AM Radio Emulator

Of course there are two parts to that:

- implementing the necessary filters that do the distortions like selective fading

- implementing a process that varies the parameters of those filters over time, preferably controlled by time-of-day (the effects are quite different during day and evening), seasonal effects, random input, etc.

Probably already has been done by someone, you only need to locate it.

When was the last time anyone in this newsgroup was accused of being gentle?

Sure. I do something like that to test the susceptibility of communications schemes in a controlled environment: Also look for TIMS or "transmission impairment test set" such as the HP4934A through HP4947A: I have an HP4935A somewhere in my mess. I don't recall what flavors of noise it will generate, so don't rush out an buy one before reading the manual.

However, that's probably overkill for what you're doing. Since you're working with clean demodulated audio, what you want can be done with a simple mixer or two. One mixer acts as an audio attenuator to cause the audio level to change, roughly simulating changes in AM signal strength. The other mixer is used to inject random levels of either digitally generated or pre-recorded noise with the audio. The combination of these two effects should convert most any form of quality audio into garbage.

The whole thing could probably be programmed using a computah, but methinks you probably want something equally vintage to AM radio. Random can be specified by how often a pattern repeats itself. If you want slow and random variations in audio and noise levels, simply use two or more potentiometers in series to set the mixer levels, driven by slow motors running at non-harmonically related speeds. Throw in a few non-linear cams to make it more random. Maybe do it optically.

When you're done with the static and fading, I suggest you add various types of interference.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Selective fading needs a bit more to include sideband phase and amplitude imbalances. You may need to modulate the signal on a carrier and then fudge the sidebands before demodulating it.

--

-TV

Of all the things I want for Christmas, noise, fading, and various types of interference are not on the list.

Mikek

Here's a free iZotope plugin that generates varying degrees of vinyl snap, crackle, pops and such. I thought of spoofing my friends with a noised up audio file and then claim the original was an example of my editing skills. ;-) For sounding like AM, DX, QRM, QRN and stuff I'd be inclined to checkout the effects of Adobe Audition

Methinks that varying the amplitude and ignoring the phase of the received RF signal is a tolerable simulation of various forms of fading (Rayleigh, Rician, Nakagami, Weibull, etc), especially since there is no attempt here to do any measuring. I suspect the OP is into nostalgia and trying to turn back the clock to the days when all forms of recorded and broadcast music sounded like garbage. I don't believe that a precise simulation of said garbage will add much to the nostalgic experience.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

About 40 years ago I converted a R101/ARN6 Direction Finding receiver to so lid state. With a half way decent antenna I could listen to station hundred s of miles away , for hours with no fading. Most days, I could hear WSM fro m Nashville, to a few miles South of Dayton Ohio until the early afternoon. Multiple RF and IF stages made a huge difference. The fun part of converti ng it were that most of the tubes were 12SN7, dual triodes so I soldered FE Ts across the bottom of the tube sockets just to see if I needed to make an y other changes. The Pentagrid converter was fun, but once I removed the 40

0Hz servo supply, the power consumption dropped from 24V/28A, to working fr om a Nine volt transistor radio battery, and feeding an external audio ampl ifier. Sadly, that radio was stolen about 32 years ago. I picked it up at a hamfest for $5, and told it was defective. It was. Somehow, it had left th e factory with one of the Heater pins not soldered to ground. Now, those ra dios go for hundreds, in damaged condition. Fair Radio still had a huge pil e, when I bought mine and they were cheap.

You must be too young to remember the glorious days of AM radio, then. For those of us of "advanced years" (the "vile boomers" as some here would call us) there is something almost magical about hearing your favourite old songs rendered just the way you heard them half a century ago.

--

"When constituencies are small their elected representatives must concern themselves with the local interests of their constituents. When political representatives are distant and faceless, on the other hand, and represent vast numbers of unknown constituents, they represent not their constituents, but special interest groups whose lobbyists are numerous and ever present. Typically in Europe a technocrat is an ex-politician or a civil servant. He is unelected, virtually impossible to dislodge during his term of employment and has been granted extensive executive and even legislative power without popular mandate and without being directly answerable to the people whose interests he falsely purports to represent."

