Meanderings – My BLOG

Meanderings are irregular BLOG postings about things that happen to and and are done by neal as he follows the spider through life’s web. It’s a sort of diary really. But let’s just see what appears here. I’m curious because I will allow it to just write itself as it were.

How Namibia got FM Radio

In 1969, years before the start of Television in South Africa, we were entertained by radio programs. In South Africa itself, we could listen on Short-wave and in all the bigger centres we had Medium-Wave. Both of these are what we call AM (Amplitude-Modulated) systems. Later on we got FM and the quality was amazing.

You cannot broadcast sound, i.e. audio, directly. Sounds produce pressure waves that attack our ears at a rate of between 50 and 15 000 pulses/second. The electrical signals that correspond to these sound waves are accordingly of  extremely low frequency when compared to radio waves and they could not be radiated from an antenna. To radiate from an antenna, we need to use signals that vibrate at a couple of hundred thousand cycles/second up to many millions of cycles/second. So what we do is, we generate these higher frequencies and push them into antennas that will radiate them into the space around and above the antenna. Medium-wave and short wave transmissions work by radiating the energy away into near-space where it is refracted (bent) by magnetic layers in the atmosphere and caused to bend back towards the earth. It then touches down again at a distance of many km from the transmitting antenna. The exact skip-distance (technical term) from origin to reception point depends on the frequency of the transmitted wave, the angle of the antenna to the surface of the earth,  and also to some degree on magnetic conditions in the atmosphere. These conditions change all the time as radiation from the sun ionizes various layers of atmosphere. In this way it is possible to shoot a signal into the sky in say Johannesburg and receive it in Cape Town or London.

Higher frequencies than those used for medium and short-wave radio can also be transmitted. As the frequency rises however, the way that the wave is propagated through the atmosphere changes gradually. When we get above the short-wave band, say roughly to wavelengths shorter than 10 metres i.e frequencies higher than 30 MHz (30 million vibrations per second), waves sent off into space will no longer be refracted back to Earth. At those frequencies the energy in the wave will simply carry it straight on out into space where it will travel forever or until it bumps into something and gets absorbed. But these so called VHF (Very High Frequency), UHF (Ultra High Frequency), and Microwaves can be used for what we line-of-sight or terrestrial transmissions between places on earth that can see each other. The transmission antenna is designed to send the waves in a beam (exactly like a torch beam) to the receive point. (A torch beam is a beam of radio energy and is only different in that it is in the frequency band that human eyes can see. Eyes are actually radio receiving antennae tuned to the extremely high frequency, GHz (Gigahertz) range. Visible waves are between 400 and 700 nm (nano-metres) in length.)

So we wanted to send some music through the air but we could not and instead we sent a steady beam of radio waves that had no direct meaning for us humans. Well, here’s the secret of transmitting the music or other intelligence we want to receive. The basic beam  of high frequency radio energy that we can transmit is called the “carrier-wave”. But we modulate the carrier wave with the sound waves we want to transmit. Modulation means that we change the shape of the carrier in some way so that when it reaches the receiver, we can detect these changes, discard the carrier and retrieve the original intelligence. Modulation can be done in any way that will change the shape of the carrier in a recognisable manner. In AM radio (Amplitude Modulated Radio), we cause the carrier to get weaker or stronger  directly in proportion to the strength of the modulating signal. i.e. the radio Graph of an Amplitude Modulated carrier
program. In this diagram that I lifted from the internet, you will see that the outer shape of the top part of the modulated carrier (the upper sideband) has the same form as the original modulating soundwave. Actually the bottom shape of the modulated carrier also carries the same information upside-down and that is known as the lower sideband. In practice we delete the carrier and the lower sideband in the average AM receiver and are left with just one copy of the original modulating sound wave.

