulation is not perfectly symmetrical but of course loudness wars on the AM dial is the game and in some ways can get your signal heard farther out. Anything beyond a certain point in positive modulation can cause distortion in modern radio receivers so there is a practical limit between loudness and outright distortion.
I found a few ideas to increase modulation at little expense to audio quality. One of them is the Kahn Symmetra-Peak used in AM broadcast studios for eons. This is nothing more than an all-pass filter that basically acts like a phase delay where the higher the audio frequencies coming in the more the phase delay. This has the effect of causing asymmetrical audio (like voice) to kind of balance out so the negative and positive peaks are mostly at the same level. This is VERY useful for voice since many peoples voice tends to either peak positive or negative from the zero point on an oscilloscope which means your voice will ride highly positive or negative. Your voice may vary. The Symmetra all pass filter simply removes any weird subsonic components in your voice that causes DC shift I think. This is useful when using your mic into a compressor or limiter since they look for the highest peak either positive or negative, and the voice naturally will peak to one side and the limiter will cause itself to kick in on one of those peaks. This means your voice is being limited or compressed before it really should and reduces overall loudness which can be a huge issue for AM broadcasting if you want your signal to be loud and clean. Equalizing the peaks, much like how a phase flipper works by picking the peak strongest to ride positive, will allow the broadcast voice to be louder within the same modulation. Many digital broadcast processors even have auto phase flipping effects now to gain loudness on talk radio.
BLAH BLAH.
Anyways TL;DR.. Thinking about building some of these things just to test out. The two projects rolling around in my head are this one here..
and.. the idea of making a simple audio limiter circuit with an opamp but ONLY limit the negative side of the audio waveform. Normally audio limiters or compressors limit both positive and negative peaks but would it work to simply limit only the negative portion before the audio enters the AM transmitter? This way it would allow +100% modulation but hard limit anything below -100% modulation.
I can imagine both of those effects can be used on AM broadcast, CB/Ham radio and basically any voice AM transmission to get the maximum sound volume within the modulation limits.
Interested to see if anyone here has any information or ideas on this. There is not a lot of information online for these topics and only one person I can find who released their own schematics for a home made AM broadcast audio processor.
Ham radio operators seem to have some information on these things but quite limited at that.
Not sure where this project will go but I have been thinking about building a neat little AM broadcast audio processor using analog parts, something that can be used for mediumwave or shortwave pirates within simplicity to create HIFI broadcast audio to maximise within reason the overall loudness and regulate modulation output much like professional units.
I am just getting heavy into studying this but I believe the chain of circuits would be..
Audio in > all-pass filtering (phase scrambler) > limiter > pre-emphasis for AM > 10kHz LPF (9kHz euro) > clipper > to transmitter
Quite simplified. I know professional processors at their basics include a high frequency limiter along with the all band limiter, and some other stuff, not even going into multi-band compression.
However the idea here is to make a step up from the current pirate transmitters I see using nothing more than a simple audio limiter.
I need a new project and was thinking about dinking around with AM audio processing.
There seems to be very little information out there on how to properly implement asymmetrical modulation where you can get up to positive 125% modulation and no more than 95% negative. Reason for this because sudden modulation dropout or negative modulation beyond 100% causes nasty adjacent splatter and audible distortion. Positive modulation above 100% can increase loudness on the dial at an expense of light audible distortion but without the effects of splatter.
So, most broadcast stations on the dial do something around -95% modulation and +105-125% modulation. In other words the modulation is not perfectly symmetrical but of course loudness wars on the AM dial is the game and in some ways can get your signal heard farther out. Anything beyond a certain point in positive modulation can cause distortion in modern radio receivers so there is a practical limit between loudness and outright distortion.
I found a few ideas to increase modulation at little expense to audio quality. One of them is the Kahn Symmetra-Peak used in AM broadcast studios for eons. This is nothing more than an all-pass filter that basically acts like a phase delay where the higher the audio frequencies coming in the more the phase delay. This has the effect of causing asymmetrical audio (like voice) to kind of balance out so the negative and positive peaks are mostly at the same level. This is VERY useful for voice since many peoples voice tends to either peak positive or negative from the zero point on an oscilloscope which means your voice will ride highly positive or negative. Your voice may vary. The Symmetra all pass filter simply removes any weird subsonic components in your voice that causes DC shift I think. This is useful when using your mic into a compressor or limiter since they look for the highest peak either positive or negative, and the voice naturally will peak to one side and the limiter will cause itself to kick in on one of those peaks. This means your voice is being limited or compressed before it really should and reduces overall loudness which can be a huge issue for AM broadcasting if you want your signal to be loud and clean. Equalizing the peaks, much like how a phase flipper works by picking the peak strongest to ride positive, will allow the broadcast voice to be louder within the same modulation. Many digital broadcast processors even have auto phase flipping effects now to gain loudness on talk radio.
BLAH BLAH.
Anyways TL;DR.. Thinking about building some of these things just to test out. The two projects rolling around in my head are this one here..
and.. the idea of making a simple audio limiter circuit with an opamp but ONLY limit the negative side of the audio waveform. Normally audio limiters or compressors limit both positive and negative peaks but would it work to simply limit only the negative portion before the audio enters the AM transmitter? This way it would allow +100% modulation but hard limit anything below -100% modulation.
I can imagine both of those effects can be used on AM broadcast, CB/Ham radio and basically any voice AM transmission to get the maximum sound volume within the modulation limits.
Interested to see if anyone here has any information or ideas on this. There is not a lot of information online for these topics and only one person I can find who released their own schematics for a home made AM broadcast audio processor.
Ham radio operators seem to have some information on these things but quite limited at that.
Not sure where this project will go but I have been thinking about building a neat little AM broadcast audio processor using analog parts, something that can be used for mediumwave or shortwave pirates within simplicity to create HIFI broadcast audio to maximise within reason the overall loudness and regulate modulation output much like professional units.
Hi there.... There is most likely only a small hand full of people still alive, with first-hand knowledge on AM-Type Audio Processing in the analog domain that can hold the Mod-Monitor at 99.5% while allowing Pos-Peaks to shoot to 125+%. That said, I am a broadcast engineer, retired as well as a circuit designer, with the same goal in mind. I worked with Ron Jones, creator of CRL - Circuit Research Labs out of Phx. AZ. Much to my surprise, the schematics of the early CRL "Boxes" are now posted on the web !!! Ron's 'phase-scrambler' worked VERY Well and his use of MPF102 FETs for gain reduction was also brilliant. Orban-CRL spent a lot of time and money perfecting these devices, one Link: ftp.orban.com/CRL/AGC400%20Manual.pdf and for the Dynafex DX-1, look at the last page of this link: ftp.orban.com/CRL/DX-1%20Manual.pdf
Feel free to e-me direct at: PCRDave@gmail.com Good Luck & GREAT DXing..... Dave....
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