brainwagon

Saw this drift by on the KnightsQRSS mailing list, looks very promising. Also makes me think that I should have gotten my own program ready for release. Sigh, I’m a bum.

>>> QRSS VD – The Free, Open-Source, Cross-Platform QRSS Spectrograph by AJ4VD « SWHarden.com.

I’m still trying to shake the worst of a cold, so the XBox 360 is getting a bit of a workout. I usually only play video games when I’m sick and/or tired, just as some relaxation and diversion. Games which are too involving, requiring long quests aren’t really in the mix for the most part, since I don’t really feel like dedicating that much time to them.

Nevertheless, Carmen picked me up a copy of Mass Effect, what is essentially a space opera based role-playing game, where you play a hero trying recover an alien artifact, yada yada… it’s reasonably well done, but the game play has a bit too much running around on side quests for my taste. And, like virtually every RPG type game, the majority of your time seems to be spent trying to find/buy upgrades to your skills and equipment. Honestly, this kind of game usually doesn’t appeal to me very much: it’s like working a job. But this game is, I must admit, better and more imaginative of its genre, so I am about four or five hours into it.

But back to the math:

Inside the game there is a casino called “Flux” which houses a bunch of machines upon which you can play a casino game called Quasar. It’s sort of like a stripped down version of Blackjack. The idea is that you try to get a score as close to 20 as you can (without going over). In each step, you can either a) hold with what you got b) choose the X strategy, which adds a number from 4-7 inclusive to your count, or b) choose the Y strategy, which adds a number from 1-8 to your total. The game costs 200 quatloos (or whatever) to play, and payouts vary according to your total.

(The reality of the game is that you actually can’t hold on any total less that 15, but since that strategy would always guarantee the maximum possible loss in all situations, it hardly matters in determining the optimal solution.

Anyway, I wrote a little Python script to compute the optimal strategy, which can be summarized
as follows:

If your total is 17 or greater, then you hold and collect your winnings.
If your total is 16, choose strategy Y.
If your total is 15, choose strategy X.
if your total is 1, 2, 6, 7, 8, 13 or 14, pick strategy X.
Otherwise, pick strategy Y.

It isn’t clear to me how the game picks your initial count. In the ten or 15 minutes that I’ve played the game, it seems to me that the initial count is perhaps uniformly distributed between 1 and 5 (inclusive). This amounts to an average net profit to the player of about 40 quatloos per round. Hence, it takes about 25 rounds to make 1000 quatloos.

The best numbers to get are 13, 7, 12, 6, and 1. The worst? 16, 15, 9, 10, 11, although only 16 and 15 have a negative expectation.

I’m doped up with cough medicine, have a sinus headache, and yet here I am, writing programs and doing math instead of playing a video game. I must be crazy.

Addendum: There are many sites that also derived the same strategy, like this one. Google for more if you’d like.

Once again, Roger G3XBM has provided me with links to interesting and inspiring reading. Roger is now interested in experimentation below 9khz. He found this very interesting blog called:

VLFradio.com

Overnight I heard and was heard by a number of VK and ZL stations, but perhaps my most interesting spot was from long time QRSS/grabber/WSPR participant W1BW. Despite the fact that he was running a mere 5mw, I managed to pick him up twice overnight:

2 spots:

What’s even more amazing is that he was heard by VK7ZL, VK2/VK6DI and distances of greater than 16000 km.

I was reading the pre-announcement for this month’s QST, which apparently includes a product review for a new tri-band Alinco HT: The Alinco DJ-G7T. It is intriguing because it includes two features which are uncommon. First, in addition to 2m and 70cm, it includes operation on 1.2Ghz. But more importantly from my perspective, it is full duplex.

You see, I like to work the FM satellites using only an HT and a small handheld yagi antenna. If you have full duplex, it is considerably easier: you can hear your own signal coming back down from the satellite, and can monitor signal quality and whether anyone is doubling with you. It is for this reason that I continue to use my TH-D7A, despite the fact that on the whole, it’s kind of a clunky and outdated (and is now discontinued) HT. If this HT checks out, it could be a reasonable replacement for my Kenwood.

Anybody have any experience with this one yet?

Addendum: Since you can apparently operate full duplex between any two modes, in theory you should be able to work mode L/U on AO-51 using this HT, although the Doppler correction might be pretty fast.

Yesterday I spent some time trying to implement a simulation of an SSB exciter that worked by using the Hilbert transform on some input audio, and then multiplying it through by a carrier signal in quadrature and summing (or differencing) the two resulting streams to produce the LSB or USB. I was getting poor cancellatio of the opposite sideband, but through a series of experiments determined that if I lengthened the FIR filter to a huge number of taps (around 600) I began to get the cancellation that I thought I would get. The lack of cancellation is worse at low frequencies, which is should be a clue. Before I went to bed, I thought I had it, but in the light of morning, now I am not so sure. I’ll ponder it more.