So much for AM but what and why do we consider FM to be a better system? Well in FM (Frequency Modulation), we have a carrier as before and we change its shape with the modulating signal as before. It’s only that we use a different method of modulation. Instead of varying the amplitude of the carrier, we vary the frequency of the carrier from moment to moment in proportion to the amplitude of the modulating signal. So a loud sound makes the carrier vibrate faster and a soft sound makes it vibrate slower. For complex reasons that I won’t try to describe here, this can only be done if the carrier frequency is very high. That is why the FM radio band tunes from 88 to 108 Mhz (Million cycles/second), many times higher in frequency that the medium-wave band and even more so than the short-wave band. The reason we use FM is that AM transmissions get interfered with by all sort of things. A car spark plug or a fluorescent light or a switched-mode electronic power supply or an electric drill  etc. all radiate signals into space and these are almost all AM type signals. When these interfering signals reach your receiver at the same time as the transmission you want to hear, your radio cannot discriminate and receives both. The result is that AM signals are greatly troubled by interfering noise sources. Frequency modulation

None of the devices mentioned above generate FM type radiation however so when we are looking for frequency variations in the receiver we can ignore all the amplitude varying interference and that means that FM radio is quiet and noise free. Here’s a graphic illustration of how FM modulation works.

So, back to Namibia and FM. In 1969, Namibia could only receive Shortwave radio from Johannesburg or overseas and shortwave does not generally produce good quality. It was decided to extend the growing FM transmitter network from South Africa to Namibia (Then called South-West Africa). Well that was all very well, but how could the radio programs be sent to Windhoek so that they could be put onto the FM transmitters? Within South Africa, there was a network of telephone lines that carried the radio station signals to each transmitter on each hill so that it could be broadcast. But at that stage, there were no telephone lines between South Africa and Namibia. To solve the problem, the SABC built a short-wave receiver station in Windhoek. A receiver hall was filled with highly sophisticated German-built receiving equipment fed off highly sophisticated aerials that could make a reasonable job of receiving the South African radio stations from the Johannesburg short-wave transmitters. The quality was very much better than any domestic short-wave receiver could manage, and whilst it was not high fidelity music quality, it was pretty good. FM transmitter stations were then built on the mountain outside Windhoek and at Oshakati on the Ovamboland border and the signals received at the Windhoek receiver station were fed by local post office lines to these transmitters. The SABC then also built radio studios in Windhoek to produce local radio programs in English, Afrikaans, Ovambo, Herero and the Damara-nama languages.

And so FM broadcasting came to Namibia and I’m pleased that I was there and could be part of getting it all going.

Bamboo incense burner

Just to complete the set of blog articles on things I have made of bamboo, this one on an incense burner.

I  sometimes burn incense to create an atmosphere and I needed a burner to stop the ash from falling all over the place. I had salvaged some lengths of bamboo that someone else had discarded & I felt they lent themselves to this little project. Some of the Joss Sticks I have are quite long and they are best burned in a holder at an angle to the vertical so that their ash falls into a suitable collection tray. So I cut a 500mm piece of the bamboo and split it in the length to form the collection tray. The burner needed to stand on a stable base and I considered ways of creating that. I could have attached small feet under my half-tube of bamboo or I could have just sanded down the underside of the bamboo to create a flat base to rest on. Flattening the bottom was an appealing approach but it would have taken too much wood off the underside and left it flimsy. Eventually I decided to use a shorter section of the same bamboo and glue and rivet that onto the actual burner. With a lick of varnish it looked sort of arty and the short under-piece could be sanded to provide the necessary flat support surface. So here are views of my incense burner from above and from the side. It goes well with my chanter quiver and music stand that I have described in previous articles.

Musical bamboo

In an earlier blog article I described the bamboo quiver I made to carry my Bagpipe practice chanter around. Well, after that I still had some bamboo left over, so I decided to  make myself a music stand on a similar theme. Actually I didn’t quite make the whole thing.

