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I guess that I must be in the minority - it seems to me that for best AM
fidelity (not selectivity, nor sensitivity), you would use a crystal set with tuned RF stages, no IF, no heterodyne of any kind. use the tubes for RF amps if needed, and for audio amplification, and use a tube diode for the detector. |
In article ,
Patrick Turner wrote: Syl's Old Radioz wrote: "Patrick Turner" a écrit dans le message I don't expect anyone to pay 3c for what I say, which could be seen as OT. You just met our village idiot it seems... There is an unspoken rule here..._Ignore_ his posts. Let him talk to himself. We don't get into fight with village idiot like you do on RAT...Keeps rar+p "clean"...;o) Syl Well, with all due respects to all gentlemen and possible idiots on all groups to whom this subject thread is cross posted to, I reserve the right to decide who I will ignore or not. I will desperately try not step on anyone's toes as I act in well intentioned freewill. I won't budge from the idea that its possible to digitise the signal from the antenna and simply apply suitable algorithms, and get digital decoding, without all the phase shift caused by consecutive tuned circuits. Chill dude. There is nothing wrong with this idea and the current technology can do it. The problem is money. It would be expensive to do this and I would not expect people to pay the price when it would be a small improvement over the current generation of radios. Heck, I would not expect people to pay the price for a large improvement. Digital techniques do not end all distortion and add there own type of noise by the way. -- Telamon Ventura, California |
william_b_noble wrote:
I guess that I must be in the minority - it seems to me that for best AM fidelity (not selectivity, nor sensitivity), you would use a crystal set with tuned RF stages, no IF, no heterodyne of any kind. Use the tubes for RF amps if needed, and for audio amplification, and use a tube diode for the detector. Actually, this setup intrigues me for local reception, since it appears to be a quite simple circuit. Are there any schematics of such a circuit -- any commercially made radio of yesteryear using this design approach? Jon |
John Byrns wrote: 1. It has been variously stated that the audio bandwidth of AM broadcasting is either 3.5 kHz, 5 kHz, or 10 kHz. In the US AM broadcast channels are 20 kHz wide, so audio is effectively limited to a maximum of 10 kHz by law/regulation. It is my impression that most AM stations transmit audio out to this legal maximum. Of course as HD-radio takes hold this will change with the analog signal cutting off somewhere around 5 kHz. I know there are at least 2 active broadcast engineers that read this group, perhaps they could fill us in on what the stations they are involved with are actually doing as far as audio bandwidth goes? We've kicked this around the block before - but I guess it won't hurt to kick it one more time. (I'm only going to address US standards here). The AM Bandwidth is 10Khz. However - what has to be taken into consideration are "real world" filters - and the "stop band" specifications of the NRSC-1. I.E. the signal must be down 15db (from 100% modulation) AT 10khz(!!!) Further - it must be: -30db at 10.5Khz; -40db at 11Khz; and -50db at 15Khz. -50db is .32% modulation (that's point 32 percent - not 32 percent). Now - depending on how good your processor / final filters are - figure your "real world" bandwidth from there. Consider "good" filters at 12db per octave - and "really, really good" filters at 24db per octave (an octave is 1/2 (going down) or double (going up) a given frequency. So if you put a 12db per octave filter in front of your transmitter - the highest unattenuated frequency through that filter will be around 4.5Khz. You can do much better with a 24db per octave - somewhere around 7.5Khz. Of course - NRSC-1 is no longer the "newest kid on the block" - the new one is ITU-R (Recommendation 328-5). To meet that spec. - the processor filters are set to 6.0Khz. Amigos and Optimods are set up to meet NRSC-1 power spectrum requirements "out of the box". The Optimod 9200 (the current top of the line digital AM processor) is adjustable from 4.5kHz to 9.0kHz in 0.5kHz steps, plus NRSC - is guaranteed to meet ITU-R (Recommendation 328-5) and NRSC-1 power spectrum specifications without the need for further low-pass filtering prior to the transmitter. And as noted -- is typically set for 6.0kHz for ITU-R. Amigos or Optimods are probably in 90%+ AM stations in the US (We're (WMER) is running an Amigo). Here is the NRSC site for those wishing to get thoroughly tech: http://www.nrscstandards.org/ And NRSC-1 itself (PDF document - needs acrobat reader) http://www.nrscstandards.org/nrsc-1.pdf 2. The idea expressed above that a "modern sophicated decompressor circuit could match the curve of the compressor" seems far fetched to me. Yeah, I agree: I can't imagine trying to "undo" what either the Optimod or the Amigo do to the audio; talk about multi-band; mutli-limit; multi-everything... sheesh. Here's Orban's Optimod 9200: http://www.orban.com/orban/products/..._overview.html You'll find a pop-up menu in the upper right of the page - you can view the features and specs. from there. Impressive stuff. 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 , Patrick Turner