But my morning reading found this website on MathWorks, which details the kind of math that I’m implementing in C. I’ll review it again.

Single Sideband Modulation via the Hilbert Transform Demo.

Addendum: Here’s a brief glimpse of the kind of thing that I worked on. I’ve taken a regular mono signal recorded at 48khz, low pass filtered it at around a 3khz frequency, computed the Hilbert transform of it, and then frequency shifted the single sideband version of it up to a carrier frequency a little above 8khz (that would be VLF, I understand, but it is the principle of the thing!). This was computed using a Hilbert transform with 501 taps (250 non zero entries):

If I try to compute the same thing with only 201 taps, you can see that some of the very low frequency signals are visible in the lower sideband opposite their USB constituents.

I need to ponder this some more.

Addendum²: Just as a little test, I tried modulating my voice up by just 350 hertz, simulating reception of a signal on USB, but tuned 350 hz too low. You get the classic Daffy Duck type speech (sans any noise that is typical on HF):

CQ CQ CQ, this is K6HX calling CQ and standing by…

I’m gonna dash off to an appointment with my tax guy in a few minutes, so I’ll have to be brief. I’ve been reviewing literature on inexpensive/simple phasing double sideband and single sideband transmitters. It dawned on me that while it was pretty simple to generate the necessary quadrature signals for the carrier frequency (the solution that most people use is to generate a carrier at 4x the desired frequency and then divide it in quadrature with flip flops) that such a solution wasn’t really that neat at audio frequencies. To get good opposite sideband cancellation, you need to generate a very accurate 90 degree phase shift for the incoming audio frequencies. This seems tricky. Fussy.

But, of course if you have some compute power handy, it isn’t really that big of a deal. You can use something called the Hilbert transform which does precisely what you want. From a given signal, it generates the quadrature signal that you need. If you plot the input versus the output for a constant amplitude signal, you get a nice circle, precisely in phase. If you apply it to a voice input signal with varying amplitude, you get a series of circular spirals, like the plot below.

It is probably only an hour or so work to generate a complete abstract implementation of the phasing transmitter/receiver. That’s coming, and I’ll document it more here and make the code available for the interested. Till then, wish me luck with the IRS.

Addendum: Tax appointment went as well as could be expected. I didn’t like the straight lines in the digital signal above, which game from its relatively low sampling rate. If you upsample the signal with a high quality, band limited up sampler, you get a much smoother result, as can be seen below (the audio input wasn’t quite the same, so it doesn’t look exactly the same):

Today I was cleaning out my office. I’m a clutterbug: I have tons of treasures, but also, let’s face it, an even larger amount of crap. When I first moved to California 19 years ago, everything I owned fit in 13 banker’s boxes. Now, I don’t even think my power adapters would fit in 19 bankers boxes. So, I was ruthlessly going through boxes and tossing stuff out. Along with endless old cds, and papers that I printed out, I found a bunch of stuff from one of my previous obsessions: telescope making. I even found a 6″ mirror, aluminized, carefully wrapped in optical tissue. I’ve pretty much abandoned telescopes in recent years, but I really should get back into it again.

Anyway, tonight I was strolling down memory lane, and encountered a link to something I hadn’t seen before: the Bahtinov Mask. It’s a gadget that you can place in front of a telescope to help you focus precisely. It uses a very clever arrangement of masks to produce a diffraction pattern which can be adjusted to a precise focus.

If you dig around, you can find all sorts of testimonials about how terrific they are. I’ll have to make one of these and give it a try. Perhaps if I can find someone who will loan me a laser cutter, I’ll even cut a really precise one.

Addendum: Here’s a link to a nice mask generating program that will output an SVG file for any scope you need.

I was reading amateur radio blogs and for the third time or so in a month, I was treated to what amounted to a diatribe against software defined radio. When I read these, I can’t help but sigh in frustration.

I understand nostalgia. We all like the cars we wish we could have had when we were teenagers. We like the music that we learned to like as young people. Heck, I still have the first computer that I bought at age 14. In moderation, nostalgia can be a good thing. It reminds us where we came from. It gives us context. It gives us history.

But I think we always have to temper a sense of nostalgia with perspective and vision. Our first cars were our loves, but they were (let’s face it) often gas guzzling, unreliable death traps. Some of our music was probably brilliant, but a lot of it was (let’s face it) crap. And my first computer, while enabling me to explore a world of computation, which eventually led a rich and rewarding career, was by any modern standards less powerful than an alarm clock.

Which brings me to software-defined radio.

