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posted by Fnord666 on Monday July 08 2019, @03:11PM   Printer-friendly
from the does-it-take-10-minutes-to-warm-up? dept.

Antique radio receivers retain a significant charm, and though they do not carry huge value today they were often extremely high quality items that would have represented a significant investment for their original owners. This guy acquired just such a radio, a Philco 37-11 made in 1937, and since it was it[sic] a bit of a state he set about giving it some updated electronics. Stripping away the original electronics, he gave it a modern amplifier with Bluetooth capabilities, and a Raspberry Pi.

One of the coolest applications for a Pi I've seen.


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  • (Score: 3, Interesting) by fyngyrz on Monday July 08 2019, @07:55PM (13 children)

    by fyngyrz (6567) on Monday July 08 2019, @07:55PM (#864664) Journal

    Get a decent SDR and some good software, and you'll never want to listen to an analog radio again unless you're a masochist at heart. SDR software offers perfectly smooth tuning, any bandwidth you want, any slope to the bandwidth edges you want, any kind of EQ you want, enormous, consistent tuning range(s), totally configurable demodulation (for instance, in FM stereo, you can control the high-blend crossover and intensity for weak signals if you like), subcarrier demodulation, notch filters. noise reduction, precision metering, nearly zero inherent noise, wideband recording (for instance, the entire AM band all at once for later perusal), precision noise blankers, signal analysis, multiple antenna diversity and hot signal selection... and given that you have the SDR, most of the software is outright free.

    If you wanted to stuff all this in a nice wood cabinet, that'd be fine. :) But your computer's monitor will do.

    A modern SDR has about the same relationship to these old tube radios as a Tesla has to a turtle. And yeah, I admit I do have a couple of the old beasts, because I have plenty of space and I love the awesome cabinets and... well, I tend to wallow in nostalgia from time to time. Plus... I like to fix things. But to actually use to listen to? No way.

    you really need to get rid of the PLL based synthesized tuning

    Modern SW receivers and ham radios (for instance an IC-7300 [gigaparts.com], which is both) typically are able to tune in 1 Hz steps. You won't hear any steps happening, presuming you're tuning at a rate used in normal scooting about the bands. However, the IC-7300 excepted, most remain superheterodyne designs of one sort or another and their performance isn't even close to what you can get out of a direct-sampling SDR.

    International shortwave broadcasters all use the standard AM format

    No. Some use SSB, and some have gone digital with the DRM format. It's rare, but the SW stations are out there. A very few AM broadcast stations are using C-QUAM AM stereo encoding. And of course there are a wealth of other broadcast, utility, military, time, fax, satellite, data, DGPS, aircraft... and many other signal types out there to snoop on.

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  • (Score: 2) by Rupert Pupnick on Tuesday July 09 2019, @12:13AM (12 children)

    by Rupert Pupnick (7277) on Tuesday July 09 2019, @12:13AM (#864777) Journal

    Wow, that IC-7300 is insane with its real-time spectrum analyzer display of the frequency band. Looks like a piece of lab gear!

    Can’t argue with a 1 Hz step size, especially if it’s got a decent sized tuning knob to go with it. The old mechanical tuning systems probably had more than 1 Hz of backlash anyway.

    If the idea is to reach as many listeners as possible, it’s hard to believe that AM short wave broadcasting will ever really go away since you can build a receiver for next to nothing, if you can get the parts— especially the big front end variable capacitor for tuning. But maybe those are hard to find nowadays.

    • (Score: 3, Interesting) by fyngyrz on Tuesday July 09 2019, @09:27AM (4 children)

      by fyngyrz (6567) on Tuesday July 09 2019, @09:27AM (#864914) Journal

      You like the IC-7300's insight into the signals, eh? Check this out, then. [fyngyrz.com] That's done with a $200 SDR that provides a frequency range from about 10 KHz (not a typo) to 1.6 GHz. I wrote the software to run on either Windows or OS X, and it's free.

      😊

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      It's not stroganoff.

      • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @02:59PM (3 children)

        by Knowledge Troll (5948) on Tuesday July 09 2019, @02:59PM (#864995) Homepage Journal

        Having a waterfall of the band is pretty nice but I always thought the most interesting use case with SDR was taking advantage of the high bandwidth signal and running N software receivers/demodulators at once. Not long after I got a HackRF I implemented a GNU Radio chain that listened to a couple 2M FM repeaters at the same time and dropped WAVs on disk of the demodulated audio. Very very cool stuff but it's just the start.