I happened to have an old paraffin torch lying around. One of these decorative devices Bamboo torchone plants in the ground to create atmosphere when having people round for a braai. (South African word for barbecue). it is basically a piece of bamboo about a metre and a half in length. The upper end is split into a number of thin fingers which are then bound around a metal paraffin torch to create a pleasant flickering orange light. I tossed the burner away and also the binding ties that kept the upper fingers cupped around the burner. I then turned the thing upside-down so that the fingers became legs on which the bamboo could stand on a hard surface. Music stand from front.To give it some stability, I put a wooden spacer in between the fingers and pulled that up to splay the fingers into a reasonably Music frame tiltedwide base. I then made a wooden collar that could slide up and down the pole and I put a threaded thumbscrew into that so I could lock it at various heights on the pole so that a musician can sit or stand whilst using the stand. I then cut slivers of bamboo and screwed them together to make the actual sheet music support. This support frame swivels on the wooden collar that slides on the upright, allowing the user to tilt the music support frame to a convenient angle. The music frame only has one screw per joint and the thing is carefully measured, so it can fold up quite small if required for transportation. In this picture of it folded, it has a terry clip for mounting it. But this was later replaced by the wooded collar and thumbscrewMusic support folded web.jpg
I mentioned higher up. The tall upright in the middle is supposed to look like a musical note (crotchet) but it has a practical purpose too. It is a counter balance to offset the weight of the frame below and make the whole assembly more stable. The head of the crotchet leans to the right because the frame is mounted off to the left of the upright pole and the crotchet shape of the counterbalance helps to distribute the weight properly. The support frame itself is also designed to resemble a # musical sharp note. At the base of the frame, I have fashioned a shelf to support the sheet music and it has three little bamboo fingers pivoted on its front edge to retain the music.

All in all it is probably more arty than practical but it does work if one is careful with it. I have also recently acquired a neat little book-light that clips onto the top of the upright pole and lights the sheet music very effectively in a darkened room.

 

Bamboos and boxes

Provide the base support for the chanter

So when I started to learn to play the bagpipes, the first thing I had to purchase was a practice chanter. The full set of bagpipes uses a hide or synthetic bag to provide a steady flow of air through the chanter and the drones. The drones are the three pipes that rest on the pipers shoulder and they just make different bass notes to support the chanter. IMG_0505The chanter itself though is the pipe with finger holes in it that makes the notes of the tune. Here is a photo of yours truly taken by my bagpipe teacher when I went for a lesson with my new Chanter reedpipes. There is a reed in the chanter that vibrates when a stream of air from the bag passes over and through it. But here’s the thing. A bagpipe chanter makes a heck of a loud sound (so,me would say noise), and so do the drones. If you practiced playing them in your apartment, some neighbour would sooner or later shoot you. So in modern times, the practice chanter was developed. It is similar to the actual chanter, but the reed is made of plastic and is relatively un-noisy. There is no bag so the air to sound the practice chanter is provided directly by the player blowing down a blow stick which is connected directly to the practice chanter.

So having set the scene, let’s get back to the title of this blog article, i.e. ‘Bamboos and boxes’.

Chanter quiver comp
Chanter quiver

Any musical instrument should be protected for transportation and since I had some bamboo lying around, I sawed off a length and used it to make a carrying case for my practice chanter. The cap and strap are made of leather. I thought it was quite arty looking and would be a convenient way of carrying the chanter around. Here is a picture of what I called ‘The Quiver’. It worked O.K. but it is quite a long and bulky object and I realised after a while that it was not very practical. So I made a container of a different design for the chanter. It is a little foam lined, hinged box with cutouts in the foam to cushion the chanter.

To transport the chanter in this box, one unplugs the blowstick from the lower part and stows them side by side. The reed has to be removed too because it would be too vulnerable if it was left protruding from the playing tube. The reed and also a couple of spare reeds get popped into a small plastic pill box with a desiccant to keep the reeds moisture free. Piper 2 nealPractice chanter reeds are made of plastic and don’t work well when wet. Real chanter reeds are made of special Indian reed and they have to be moist if they are to be coaxed into making any sounds at all.

Tape-recording Video

We saw that when recording analogue sound on tape, head to tape speed was critical. Some musical notes sound higher pitched to us because their wave patterns have higher frequencies and shorter wavelengths. If the wavelength becomes so short that it starts to approach the length of the gap between the recording head poles, the waves are cancelled out as they pass the head. We can increase the maximum recordable frequency by reducing the width of the head gap or by speeding the tape up so the magnetic flux is spread over a longer section of the tape.

But head gaps can only be made so small and tape speed cannot be increased 300 times to make it possible to record the 5 million cycles per second (5MHz) that video requires as opposed to the 15 000 Hz that good quality audio requires. To record analogue broadcast video on a tape moving across a fixed head, the tape would need to run at about 140km/hr. Giant reels of tape would have to be managed and the whole thing would be highly impractical. So how do they record analogue video then?