- Sir James Goldsmith (Member of the European Parliament) 1933 - 1997

"Glorious days of AM radio"? When was that, 1948?

And of course the hiss was much crisper back when we had hearing above 8 kHz. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

IMHO, simulation of multipath is essential to get the high distortion during deep fades. When a multipath notch takes out just the carrier but not sidebands, a simple envelope (diode) detector doesn't like it and produce strong overmodulation distortion, With carrier strongly attenuated and the sidebands at different phase this will cause all kinds of interesting effects :-(.

To simulate this, you would need an AM modulator, a very deep tunable notch and a half wave rectifier. A similar effect without a sharp filter cold be tested by attenuating the AM modulator carrier with full audio (to cause strong overmodulation) and attenuate the whole produced signal, Of course all this can be done on a computer program,

Sigh. This is beginning to sound like an audiophile discussion on the accuracy of various audio compression techniques. At some point, one has to cease trying to achieve perfection and be satisfied with whatever might be considered adequate. Some people are never satisfied, while others, like me, will graciously accept mediocrity so that they can move on to something more interesting. In this case, the OP is starting with demodulated audio coming from a streaming media feed. Looks like the original URL is not quite correct. Try: or see: for other streaming options. Looking at the HE-AAC 32 kbps, 22.05 kHz stream using Spectrum Lab: Eyeballing the spectrum display, I see about 60dB dynamic range (or about 30dB better than my 24 bit sound card). The streaming music being played has probably never seen an AM broadcast station, which compresses the audio as much as possible to make it seem "louder". To add additional realism, perhaps the OP should also add an overdose of over-compression.

Yep. The OP is working from demodulated AM audio. The carrier and sidebands are long gone (as if they were ever present on the stream). It would be necessary to rebuild the original RF AM modulation scheme by feeding the audio stream to an AM modulator, separating out the sidebands and carrier, introduce the necessary fading effects, and demodulate the results with a cat whisker detector (or worse), in order to accurately reproduce the desired distortion. I don't believe the result is worth the effort when compared to simply using the amplitude component of a fade simulation. If distortion at low levels is deems a requirement, then it can more easily be simulated by adding some non-linear distortion components when the fade simulating attenuator is operating at high attenuation.

A computah simulation would certainly make things easier, but based on previous discussions in this newsgroup involving the OP, I suspect that something more primitive is more to his liking. That's why I suggested asynchronous motorized attenuators for generating amplitude fading and pre-recorded noise, instead of a computah simulation.

I've been listening to the Zenith Classic Rock feed for the last hour. I can see what attracted the OP. Except for the UK version of English, it takes me back to the days when I was an aspiring juvenile delinquent and general misfit. At the time, I had an FM radio, which does not provide the desired fading and distortion effect. I guess I missed that, somehow.

Ah, nostalgia...

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I'm afraid the use of 'puters wouldn't be authentic; it wouldn't sound quite the same as an analogue method. For example, there's a particular form of fading we used to get on good ol' AM which sounds exactly as if the tape is poorly tracking the playback head and about to come off altogether. It would take an awful lot of hard work to reproduce that with a 'puter - and it just wouldn't sound the same anyway.

--

"When constituencies are small their elected representatives must concern themselves with the local interests of their constituents. When political representatives are distant and faceless, on the other hand, and represent vast numbers of unknown constituents, they represent not their constituents, but special interest groups whose lobbyists are numerous and ever present. Typically in Europe a technocrat is an ex-politician or a civil servant. He is unelected, virtually impossible to dislodge during his term of employment and has been granted extensive executive and even legislative power without popular mandate and without being directly answerable to the people whose interests he falsely purports to represent."

- Sir James Goldsmith (Member of the European Parliament) 1933 - 1997

Jeff Liebermann wrote in news: snipped-for-privacy@4ax.com:

http://148.163.81.10:8006

Zenith is the zenith of classic rock!