I’ve seen lots of people write negatively about software defined radio. Often it’s just pure nostalgia. “Real radios glow.” “Real radios have KNOBS.” “Real radios don’t require your laptop.” “My old radios just sound better.”

I think we as hams should be more visionary than these statements. Software-defined radio is enabled by the remarkable evolution of computing hardware. The speed of computation has in the time that I’ve owned computers increased by a factor of about 200,000. The cycle time of most desktop computers is now low enough that an inexpensive computer can execute dozens of instructions for every cycle of an HF signal. This enables the traditional features of radios like mixing and filtering to be done in software, instead of being cast in hardware. This means that we can have unprecedented control over these processes. And, potentially we can even change many aspects of operation even after the soldering of our radio is complete. Software isn’t a panacea, but it is the source of such great power it can’t and should not be ignored.

Most of the criticisms I see about radio interface seem curiously misplaced. I agree that using your laptop as an interface is often sub-optimal. But if you’ve played with any radios at all, I bet you had some criticism about the conventional radios that you had as well. Modern radios are complex, and this complexity is not often controlled by careful user interface design, whether in software or in conventional hardware front panels. Even if you thought that some radio you had in the past was perfect, I would submit that it would still be beneficial to use software defined radio technology inside.

But even beyond that, I have been playing with an SDR-IQ for the last couple of months. Its ability to monitor all signals on an HF band is enormously compelling. Using digital displays, we can give the user a perspective on the entire band which is undeniably useful. Technology like CW Skimmer can track dozens of Morse signals in real time. Programs like HRD and fldigi can do the same with multiple PSK signals.

I’ve seen a few people abandon the idea of software-defined radio simply because they aren’t adept at software design or have no knowledge of the mathematics and algorithms that underly software-defined radio. They fatalistically claim that they will never or can never understand or learn how software-defined radios work. I am kind of flabbergasted by this attitude. It’s not that it is easy: far from it. Studies show that to become expert in a subject takes the better part of a decade. But as hams, we are supposed to be about self-training and experimentation. Sure, there is no mandate for you to do so, but let’s not be so fatalistic and condemn it merely because it doesn’t coincide with our own abilities or interests.

There is no reason to forget the past, but let’s not worship it either. Let’s look at the best of what was, the best of what we have now, and the best that we can imagine, and experiment, design, build and use the radios that we have yet to invent.

Well, yesterday I was out shopping at HRO, and couldn’t resist the allure of a new Signalink USB sound interface for my FT-817. This is a sound interface that looks just like a USB sound card when you plug it in, and has rx and tx volume controls on the front.

So, of course, I hauled my FT-817 out of the box it has been in since before the holidays, and had to test it out. The first impressions are a bit mixed. I didn’t have any difficulty at all getting it to work with fldigi or Mixw or Spectran, but for some reason WSPR required a bit of tweaking. It would sometimes just act as if it wasn’t getting any receive audio: the RX level would stay at -30. I suspected that it might have something to do with applications which open the sound card exclusively, so I made sure those were all turned off, and eventually ended up running it in Windows Vista compatibility mode (I have a Windows 7 laptop) and then all was well.

And.. since all was well, I decided to do a bit of WSPR transmitting for the first time in months.

I was heard by VK6DI from his new VK2 location, which was nice to see. David was one of my most distant spots, and his old grabber was the funnest thing for me to check out while testing my QRSS/MEPT beacon stuff before. He’s a few thousand km closer now, but still, Australia is nice. ZL2TLD and I managed to exchange packets. But most interesting was a new station I hadn’t seen before: 4Z4TI. That wasn’t a DXCC prefix which I had seen before in my WSPR spots, so I had to look it up. Yep, Israel. Very nice. Toss in EA4BUL and Japanese stations JA2GRC and friendly face JQ2WDO, and that rounds out my list of DX for the night.

You’ve had a C compiler sitting in front of you and some communications textbooks, and so you’ve done some hard work and created a new digital mode that you’d like to see widely deployed. What should you do? Well, let’s begin with what you should not do:

1. First, you shouldn’t presume to dictate what frequencies it should be used on without paying some close attention to the bandplans, both voluntary and involuntary, that we as hams have established. A particularly bad choice would be to recommend frequencies such as 14.101, which are uncomfortably close to the 20m beacon that many hams rely on.
2. You should not recommend frequencies that are likely illegal. Modes wider than 1khz, or which use unspecified coding aren’t legal on 30m in the U.S.
3. You should not call your mode something which is either incorrect, or at best misleading. If you say your mode is spread spectrum, it’s going to make a lot of people upset, and you could have avoided that by knowing what words mean.
4. You should not keep the details of your scheme private. As amateurs, we are tasked with improving the radio art through experimentation, and trade secret modulation does nothing to encourage or aid that kind of cooperation. I’d suggest that you make your scheme open source to achieve maximum impact. Private protocols and encodings are basically just encryption.
5. You should not make vague assertions about the mode’s performance. Comparisons of a mode which runs at (say) 16 baud and take 2.25khz of bandwidth have to be carefully made when compared with modes that run at (say) 31.25 baud and only 80hz or so of bandwidth.
6. Lastly, you shouldn’t fabricate communications between yourself and the FCC in support of your new mode. It doesn’t lend much credibility when you later retract all evidence that you did so by deleting posts from your blog.

