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"John Byrns" wrote in message ... In article , 2. The idea expressed above that a "modern sophicated decompressor circuit could match the curve of the compressor" seems far fetched to me. In the days of yore when audio processing consisted of a single broad band compressor, and a broad band "peak limiter" one might have contemplated this, at least as far as the compression part went, but today's audio processing is much more complex. Processing today involves broad band AGC, multiband compressors, plus multiband and broadband clippers in place of the old "peak limiter". It isn't clear to me that this would be easy to undo, or even possible. I don't know if the multiband aspect creates problems for reversing the process or not, but how do you undo clipping, and if there are any feed forward compressors involved it is possible that the output isn't even a single valued function of the input, making recovery mathematically impossible. Sure, the modern decompressor might be farfetched. There's no doubt that some things can't be recovered. Maybe the best that can be done is to more or less de-process the AM radio signal. I think audio processing is a bigger problem than bandwidth, distortion or noise for a real high fidelity AM tuner. And I think it's a problem which deserves it's fair share of consideration. Frank Dresser |
Have you considered a test bed for your hi-fi AM radio experiments? Bill's
comments about IF transformers got me thinking that one approach would be to get an old 70s era stereo receiver with IF transformers and no crystal/ceramic filters in the IF strip. These things are pretty inexpensive at the Salvation Army type stores now. You could try stagger tuning the IF transformers for a wide bandwidth. Ideally, you'd use a sweep generator, but you could probably do a passable job with a standard signal generator and some patience. It's not a tube setup, of course, but you'd get wide bandwidth AM with a reasonably low noise wideband amplifier. Frank Dresser |
John Byrns wrote: OK, now we are getting down to brass tacks as my Grandmother used to say, whatever that may mean. The $64 question is does the Amigo as used at WMER only do NRSC, or does it have a selection of cutoff frequencies like the Optimod 9200, and if it does what is the cutoff frequency set to at WMER? Yes, only NRSC - but you can "tailor" for various "ends" (i.e. talk; "punch", etc.). - it's the "cheap seats" version - (I.e. CLR) vs. the optimod (Orban - though now they are the same company). best regards... -- randy guttery A Tender Tale - a page dedicated to those Ships and Crews so vital to the United States Silent Service: http://tendertale.com |
Phil B wrote: Randy, You get the award for most informative post concerning the "broadcast standards" in this thread. I was waiting for you to come through. It takes someone with real broadcast experience to give us the real scoop. Thanks. (as goofy would say - garsh!)... Hey - when one must keep the station owner's butt (well - at least his wallet) out of the jaws of pain (in spite of the FCC's reputation - they can and DO still bite on occasion) one sorta has to keep up - whether one wants to or not. And in today's internet published world - finding technical details is (usually) just a search engine away. Besides I'm none too proud to ask some of my friends and business acquaintances (like Olen Booth of Broadcast Engineers) who are eye-ball deep in this stuff everyday of the world. Guys like him DO know it like the back of their hand. best regards... -- randy guttery A Tender Tale - a page dedicated to those Ships and Crews so vital to the United States Silent Service: http://tendertale.com |
Brian wrote: Randy, those figures are not characteristic of modern processors that use DSP filtering, which is capable of extremely rapid rolloff. Take a look at http://n2.net/k6sti/speech.jpg . This is a screen shot of my HP 141T/8553B/8552B spectrum analyzer tuned to a local AM radio station broadcasting speech. The analysis-filter bandwidth was 300 Hz, the vertical scale 10 dB/div, and the horizontal scale 5 kHz/div. Yep and it shows that at around 7.5 it's down 10db - and not symmetrical at all- I.e. their pushing the positive modulation WAY harder than negative - which is probably ok for voice - but would sound pretty lame for music. Their "tilt" would make them "punchy" all right - (make most listeners punch-drunk - esp. women who (on average) dislike tinny sounding stuff - and have the ears to hear it. http://n2.net/k6sti/music.jpg shows a different AM station broadcasting classical music. The music spectrum is evident, but so is the brick-wall filtering at 10 kHz. These spectra are typical of what I observe for AM stations here in Southern California. So is the -25db roll-off by 7.5khz - this indeed looks like good conformance to NRSC-1. If you have a receiver capable of SSB reception, Yeah - a few - but I also have an Empire NF-105 calibrated RF / Noise meter that goes from 14Khz to 1000Mhz. It tells me what I need to know. best regards... -- randy guttery A Tender Tale - a page dedicated to those Ships and Crews so vital to the United States Silent Service: http://tendertale.com |
In article ,
says... wrote: Jon Noring wrote: Well, being the "OP", I want a high-audio performance, modern design AM tuner to integrate into my audio system -- and I believe a lot of tube-o-philes likewise want that -- but not everyone obviously. There are several reasons why most higher-grade audio systems use separate components, the reasons of which are obvious to most everyone. The AM tuner is no different than other audio components in this regard. Maybe what you want is the old JW Miller passive AM tuner. No active devices at all, just a bunch of tuned circuits and a detector diode. Yes, possibly. John Byrns has written about the J.W. Miller TRF AM tuner quite a few times Back in the 70's I built a passive diode detector receiver for the broadcast station I worked for. The transmitter was only a mile away. It was flat out to 10 KHz, and we used it for a line-level signal that was usually used for the control room monitor speakers. We could A-B the line out going to the transmitter and the received signal. It was good for understanding how the audio limiting at the transmitter was affecting different kinds of program material. -- Regards, Terry King ...In The Woods In Vermont "The one who dies with the most parts LOSES! What do you need??" |
