More early reading on vacuum tubes. To be honest, I never really understood (or frankly studied) the physics of semiconductors, which always seemed a bit abstract to me. Having read some of these early texts on how vacuum tubes work, they seem fairly straightforward by comparison. I wonder if I go back and read some introductory semiconductor texts, whether they will seem more straightforward now?

Anyway, another nifty classic text on Google Books:

I believe that it was the Amateur Radio newsline podcast which mentioned the following:

A Miami based Radio Station – The Disco Palace – has started broadcasting a DRM SW channel of ‘best of Disco music’ for listeners in Europe and North America.

via New DRM channel of Disco Music  :: DRM.

So, today I tuned in and recorded some of it using the SDR-IQ, and then processed it with GNU DREAM, the GPL’ed DRM decoder. The signal was very strong, no drop outs, and well, it’s disco. Here’s a couple minutes of the decode.

The Disco Palace broadcast using Digital Radio Mondiale on 17.755Mhz

As part of my delvings into things vacuum tubes, I of course found that many others have been down this road, including this rather interesting receiver built by Alan Yates. Being a novice at this, I was intrigued by the fact that his circuit used the 12DZ6 tube, which could apparently be powered by ordinary 12 volt supplies. Its application was in automobile radio circuits, where high voltages were inconvenient (when needed, they were often generated by “vibrators”, which created an AC voltage from a DC battery voltage, which could then be stepped-up by a transformer). When I mentioned this to Tom, he thought (as I did) that it was odd, and that he didn’t understand how tubes could operate with such low plate voltages. But of course, if you search, you shall find, so I uncovered this terrific page that describes some theory and circuits that use these low voltage (and presumably safer) tubes in radio circuits. Nifty.

Low Voltage Tubes

For the last week, I’ve been embarking on a ham radio “trip down memory lane”. Well, it would be memory lane if I had any real memories of the tube-based equipment that were staples of the ham shack until probably the 1970’s or so. But if I have a personal philosophy of my little projects, it’s that one has to look back to gain perspective about our current technology. Or something like that. Perhaps that’s just a rationalization for spending time reading old books about vacuum tube design. Or perhaps this is all motivated by the idea of building a radio that glows from scratch. Or, perhaps to demonstrate that I understand the similarities between tubes and FETs.

Whatever the motivation, I’ve been looking around at projects that people have done. A popular project seems to be “twinplex” radio, which uses a single dual triode tube in a regenerative receiver. Staring at circuits and reading Basic Theory and Application of Electron Tubes, I’m beginning to understand the functioning of these circuits. And, it turns out youtube has a lot of nice inspirational videos of people’s projects, such as the following:

httpv://www.youtube.com/watch?v=PYeB7nGwGv0

Now that the winter is over, our local flea markets at Livermore and De Anza should be starting up again soon. Perhaps I’ll keep an eye out for the components.

Addendum: Here’s another link for inspiration.

While over at Ham Radio Outlet the other day, I noticed a new QRP/homebrewing book on the shelf published by the RSGB:

International QRP Collection, edited by Rev. George Dobbs and Steve Telenius-Lowe.

It’s not an ideal book from the homebrewer/QRP viewpoint: it includes a bit too much operating/equipment reviews for my taste. Still, it has a number of interesting construction articles, including the “One Transistor Marvel” from Dave Ingram, K4TWJ (SK, sorely missed) which I hadn’t seen elsewhere, but obviously inspired things like G3XBM’s XBM80-2 transciever. It also includes KK7B’s articles on designing and building linear transistor amplifiers that appeared in QST recently. I’d say that’s a waste, but the articles are so good, I’d like to see them in book form (I always seem to lose magazines). It also includes details on some simple minimal rigs that I hadn’t seen before, such as G3MY’s Pippin, and some novel variations on the Pixie.

Overall, I give it an 8/10. Worth having, but not as good as DeMaw’s QRP Design Notebook.

In addition to the checkers books that I got the other day, I also picked up a couple of radio books. One was an old book on electric circuits (perhaps the subject of a post some time in the future) and the other was this book from Lindsay books on constructing tube-based regenerative receivers.

Lindsay: Build Your 1st Vacuum Tube Regenerative Receiver

It’s a reasonable question as to why you might be interested in trying to build such a thing, but frankly, I don’t have a reasoned answer. I just think it would be cool to experience this technology by actually creating a working example of this kind of art. And it’s kind of cool to build a radio that glows.

We’ll see how far this project goes.

Roger, G3XBM once again turned me on to an interesting link, this one on a group of hams who are experimenting with communication over optical frequencies. This is a topic that has interested me greatly in the past (I seem to be interested in the extremes of amateur radio, both in terms of long wavelengths and short) and seems to be a rich area for experimentation. Check it out:

The Carolina Flashers Photonics Group – Welcome.

Addendum: They link to a fascinating demonstration of a German WW2 light-based transceiver system. Very cool.

httpv://www.youtube.com/watch?v=vR4N6MTx_vw

I’ve been a little too busy to fire up the soldering iron and build anything, but I’ve been pondering putting together one of Roger’s G3XBM radios. I ordered a couple of high impedance crystal earphones just to be ready, but in the mean time, I’m studying the circuit diagrams fairly closely:

XBM80-2 An Experimental 80m CW Transceiver G3XBM.

The radio is simple enough that I should be able to understand it completely, but I must admit that I don’t understand the principle of regeneration very well, so staring at radio, it’s not particularly obvious to me how T1 serves as both mixer and oscillator. I also am not certain I see how the oscillator doesn’t radiate when the radio is in receive mode. I’ll ponder it some more, and maybe fire up LTSpice over the weekend to test my understanding.