        With enough CPU the value of N can be high and include M receivers dedicated to searching for signals. Then known stations get a receiver and the thing basically also scans non stop for more transmissions and parks a software receiver on one when it finds it. Now you are spying on a whole band at once. You can generate reports of activity from that or just archive stuff since audio is easy to store.

        The most refined I took this concept was to wrap the whole thing up into a UI that esentially had memory slots like most radios but instead of being something that 1 receiver can use the memory slots are running receivers. With the right UI a single operator could visually keep track of many existing stations at once. Throw in a little DVR technology so you can go back in time and see what part of the conversation you missed once you clicked over to that transmission you are listening to now.

        I'd love to have something like that but I have no SDR rig that tolerates real world VHF signals well. The HackRF just isn't good at dealing with how loud stations get and making filters for VHF is pretty damn hard.

        • (Score: 2) by fyngyrz on Tuesday July 09 2019, @04:25PM (2 children)

          by fyngyrz (6567) on Tuesday July 09 2019, @04:25PM (#865032) Journal

          Having a waterfall of the band is pretty nice but I always thought the most interesting use case with SDR was taking advantage of the high bandwidth signal and running N software receivers/demodulators at once.

          I have heard a few people talk about doing this in a positive way; there's a number of SDR applications on the market that offer N demodulators as a feature. I've tried it, and it drives me batty — I'm just not very good at listening to more than one thing at once. 😊 I think you have to have a particular kind of mindset to enjoy it, which I guess you might have!

          Throw in a little DVR technology so you can go back in time and see what part of the conversation you missed once you clicked over to that transmission you are listening to now.

          Yes, I implemented wideband recording, and so you can record and go back to listen to anything within the SDRs overall active bandwidth. That's my way of doing multiple coincident signals. One at a time, one per replay. Here is a link [flickr.com] to where I used wideband recording to grab the station ID of many AM broadcast stations right around the top of the hour. That kind of thing is really fun. But listening to more than one at a time? Not for me!

          I'd love to have something like that but I have no SDR rig that tolerates real world VHF signals well. The HackRF just isn't good at dealing with how loud stations get and making filters for VHF is pretty damn hard.

          I think we're all waiting for high speed, 32-bit or better, inexpensive A/D converters. Once the dynamic range of the A/D is sufficient, needing filters will be a thing of the past.

          It's not just VHF, either. I had to put a notch filter tuned for our local AM broadcast station at 1240 KHz in my antenna line, because the tower is only about a mile from here, if even that. It was causing the SDR to alias, and when that happens, you're hosed. Once I shut that garbage down — and it is garbage... probably one of the worst radio stations in the country, no loss at all — is was smooth sailing throughout the SDR's entire low frequency range, 10 KHz to 30 MHz. There's a separate input for 30 MHz to 16. GHz, and that I treated with a stepped attenuator, more on that below.

          Going back to Icom ham radios for a moment, the IC-7610 [gigaparts.com] is dual-receive, which is a small start on what you're talking about. But it's HF/50 MHz only. The R-8600, [gigaparts.com] which actually covers into the VHF range and above, is one signal at a time as well, so not along the lines of what you're talking about.

          If you're doing work with (for instance) the HackRF or something like it in a strong signal environment, one available compromise is to (a) turn off the SDR's AGC (assuming it has AGC), set the SDR's gain to a fixed max setting, and (b) put a stepped attenuator in line and adjust it so that the strongest signal sits within the top edge of the unit's dynamic range. I did that for the HF input before I built my notch, and was surprised at how little the small signal reception was affected... at least on HF, the noise level is pretty high, and I don't need to hear beneath the noise level anyway. All my VHF and above stuff is local, so that was no problem. Everything is relatively strong up there, so there's still a stepped attenuator on the VHF-and-up antenna input of my SDR.

          --
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          • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @04:50PM (1 child)

            by Knowledge Troll (5948) on Tuesday July 09 2019, @04:50PM (#865047) Homepage Journal

            I'm just not very good at listening to more than one thing at once.

            I don't think anyone can listen to more than one thing at once and make sense of it unless what they are listening to is silence. I often have multiple radios receiving at once but not all of them are making noise. Note too that my example was putting wavs on disk not listening to many repeaters at once. The use case for multiple receivers I want is monitoring multiple frequencies for activity or running digital decoders like listening to four 2M packet frequencies at once and monitoring multiple repeaters and the simplex call channel. Using audio ducking you can assign priority to each audio source and then make them mutually exclusive. I already run audio chains that use multiple speakers and duck audio inputs so I can use my desktop PC, listen to music, listen to system event sounds, listen to the ham radio, and I even cut the music when I hit the PTT so I'm not sending that audio out onto the air.