In article , wrote:
John Byrns wrote: 3. TRF receivers have been mentioned, and everyone seems to assume that a TRF receiver would consist of cascaded single tuned resonators with RF amplifier stages between. There is no reason why double tuned circuits, similar to those used in the IF transformers of a superhetrodyne can't be used in a TRF receiver, with all the selectivity/bandwidth benefits that brings to the party. For examples see the Western Electric No. 10A receiver, the J.W. Miller TRF receiver, the early Altec AM receiver, as well as others. I did a cursory check on the Internet, but did not yet find any schematics for the mentioned receivers. Are they online somewhere? Anyone? I also found the following article from John posted back in 2000, where he talks about the double tuned TRFs, such as WE-10A, J.W. Miller, Collins (which I assume is the same one Volker Tonn mentioned today), Meissner, and the Weeden (the last of which John noted to be the best designed of all of them): http://groups.google.com/groups?selm...&output=gplain Unfortunately the URLs to the TRF schematics at John's site are not working. Try this URL: http://users.rcn.com/jbyrns/BP-TRFs.html Another bandpass receiver which uses J.W. Miller coils is the Altec 101(B), the schematic for the 101(B) is available on the "Nostalgia Air" web site. After reviewing this thread, and considering that the proposed tuner is to be an audiophile tuner, and given the great affinity of audiophiles for Western Electric audio equipment it is clear to me that the tuner of choice would be an updated version of the Western Electric No. 10A receiver. The tubes used by Western Electric receiver are obsolete and should be updated to more modern tubes in the revised design. The detector in the Western Electric design is also inadequate having high distortion, and due to the square law characteristic it modulates the AGC voltage at the syllabic rate of the audio signal. Modifications to update the tubes, resolve the detector and AGC problems were available by the early 1950's, or earlier. There is one remaining problem with the W.E. 10A that hasn't yet been addressed to the best of my knowledge, and that is the aperiodic antenna input circuit, that is likely to be a source of RF 3rd order IM problems which will wipe out some weaker signals if you live near a number of 50,000 Watt flame throwers, or even 5,000 Watt stations. This problem could be eliminated by deleting the first RF amplifier stage, which is probably serious overkill with modern tubes anyhow, and making the first bandpass filter a tuned antenna circuit. This is the reason that I prefer the Weeden and Altec designs, they both have tuned antenna circuits to minimize RF IM problems. If one wanted to duplicate an existing circuit the Altec is probably the best choice as it has a tuned antenna circuit and uses modern 7 pin miniature tubes. One addition that would be necessary for any of these tuners today would be the addition of a "NRSC" deemphassis network to compensate for the preemphassis that is used in AM broadcasting today. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
Jon Noring wrote: [New Yahoo Group started: "AM Tube Tuners". See end of this message for more info.] In the last couple of years I've posted various inquiries to this and related newsgroups regarding high-performance, tube-based AM (MW/BCB) tuners, both "classic" and modern. I'm very interested in building such a tuner to match with audiophile-grade tube amplifiers and pre-amplifiers now being built by hobbyists (as well as those sold by commercial vendors.) There are quite a few nice kits now being marketed for audiophile quality tube amps/pre-amps, such as those made by diytube (http://www.diytube.com/ -- there are many others like diytube.) So why not similar kits (or workable designs) for a tube-based AM tuner? My Hi-Fidelity AM Tuner is a Eico HFT94. I have two of them. Built both around 1960. Not many AM transmitters left broadcasting a worthwhile signal in North America. But they do well on what is left in this area, away from interference. If anyone is living in an apartment, don't bother trying for good AM. Even the manufacturers of SS receivers do a poor job with their AM section, recognizing the many problems external & out of control of the tuner. Cheers, John Stewart |
Yep and it shows that at around 7.5 it's down 10db - and not symmetrical
Take a look at the screen shot again. Don't the extremely steep spectral walls--the sudden drop of tens of dB at 10 kHz--suggest something to you? The inner spectral detail is interesting, especially since the lopsided spectrum did not occur later in the day on different program material (see http://n2.net/k6sti/later.jpg), but it's the spectral boundaries that tell the story. So is the -25db roll-off by 7.5khz - this indeed looks like good conformance to NRSC-1. That spectrum was recorded during classical music with little high-frequency content, hence the sloping spectrum. What's interesting is where it suddenly vanishes at 10 kHz. For preemphasis shape, take a look at http://n2.net/k6sti/noise.jpg . I recorded this later on the same station during a quiet piano passage. The spectrum beyond 3 kHz is background noise. It shows the expected preemphasis shape right up to 10 kHz where the spectrum suddenly vanishes. I did discover two local signals whose spectrum did not extend to 10 kHz. Both rolled off very rapidly at 8 kHz. When I realized that both transmitters were in Mexico, I thought I was on to something. I found a third strong Mexican signal, but its spectrum extended right out to 10 kHz like the US stations. Brian |
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