Simple really. The tape does move, as before, at about 38 cm/sec but the head is not fixed. TRacks on quad tapeThe head is a tiny construction on the rim of a lightweight wheel and spins at a very high across the width of the tape. i.e. If we dip the recorded tape into alcohol containing very fine iron filings in suspension, and then dry it out, the recorded track appears as a series of stripes running almost transversally across the moving tape. Professional recording tapes were 2″  (about 5 cm) wide, and were sucked into a curved guide by a vacuum pump whilst they moved past the spinning recording head. Looking at the tape guide side-on, it looked something like this. This curving of the moving tape enabled the head-wheel to spin in an arc making contact with the tape throughout .

Quad head guid

The spinning head would start touching the tape at the top and lay down a track till it reached the bottom (Not strictly true but it works for now). You will see that there are 4 heads on the spinning wheel and each one would record a quarter of a complete TV picture onto the stripe on the tape. By the time the next head reaches the tape, the tape itself will have moved on a bit and the second head then records its stripe.After a full circle of the wheel, there would be four parallel recorded stripes across the tape containing the information for one whole TV picture. This ingenious approach produced a head to tape speed fast enough to record the 5 MHz waveshape which described the frequency modulated, broadcast standard,  video signal.This was known as a quadruplex system. There were other systems such as helical alpha-wrap etc but let’s deal with one at a time. They are all variations on the same principal.

During playback, the tape was synchronised by pulses recorded along the bottom edge so that the heads would find the recorded patterns and be able to track them and read the magnetic record on the tape. There was a lot of circuitry involved and the process required extensive mechanical and electronic stabilising, but it was done and high quality video pictures were recorded and recovered.

Acknowledgement: pictures taken from website http://archive.totterslane.co.uk/tech/quad.htm. Content is my own.

An electronic metronome

I am retired and live in a small apartment, so the days of having large workshops and rooms full of electronic bits are all gone. I do occasionally build a little gadget or a box though, just because I like doing that. As my current preoccupation is learning to play the bagpipes, most of the things I have made lately are to support my music making.

Metronome circuit diagram

I needed a metronome to provide rhythmic clicking sounds and flash LED lights in time to the music, or rather to keep me playing at the speed the music should be played. 

One can of course just download a metronome app on one’s cellphone but that is not in the spirit of building things. So I scratched available components out my spare parts box and reassigned a plastic container from my kitchen to build the thing into. Here’s how it all went together. Above is the circuit diagram. It is a  pretty standard design except that component values have been selected for this specific purpose.

Board drilled inc 555I used ‘Veroboard’ to mount the components onto. It consists of copper strips on the surface of an insulating board. The strips can be cut into sections using a drill, This provides wires that connect those things that need to be connected. Here’s a view of that process. The components mount onto the other side of the board and their wires are poked through holes in the tracks and soldered to the tracks.

Here is a view of  the underside of the board with all the components soldered into place. The purpose of the terry clip is to hold the 9V battery. The holes in the corners are for mounting screws. The wires you see connect to the controls which are mounted on the lid of the plastic box. Nothing is mounted to the bottom or sides of the box , so the whole thing can be accessed by simply removing the lid from the box.  Board underside soldered & bat clip compHere is a view of the component side of the board. It is shown in the maintenance position where everything can be accessed. During operation it moves inside the box next to the speaker and above the controlks so the box can be closed and clipped together.  Top of board in maintenance position

Here is a view of the whole unit, assembled and ready to operate. There is an ON/OFF switch and the click speed and volume can be set on the controls.

Completed unit - angle shot

My Blog does not support videos so I cannot show you how it looks and sounds. But because it makes a cheerful clip-clop sound at the pace of the music, I have nicknamed it “Pony”.

 

 

Chapter 2
A friend of mine once learned to do morse code and I also used to do that when I was a practicing Radio Ham. So the friend asked if I could build an oscillator which we could use to practice Morse code with. But a code oscillator is nothing different from a metronome except that it runs about 1 000 times faster. If I set the metronome to run very very fast, the rapid clicking sounds to the human ear like a musical or whistling tone. Click here t6o get the effect.   The metronome controls don’t adjust the speed of the clicking to go that fast, so I just added a switch to the top which switches in different timing components and turns the metronome into an oscillator. I then also installed a jack socket on the box (not jack-in-the -box), into which I can plug a morse key. It is just a contact on a spring loaded arm and when you tap the contact arm. it completes the circuit and sends out a burst of tone. A short burst becomes a dot and a longer burst becomes a dash. The metronome speed control becomes a pitch control to vary the note it gives out up  and down the scale.