wrote: John Byrns wrote: 7. It has been suggested that using a 2 MHz IF frequency would allow wider bandwidth than the standard 455 kHz IF frequency. I fail to see why this should be true. Because for the same Q value, the pass band would be 4 times wider Where is it written that the same loaded Q must be used for both filters? If you can change the center frequency, why can't you change the loaded Q? Within reason, for bandwidths typical of audio receivers, you should be able to build a filter at 455 kHz that has effectively the same response as a 2 MHz filter. There is no need to throw out the 455 kHz IF just to get wide bandwidth. Its difficult to make a 455kHz typical old IFT produce a nice flat topped 20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical coupled, or over critical or rabbit eared. I have tried all that. So you have tried all that and rejected the "pointy nosed", "flat topped", and "rabbit eared" response curves. I am left to wonder what sort of response curve you were looking for? Why not settle for a nice "flat topped" response curve and be done with it? Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
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. Phil B "Randy and/or Sherry" wrote in message ... John Byrns wrote: 1. It has been variously stated that the audio bandwidth of AM broadcasting is either 3.5 kHz, 5 kHz, or 10 kHz. In the US AM broadcast channels are 20 kHz wide, so audio is effectively limited to a maximum of 10 kHz by law/regulation. It is my impression that most AM stations transmit audio out to this legal maximum. Of course as HD-radio takes hold this will change with the analog signal cutting off somewhere around 5 kHz. I know there are at least 2 active broadcast engineers that read this group, perhaps they could fill us in on what the stations they are involved with are actually doing as far as audio bandwidth goes? We've kicked this around the block before - but I guess it won't hurt to kick it one more time. (I'm only going to address US standards here). The AM Bandwidth is 10Khz. However - what has to be taken into consideration are "real world" filters - and the "stop band" specifications of the NRSC-1. I.E. the signal must be down 15db (from 100% modulation) AT 10khz(!!!) Further - it must be: -30db at 10.5Khz; -40db at 11Khz; and -50db at 15Khz. -50db is .32% modulation (that's point 32 percent - not 32 percent). Now - depending on how good your processor / final filters are - figure your "real world" bandwidth from there. Consider "good" filters at 12db per octave - and "really, really good" filters at 24db per octave (an octave is 1/2 (going down) or double (going up) a given frequency. So if you put a 12db per octave filter in front of your transmitter - the highest unattenuated frequency through that filter will be around 4.5Khz. You can do much better with a 24db per octave - somewhere around 7.5Khz. Of course - NRSC-1 is no longer the "newest kid on the block" - the new one is ITU-R (Recommendation 328-5). To meet that spec. - the processor filters are set to 6.0Khz. Amigos and Optimods are set up to meet NRSC-1 power spectrum requirements "out of the box". The Optimod 9200 (the current top of the line digital AM processor) is adjustable from 4.5kHz to 9.0kHz in 0.5kHz steps, plus NRSC - is guaranteed to meet ITU-R (Recommendation 328-5) and NRSC-1 power spectrum specifications without the need for further low-pass filtering prior to the transmitter. And as noted -- is typically set for 6.0kHz for ITU-R. Amigos or Optimods are probably in 90%+ AM stations in the US (We're (WMER) is running an Amigo). Here is the NRSC site for those wishing to get thoroughly tech: http://www.nrscstandards.org/ And NRSC-1 itself (PDF document - needs acrobat reader) http://www.nrscstandards.org/nrsc-1.pdf 2. The idea expressed above that a "modern sophicated decompressor circuit could match the curve of the compressor" seems far fetched to me. Yeah, I agree: I can't imagine trying to "undo" what either the Optimod or the Amigo do to the audio; talk about multi-band; mutli-limit; multi-everything... sheesh. Here's Orban's Optimod 9200: http://www.orban.com/orban/products/..._overview.html You'll find a pop-up menu in the upper right of the page - you can view the features and specs. from there. Impressive stuff. 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 |