            Yes, I implemented wideband recording, and so you can record and go back to listen to anything within the SDRs overall active bandwidth

            I had not thought of time shifting the quadrature data from the SDR receiver that's a really good idea! I had only thought about time shifting the demodulated audio but I think both features are pretty good ideas.

            Going back to Icom ham radios for a moment, the IC-7610 [gigaparts.com] is dual-receive, which is a small start on what you're talking about.

            I've got a 7610 sitting next to me here. Dual receive in it is not implemented the way you think it is: it's not done with 2 software receivers and demodulators running at once, it's done with two complete sets of RF gear each with their own filters and ADC. Though you can, finally, access the quadrature sampled data from the 7610 via a dedicated USB port I haven't yet tried to do it. The idea of bringing an entire HF band in at once to the PC is enticing but because the receiver does per band filtering like a normal HF rig I won't be receiving all of HF entirely, just the specific individual amateur bands.

            Anecdotally I used an Icom IC-7100 for HF before the 7610 and after i switched over to the 7610 my audio quality reports from other stations increased dramatically. It went from "you sound good" to "that is a beautiful sounding radio" and each time it was just using the vendor supplied chicken choker mics. Icom also has the new SDR direct sampling VHF and UHF rig called the IC-9700 which is basically the VHF/UHF version of a 7300. It direct samples even the 70cm part of the band! There's a few hams around here that got one recently and they love it though it's got buggy firmware right now.

            If you're doing work with (for instance) the HackRF or something like it in a strong signal environment, one available compromise is to (a) turn off the SDR's AGC (assuming it has AGC), set the SDR's gain to a fixed max setting, and (b) put a stepped attenuator in line and adjust it so that the strongest signal sits within the top edge of the unit's dynamic range.

            That's a good idea though there are also off the shelf SDR amateur receivers offering 14 bits and a few mhz of bandwidth and still come in at a reasonable price around $130 or so. When I talk to people they report that 14 bits is nice and that 12 bits doesn't quite cut it. Both of those units are available with a small price difference. If I was dedicated to running a more custom SDR setup in the shack I'd look to one of those things I think. Something like an SDRPlay.

            • (Score: 2) by fyngyrz on Tuesday July 09 2019, @06:45PM

              by fyngyrz (6567) on Tuesday July 09 2019, @06:45PM (#865096) Journal

              Dual receive in it is not implemented the way you think it is: it's not done with 2 software receivers and demodulators running at once, it's done with two complete sets of RF gear each with their own filters and ADC.

              Well, that buys you things too. Means you can have legit reception on bands outside the bandwidth of the one SDR. And you should still be able to run them both on one band if you like. It just takes more hardware.

              Though you can, finally, access the quadrature sampled data from the 7610 via a dedicated USB port I haven't yet tried to do it.

              On the 7300, what you get is very narrow bandwidth — I think it might be after the demodulator bandpass, but before the demodulation. It's only a few KHz worth of sampling. It would be terrific if it was the full wideband sample... but it's not. If it is on the 7610, that's a very worthy feature.

              Icom also has the new SDR direct sampling VHF and UHF rig called the IC-9700

              But... no general coverage. 🙁

              When I talk to people they report that 14 bits is nice

              It is. Among others, I run an AFE822x, which is a $200-ish, 14-bit SDR with 10 KHz to 1.6 GHz coverage. I really like it. The Ethernet connection is da bomb; you can (and I have done) remote it far away from the computer(s), and so keep the dense RFI that comes out of computers, routers, and all that junk down to a dull roar. USB connections are tethered right to the computer which does not make me happy. But as I said, because of those 14-bits, I had to go with a notch filter to make sure that everything it heard coming in from the antennas was within its dynamic range. Otherwise... clipping, aliasing... awfulness. On VHF I use a stepped attenuator because all that local stuff is quite strong and I have no need for much sensitivity anyway. If it isn't within ten miles, it's over 75 miles away in some other small town and I just don't care. I just use a discone. Hears satellites really well, all the local stuff too. VHF is fun. 😊

              --
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    • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @02:48PM (6 children)

      by Knowledge Troll (5948) on Tuesday July 09 2019, @02:48PM (#864991) Homepage Journal

      I have an Icom IC-7610 [icomamerica.com] sitting next to me which is the 7300's big brother. It's basically 2 7300 receivers and 1 7300 transmitter all stuffed together into one chassis with a larger touch screen, more knobs, and native ethernet and IP networking with a server for remote operation built right into it. The thing even talks NTP and sets its own clock.

      • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @03:01PM (5 children)

        by Knowledge Troll (5948) on Tuesday July 09 2019, @03:01PM (#864996) Homepage Journal

        Not yet enough coffee I suppose.

        Other really neat features of the 7610 include the ability to use a mouse to interact with the content on the display (though it is minimal interaction), a keyboard for supplying text that is sent out over the air as Morse code or a digital signal called PSK, and you get a DVI port so you can use a larger monitor instead of relying on the 7 inch wide touchscreen.

        • (Score: 2) by fyngyrz on Tuesday July 09 2019, @04:36PM (4 children)

          by fyngyrz (6567) on Tuesday July 09 2019, @04:36PM (#865038) Journal

          The 7610 has a lot to recommend it. But.

          The only reason I don't already own a 7610 is because they screwed up the spectrum/waterfall vs. noise blanker interaction the same way they did on the 7300; the noise blanker processes the signal chain after the spectrum/waterfall, so turning it on in a high noise environment (of which I am the not-very-happy subject of) does absolutely nothing to clear up the display.

          Fortunately, since I write my own SDR software, I didn't make the same mistake, and so I can see the spectrum/waterfall on my SDR software via an AFE822x SDR even when the 7300 is blindsided by Icom's error — turn on my noise blanker, and the display clears right up. Plus it's a better noise blanker than Icom's. 😊

          But for the money they want for the 7610, I'm just not inclined to forgive the whole "spectrum becomes useless" thing. I'd probably overlook it in a heartbeat if I was in a lower noise environment and didn't have to run with the noise blanker on almost all the time.

          --
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          • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @04:57PM (3 children)

            by Knowledge Troll (5948) on Tuesday July 09 2019, @04:57PM (#865051) Homepage Journal

            the noise blanker processes the signal chain after the spectrum/waterfall, so turning it on in a high noise environment (of which I am the not-very-happy subject of) does absolutely nothing to clear up the display.

            Hrm I'm sure they put the noise blanker on the demodulated audio so I'm not surprised it is after the waterfall on the 7610. The 7610 is a weird radio for being SDR - it's SDR architecture used to implement the traditional ham radio with out really treating it as SDR. I'm curious though what kind of noise blanking you use for the wideband signal. How does your blanker work?

            I don't have a lot of noise issues but i did need to spend some time tweaking the waterfall configuration to get it to generate graphs I could read well on all bands with the varying noise floor. I don't usually have to do something like run a noise blanker and the noise remover doesn't help either. Being able to noise remove the waterfall though is a great feature!

            Icom did miss the memo about SDR in a lot of ways. I'm sure if I could have waited longer to get an SDR rig I'd have something a lot more SDR like available to me in a few years. There is a new HF and VHF SDR ham radio that just became available which is just a windows computer in a box with knobs and a display and a full screen touch interface for the radio UI, which you can then minimize and open up the start menu and launch fldigi or what ever. I don't want something integrated like that but for the average ham I bet they got the right idea. These are neat times for radios!

            • (Score: 2) by fyngyrz on Tuesday July 09 2019, @06:25PM (2 children)

              by fyngyrz (6567) on Tuesday July 09 2019, @06:25PM (#865091) Journal

              Hrm I'm sure they put the noise blanker on the demodulated audio

              No, it's definitely an RF blanker; you can tell because in a noisy environment, with the blanker on, the AGC turns the RF gain up. If they were only reducing audio noise, they couldn't control the RF levels with it effectively. So the blanker is reducing the peak RF, which in turn causes the AGC to pump more gently.

              What they're probably doing is running the blanker on the RF that's already been bandwidth-limited, just before the demodulator in the signal chain. What that buys you is a much lower bandwidth sample to process, basically a lot less data to look at. IMHO, there's enough CPU and/or FPGA horsepower available at this point to forgo that kind of savings. My software, which does full wideband blanking while doing a whole lot more as well, can run without problems on an old-dual core. Just barely, but it will run. You can't do anything else, though...

              I'm curious though what kind of noise blanking you use for the wideband signal. How does your blanker work?