If the multipath takes out the carrier (and possible other sideband), the AM transmission is received as DSB (or even SSB). Try listening on a SSB transmission with a simple AM envelope (diode) dtector. It sounds just like Donald Duck speach.

During a fade, the perfectly good AM signal gradually becomes a DSB signal and later on returns to a normal AM signal.

On a computer if you multiply the carrier signal sin (2 * pi * fc * t) with the audio signal and you get sin (2 * pi * fc * t) * audio a double sideband (DSB) with two sidebands but no carrier. With the phasing method, either sideband can be canceled resulting in SSB. Assuming the audio signal can be anything between -1 and +1, the product has the same phase as the carrier when audio is positive but reverse phase when audio is negative. The product amplitude is the absolute value of the audio amplitude.

In an envelope detector, the phase information is lost and negative signals are mirrored to positive values, thus functioning as a frequency doubler and speech sounding Donald Duck and some music transposed one octave upwards.

In AM, the audio is DC biased so that it always positivem i,e, -1 ..

+1 to 0 .. 2 thus sin (2 * pi * fc * t) * (+1 + audio ) thus the product has always the same phase as the carrier and the envelope is between 0 and 200 % (corresponding to 0 ,, 400 % carrier power),

The only difference between the two formulas is the +1 DC bias, so if thatt DC bias is made gradually variable from +1 to 0 the signal changes gradually from AM to DSB, with known effects in the AM detector.

When listening on an AM transmission with a proper SSB receiver, it doesn't notice that the carrier is missing. With proper AGC, even the audio level doesn't change, only the vbackround noise will increase during the fade :-).

What, wait, you can 8kHz? I'm down about 60db at 8kHz! I just had a hearing test last week, thinking it might be time for a hearing aid. The ear doc said it would be a waste. I was surprised and expected a hard sales pitch, but didn't get it. I have trouble with my wife, she mumbles! ;-) And when I'm in a noisy environment I have difficulty. But zero problem at breakfast with the guys. If I went to a lot of social events where the noise around me caused a problem, I might push it, but that's more of a rarity.

Mikek

Here's a recording that has limited audio, but no fading. >

Listen to a lot of OTR (Old Time Radio) Mostly 20th Century Radio,

And I like the programming on Hanks Gumshoe, but the audio there often is poor, I have complained, and it got better, but after time I think he starts turning those knobs back to 11, because that's better. :-(

Mikek

The WSM was/is at the low end of the MF BC band so the ground wave propagation is quite good. Apparently there was much less man made interference than today.

Of course there were no multipath fading during the day, since the D-layer took out the sky wave, so there were nothing that could cause multipath during the day.

solid state. With a half way decent antenna I could listen to station hund reds of miles away , for hours with no fading. Most days, I could hear WSM from Nashville, to a few miles South of Dayton Ohio until the early afterno on. Multiple RF and IF stages made a huge difference.

Normally, the signal would start to fade just before sunrise.

There were no switching power supplies to speak of, at that time. Just inte rference from TVs left on after stations went off air at night, that let th e harmnonics wipe out 650 KHz. I had the same problem 10 years later with a neighbor's TV wiping out 60 KHz. My WWVB antenna but out seven Volts P-P a t 60 KHz but that TV would drive it into clipping and I would lose my frequ ency standard until 5:00AM

You've just verified that you're a true audiophile. No matter how good the reproduction, the original is always better. Allowing this to continue, you should eventually be complaining that the reproduction doesn't smell (burning dust and rubber insulation on the tubes) like the original.

Hardly. I have an old Fidelipac NAB cartridge player stored somewhere that will easily reproduce the sound of a misaligned and loosely secured head. Just add it to the computerized fading and over-compression simulator and the reproduction should be indistinguishable from the original by anyone except a genuine audiophile. The only problem will be that you'll need to pre-record a pile of cartridges for the days listening. I don't think it can do simultaneous record and playback. However, it should be possible to add a record head and record electronics.

Your new motto should be: "Perfection can always be improved".

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558