John Byrns wrote:
In article , Patrick Turner wrote: John Byrns wrote: 7. It has been suggested that using a 2 MHz IF frequency would allow wider bandwidth than the standard 455 kHz IF frequency. I fail to see why this should be true. Because for the same Q value, the pass band would be 4 times wider Where is it written that the same loaded Q must be used for both filters? If you can change the center frequency, why can't you change the loaded Q? Admittedly it might be a bit more difficult to achieve the same Q at 2 MHz as 455 but we are talking about homebrewing and experimenting here. Its difficult to make a 455kHz typical old IFT produce a nice flat topped 20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical coupled, or over critical or rabbit eared. I have tried all that. So you have tried all that and rejected the "pointy nosed", "flat topped", and "rabbit eared" response curves. I am left to wonder what sort of response curve you were looking for? Why not settle for a nice "flat topped" response curve and be done with it? Here's a wacky idea that I'll toss out just to see if it flies... Could one use two garden variety 455kc xfmrs in series, one tuned at center plus and the other tuned center minus? Impedance matching would be an issue but maybe such a scheme offers a not so glamourous method of achieving the wider bandwidth and maintaining the flatness with little ado. Re wider bandwidth as a whole. On AM sets that I have owned with excessively wide bandwidth they all tend to sound like crap. I don't have a wealth of local stations that might be enhanced by the wider width but on weaker stations the amount of noise and all those AM "artifacts" seems to go way up making it very unpleasant to listen to. As a result I think it would make the most sense to use a switchable or continuously variable bandwidth scheme so as to not be left with an all or none scenario after so much effort. John B, you may remember one of my Tandberg receivers that had 4 positions of bandwidth ganged with a switch that somewhat tailored the audio accordingly. To the ear (or rather to my ear) this seemed very effective. -Bill M |
Now - depending on how good your processor / final filters are - figure
your "real world" bandwidth from there. Consider "good" filters at 12db per octave - and "really, really good" filters at 24db per octave (an octave is 1/2 (going down) or double (going up) a given frequency. 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. I set the storage-screen persistence to maximum and accumulated spectra for 10-15 seconds. It is easy to see the extremely sharp rolloff at 10 kHz. 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. If you have a receiver capable of SSB reception, you can easily check the spectral limits of any AM station. Put the receiver in LSB mode and tune down frequency from the carrier (or use USB and tune up). Regardless of program content, it will be obvious where the response ends. You'll hear the modulation sidebands suddenly vanish. Whenever I've tried this, my dial has always read more than 9 kHz away from the carrier. Brian |
Patrick Turner wrote:
Volker Tonn wrote: Jon Noring schrieb: 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. Have a look into the "Collins" S-series. These are state-of-the-art tube sets 'til now. At least it's not the tubes alone but the fabulous mechanical IF-filters giving outstanding results for a tube set. Manuals with layout diagrams should be available on the web.... Since Mr Noring says he has regularly trawled the Net for everyone else's expertise on AM reception, but got nowhere, because he's still doin it, why doesn't he gird his loins and put his shoulder to the task of learning all about AM and radio engineering as spelled out so clearly in all the old text books, and then damn well build his own perfect AM radio??? Thanks for sharing your frank coments. They are acknowledged. The important thing is that the replies to my "trawling" have been very informative, including yours Patrick, and are not only benefitting me, but are benefitting many others who are following this thread in real time. Whether my trawling is successful or not for my purposes is immaterial -- if I fail, I fail -- I don't fear failure as some do -- the discussion is further adding to the information pool for the community of those interested in some aspect of tube-based AM tuners, and in that regards I think it has been successful. (With Google archiving the newsgroups, this information is