This wasn’t quite a personal DX record for me, but it was close, and the first time I’ve heard South Africa in quite some time. I spotted ZS1LS twice this morning on 30m, from grid square JF96fd and a distance of 16477 kilometers. I had previously heard ZS6Y (16927 km) and ZS1TX (16492 km), and these three stations are now my most distant received stations via WSPR. Immediately below them are a smattering of VK6 stations (VK6ADF, VK6WS, VK6GOS, VK6RO, VK6BMW, VK6BN, VK6POP) interspersed with DP1POL, who was operating from grid IB59uh in Antartica.

Cool.

I haven’t been operating in transmit mode much lately, but if I look at stations that have heard me, I see the most distant station is VK6WS, at a distance of 14842 kilometers.

A recent issue of Make magazine had an article about launching water rockets via hydrogen/oxygen combustion: basically an electric current is used to break water into its constituent hydrogen and oxygen, which then bubbles up in the rocket, forcing out some additional water. To launch, this hydrogen is ignited, and recombines quickly into water vapor, but also generates a huge burst of pressure and launches the rocket.

And here was my question: how big of a burst of pressure does it really generate? Is it safe? 2 liter bottles have a burst pressure of around 120psi or so. That’s actually reachable with ordinary mechanical means, and it is pretty easy to understand and monitor: the pressure increases slowly, and can be monitored if necessary by an ordinary pressure gauge.

The chemical reaction isn’t so easy to wrap one’s head around. You actually need to know some physics. Sadly, I’m mostly self taught, so I have to work through these things slowly. A discussion with Michael and Tom over lunch the other day reminded me of the ideal gas law. One mole of hydrogen and one half mole of oxygen combine to create one mole of water vapor. But I must admit: I know very little about the realities of combustion physics.

So, I did what I always did: I googled. And I found this interesting page. It estimates the pressure to peak at around 160psi, well beyond the burst pressure of the common 2L bottle. To keep the rocket from bursting, it is vital then to dilute the reaction by the introduction of ordinary air. The nitrogen won’t combust, and should limit the overpressure.

I’ll have to muddle over the details more sometime, but it’s good reading.

Powering Water rockets with hydrogen combustion

While tuning around, I found some of the VOLMET traffic from NY this evening. In trying to identify other aeronautical signals on HF, I discovered this rather nice map, which I thought I’d archive for future reference. I should print this out and get it laminated.

HFAERO.jpg (3397×1781).

There has been a lot of publications lately about water rockets. These are rockets which are usually constructed of empty plastic soda bottles, pressurized by a bicycle pump and launched into the air. I haven’t done any of this, but it sounds like great fun. I even picked up a copy of Soda Pop Rockets by Paul Jarvis, which is a colorful if rather arts-n-craftsy book on the subject. While reading it, I couldn’t help but muse about the physics involved. How do things like diameter of the rocket, diameter of the exhaust nozzle, and the amount of pressure affect the height that the rocket might achieve?

I mentioned this to Loren over lunch the other day, since I know that he has done his fair share of water bottle rocket launches, usually using liquid nitrogen as the propellant. He had a few interesting insights, namely that the nozzle design which is common in ordinary rockets isn’t really useful in water rockets, as water is essentially incompressible. This means that you can use Bernoulli’s equation to compute the force generated. Of course, the mass of the rocket is continuously dropping as water is expelled, but that’s not too hard to deal with in the simulation.

What wasn’t clear to me was that after the water is exhausted, there is still residual air pressure in the rocket, and this pressure is significant and must be dealt with differently since the air is compressible. A bit of research led me to this rather nice website:

Rocket Science.

He has an interesting example of a 2 liter bottle which is pressurized to 100 psi and filled 20% with water. It’s fascinating: the “water burnout” (when all water has left the rocket) occurs only 0.042 seconds after launch, when the rocket has an altitude of only 3.2 feet (!). It continues to accelerate though as the air pressure equalizes. In this example, one third of the velocity of the rocket is obtained after water burnout.

It might be fun to make an implementation of this.

Addendum: Here’s another link to a water rocket simulator.
Addendum²: Another simulator.