              I take large samples of the baseband RF after it's been downsampled, examine it for spikes, and then remove them, while replacing the removed interval with a splined interpolation between the level just before the spike and the level just after the spike, using enough samples on both sides to set the spline curve up so the interpolation is somewhat sane. Then I pass the baseband RF block up the signal chain. You have control over the width and the depth of what is considered a spike. Once the spikes are removed, it is passed along to the demodulator, scope and spectrum/waterfall signal chains.

              You can also switch to the spectrum/waterfall being taken prior to the noise blanker; I did this mainly to demonstrate why you pretty much always want it after. 😊

              There are exceptions, to be sure — strong noise blanking on baseband RF can cause distortion in the wideband signal and leave you with some visual artifacts, but generally speaking, you do want it after the blanker.

              the noise remover doesn't help either.

              Yeah, so far, correlated noise removal tech — which is what Icom uses — is pretty sad. The more you remove, the worse the audio gets, and at least in the 7300, the quieter it gets, so I have to turn up the gain too. I went a different way in SdrDx, but it also has its drawbacks. I create a spectrum of the noise which, when it actually is noise, is generally pretty flat. I call this condition balanced. Then, when the spectrum becomes unbalanced, meaning that there is more on one side of it than the other, the implication is usually that there is audio in it where the audio frequencies are dominating. The balanced / unbalanced condition is used to modulate the audio gain, and it's really pretty good — no distortion at all — when there's actually audio. You have control over attack, decay, and depth of gain variation.

              However, when the signal is really just noise, it tends to pump now and then when the spectrum contains a stronger spike of one frequency or other. Makes for little noise pulses. Nothing's perfect, I guess. I've been thinking about using a correlated noise remover to generate the gate/nogate signal on a slightly (maybe 20ms) delayed audio stream so it can trigger ahead of the audio start, rather than using the correlated product as the audio. It's on my list to try. Might be pretty good.

              Being able to noise remove the waterfall though is a great feature!

              I just hate it when I don't have it. I am always looking at the waterfall and spectrum. There's only one direction I can point my beam or orient my wideband loop to reject all the crapola around here. When I'm looking off that way, everything is great. Any other direction... bah. Fortunately (or it would be fortunately if the sunspots weren't gone) that direction is right over the pole towards Europe. I guess it won't matter for the next 5-10 years... or longer. Europe is simply not reaching Montana. Not even the big SW blowtorches, much less some poor ham with 100 watts. Or even a kW.

              I do lots of signal processing specifically in the spectrum / waterfall chain designed to make the waterfall clearer, easier to read, etc. It's all switchable, of course... sometimes you just want to see what's there without any fooling with it. But the convenience of auto removal of lightning strikes (which would otherwise create great bands of noise in the waterfall), control over the amplitude of the noise floor separate from the amplitude of the signals... these kinds of visual amenities are really nice. Might be hard to imagine until you see them at work, but... nice. I should make some more Youtube videos, I guess.

              These are neat times for radios!

              Oh yeah. What's sad is this didn't happen before a lot of the SW stations went off the air and the solar minimum arrived like a ton of bricks.

              --
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              • (Score: 2) by Knowledge Troll on Tuesday July 09 2019, @09:06PM (1 child)

                by Knowledge Troll (5948) on Tuesday July 09 2019, @09:06PM (#865152) Homepage Journal

                I take large samples of the baseband RF after it's been downsampled, examine it for spikes, and then remove them, while replacing the removed interval with a splined interpolation between the level just before the spike and the level just after the spike, using enough samples on both sides to set the spline curve up so the interpolation is somewhat sane.

                Damn that is very cool. I would love to try something like that as an audio soft clipper. it sounds like the performance would be really good for transients which are a frequent headache.

                Would you be interested in joining a chat system for a digital radio club? It's focused on getting fully qualified open source ham radio nerds together into one place in a way that's familiar for us. There's another guy there that does DSP professionally, a few competent programmers, and a couple power users / hobbiest programmers. It's a nice place to find people who have a clue to talk about topics exactly like your noise blanker. I know other people in there would go "that is really cool" and basically everyone in the club has big dreams of SDR. I can get you details outside of Soylent news if you like.

                • (Score: 2) by fyngyrz on Tuesday July 09 2019, @10:39PM

                  by fyngyrz (6567) on Tuesday July 09 2019, @10:39PM (#865193) Journal

                  Thanks for your kind words.

                  You can contact me by going to section 39 [ourtimelines.com] on that page. Edit your subject accordingly.

                  Might be fun to lurk a bit in your group. 😊

                  --
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