now being preserved, and is searchable.) There are obviously two sides to the engineering of radio receivers: 1) the basic theory and the basic categories of design approaches (which I am studying -- it helps in that back in 1974 I had the equivalent of one years' worth of basic electrical engineering courses at the University of Minnesota, which is now all coming back to me), and 2) the real-world engineering of receivers/tuners, using real rather than theoretical components, and the attendant compromises and work-arounds which inevitably result. I do agree with Patrick's implying that there is no such thing as a "perfect radio". I am not seeking the "perfect radio", but a modern-design tuner "kit" sufficiently meeting the various requirements I have previously set forth. I believe once a good design results, that PCB boards can be made, coils can be built by someone or some company experienced in doing that (I mention coils since that is the one component difficult to buy right off the shelf -- thank god no one has to build their own tubes!), and the schematic with detailed instructions and guidelines sold through diytube (as an example.) The target market for the "kit" are those who build their own tube- based components for their audio system, and want every component to be a high-performer, approaching audiophile-grade in performance (yes, AM broadcasts are not "audiophile", but audiophiles want a tuner that brings out the best in what is there in the signal.) They don't want to spend their limited time building junk, they don't want to build a Radio Shack beginners' crystal set. They want very good performance (which is admittedly a "fuzzy" word), commensurate with their other components. They just want the tuner kit not to be overly complicated in design, to work if they follow the instructions and guidelines, and to meet their (collective) expectations. And these kit builders are not novices, either, at wielding a soldering gun, and in chassis and cabinet design -- they are mechanically- and electronically-inclined, and are now building audiophile-grade amps and preamps from the many kits now out there. I also believe that some of the vintage radio collectors, who are experienced at restoring radios, will also take an interest in the AM tube tuner kit. (For those who don't know, I'm now restoring a Philco 37-670 console, so I'm not exactly out-of-touch with the radio collecting world.) Based on my experience with building audiophile-grade tube amps and plugging into that community, I think I've laid out pretty well what they want and expect. Most are not going to become radio design enthusiasts, they will not live and breathe tuners, building hundreds of circuits on cake pans in their basement (and I am not disparaging those who do!) They simply are going to listen to the tuner they laboriously built from the kit, happy with its performance, and happy for what they have learned about how radios work "under the hood", in a general sense. Some will no doubt get the radio bug, and join the people here, rescuing old radios from the landfill, and restoring them. Maybe my focus on TRF-based designs and "channel-based design" have been diversions. But, from what I've read about real-world TRF designs (John Byrns messages have been great here), a TRF-based design has some nice attributes from the audiophile kit perspective, and there are clever real-world solutions around the selectivity and gain limitations of the "what's taught in textbooks" regarding basic TRF design, as John Byrns and others have noted many many times, but which seems to fall on deaf ears of those who believe that the best high-performance receiver (however "high-performance" is defined) *must* be super-het in basic design. But obviously, the vast majority of commercial designs of high-performance tube-based radio receivers from the mid 1930's to the 1950's are super-het designs, and many of them are great performers, so I'll post a parallel message with another call for candidate radios to inspire the AM tuner kit. After all, if one is to put together an AM tube tuner kit, it makes a lot of sense to base it on a proven design from the past -- why reinvent the wheel? For example, diytube (at http://www.diytube.com/ ) has taken the venerable Dynaco ST-35 amp design, modernized it some (and to further improve its audiophile performance), and is now selling the PCB board with schematics and instructions to diy audiophiles. it is an excellent performer (I know firsthand -- it is a *very nice* sounding amp.) diytube is now working on a high-power monoblock tube amp kit based on the Eico EL34 amp of old -- can't wait until it is released. Just some thoughts... Jon Noring |
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