This month’s QST had a pointer to a potentially interesting voice codec that I hadn’t seen before. It appears that Broadcom has developed a voice codec, and released it under the terms of the GPL for royalty-free use. It’s actually two codecs: one at 32kbps, and the other at only 16kbps. I haven’t done much reading on them yet, but there are certainly a number of possible applications that I can think of where having open codecs like this would be useful. Unfortunately, we probably need a codec with a data rate more down in the 2400 baud range to make digital voice on VHF comparable to DStar, but it’s still interesting.

Check out the links:

Broadcom.com – BroadVoice® Speech Codec Open Source C Code.

Addendum: In listening to the voice samples, I must say that they seem quite good, although not perhaps any better than the already free Speex based codecs. It’d be interesting to do an apples/apples comparison of the two, but look at these Speex encoded examples which sound very good at approximately half the bitrates of the BroadVoice codecs.

Addendum²: K3NG mentions the “white elephant in the room” with respect to D-Star: namely that it uses a proprietary codec. I must admit that I’d probably own a D-Star radio if the speech codecs were open. I find it really annoying when amateur radio adopts techniques which are covered by patents: they serve only to protect business interests, and to discourage free experimentation and deployment of new radio techniques. If D-Star used a freely available audio codec, there would be less to keep other manufacturers from supplying compatible radios (they wouldn’t need to license technology or single source chips from a single manufacturer) and there would be a great deal more experimentation (echolink->D-Star gateways anyone?). As cool as D-Star is, I can’t help but think that we are sending the wrong message to equipment manufacturers by adopting them widely.

Looks like an RTTY contest is currently going on, and nothing is better for revealing band openings. I tuned up to 15m to see what was going on, and the normally quiet 15m band was alight with stations:

Imagine what the band would look like if I had a real antenna!

Addendum: 20m is even more alight:

Here is the tiny clever bit from last night’s demodulation experiment using HF radiofax. For the purposes of this experiment, I record a single channel audio of the HF fax transmission. The audio is centered at an audio frequency of 1.9 Khz, and varies (except for start/stop pulses) between 1500 and 2300 Hz. I begin by band pass filtering using a filter designed with the applets you can find at this website, then split the signal into two halfs, and multiply one half by a sine at 1900 Hz, the other by a cosine at 1900Hz. This works just like a I/Q mixer in a software defined radio (in fact, it is just such a mixer). This frequency shifts the signal down so that now all signals occur between -400 and +400 hz.

From each pair of I, Q values, we can easily determine the squared amplitude of the signal (just I² + Q²) or the instantaneous phase of the signal (atan(I/Q)) but we are interested in the frequency: the _change_ in phase between two samples. I’ve sen this derived before, but this way makes the most sense to me, so I thought I’d write it down.

Let’s consider two adjacent I/Q samples, I₀ Q₀ and I₁ Q₁. The question is, what is the angle that will rotate sample zero to sample one? Well, first, we must realize that they may differ in amplitude. We’re not concerned about their amplitude, so we might as well get to normalizing them. We simply divide them through by their length. That’s no problem. But then how do we compute the angle between them?

I chose to use the geometric interpretation of cross products to figure out the math. If we have two three-dimensional vectors, and compute their cross product, the resulting vector is perpendicular to both, and has length proportional to product of the length of the two vectors and the sine of the angle between them. In our case, our vectors seem like two-dimensional vectors, but let’s fix that simply: tack on an extra zero to each pair. They are now obviously both contained in the X-Y plane, we know that the normal is perpendicular to that plane so has zero components for both the X-Y entries, and the Z component contains all the interesting bits. In particular, we know that its proportional to the length of the two vectors (which are both one, since we normalized, and can be ignored) and the sine of the angle between them. Hence, we could just take the arcsine, and recover the angular change (which would be between -π/2 and &pi/2). We can then map this back into frequencies if needed (-samplerate/2 to +samplerate/2) or in our case, map that into grayscale values.

But surely you are balking at this ugly arcsine call in the middle of our program, aren’t you? Well, it’s not like we haven’t got CPU to burn. The implementation I have processes 15 minutes of recorded audio in just a few seconds, so even using an arcsine results in a program which is 100 times faster than realtime on current hardware. But you are right: it worried me a bit, so I scratched a little deeper. When you have sines and cosines in things, it’s often useful to use small angle approximations. These are derived from the Taylor series expansions for sine and cosine.

For small x (values near zero) cosine(x) is approximately one, and sine(x) is approximately x (just take the first term of each of the above equations. But, if sin(x) is just about x, that means that the asin(x) is just about x too. Thus, we can choose to not use asine at all, and instead just use the output of our cross product calculation as our frequency, and we might expect it to work just fine (it has range -1 to 1, whereas asin has range -π/2 to π/2, but the frequencies we are interested in are confined to a small portion of the audio spectrum anyway, so the error is likely to be minimal).

If you want, you could try to use more terms from the series for arcsine:

but in my experimentation, it made no difference in image quality.

Anyway, once you recover frequency, you want to map values from near -400 hz to black, and 400 to white. You need to normalize for sample rate, some clamping, and some resizing, and you are done. Okay, I haven’t covered rectification/slope correction, and the aspect ratio stuff is a bit tricky to get exactly right, but I’m still working on them.

Hope this wasn’t too boring.