Here are the Morse code alpha-numerics in case you would like to practice your Morse code. LOL.Morse code

The rise & fall of recording tape

Part 2 – 0 Audio tape basics

During the 1950s and 60s, magnetic audio recording tape came very much into its own. It had been around for a long time but it was not robust and the quality was poor. The detail is not for a blog like this.

The modern tapes of the time were 1/4″ wide single track tapes made of a plastic called cellulose acetate. It was strong and flexible and did not easily stretch. If a recorded tape is stretched out of shape and then played back, the sound will be distorted. To make recording tape, small magnetizable metal-oxide particles, suspended in a lacquer that hardens into a flexible layer when it dried, is spread on the surface of the acetate ribbon. I have heard such tape laughingly described as a strip of plastic with rust painted onto it.

Now whilst the particles in the emulsion can be magnetised to remember the music waveform that had been recorded, the magnetism does not spread on the tape as it did in the old wire recorders. This is because the particles tape-headare discreet from each other and not contiguous. Tape from the supply reel is drawn over the recording head and taken up on another reel.The head itself is a metal ring cut through at a point to form an extremely narrow gap. A coil of wire is then wound around the head ring and the audio signal that needs to be recorded is passed through the wire. The signal current flow causes a magnetic field across the head gap with a magnitude proportional to the audio signal being recorded. Louder signals cause a stronger current in the head and magnetize the tape more. As the tape is drawn across the head gap, the particles on the tape are magnetised and when that tape is again passed over the head later, it will induce a current into the coil which can be read out as the original music.

All was well in theory but in the original systems the quality was poor due to losses and distortions. For one thing, the amount of magnetism induced onto the tape was not directly proportional to the magnetising force. Small magnetizing signals magnetized the tape to a certain extent but doubling the input signal did not double the magnetization. This led to a distorted copy of the original when played back.  The problem became less severe at higher tape magnetization levels and the reproduction was reasonably good in the middle range, or so called linear part of the tape’s magnetization curve. So a way had to be found to use just that linear range of the tape magnetization curve over which the recorded signal remained a fairly accurate facsimile of the original signal. This was done originally using either a fixed magnet  or a small DC magnetizing current through the head-coil, to pre-magnetize the tape to the point that it started operating on the linear portion of its magnetization curve. This was called DC bias.  The varying signal current was then also passed through the head to create the recording using only the linear part of the magnetization curve. The trouble with that approach is that there is a limit to how much you can magnetise the particles on the tape. At some point the tape is fully magnetized or ‘saturated’. So if the DC bias is too strong, there is not much headroom between where the DC bias leaves the tape magnetization level and and the saturation point of the tape. To fix this, the DC biasing system was replaced by a high frequency AC biasing signal. To understand how this works though we need to understand the importance the head gap and tape speed.

The audio signals we want to record are alternating electrical signals. i.e. they swing from positive to negative and back to positive rapidly. We say they are AC (alternating current) signalssine. If we draw a graph of a pure musical tone it will be a simple Sin wave, i.e. one that varies in intensity in a constant repetitive fashion in time, it could look like this. The sound we hear when we strike for instance a middle A note on the piano changes polarity 440 times per second. The horizontal axis of the graph indicates the point of zero magnetising current. As the current gets more strongly positive it will rise above the line and as it becomes negative it will be shown below the line. But this graph is a snapshot of a moment in time. If you look at it a moment later it will all have moved a bit to the left. So the graph depicts how positive or negative the current is at various moments in time. Click on the arrow below to hear  what it sounds like. . So if you look at this graph you will see that one cycle of the sound or one wavelength if you prefer that term, is from any point on the graph through till when it gets back to the same point and starts repeating itself. If there are 440 repetitions per second as in the case of middle A on the piano, each cycle will last for 1/440th of a second. i.e as the tape moves past the record/playback head, 440 wavelengths will pass the gap each second. It all works fine unless the head gap is too large. If the gap is the size of a wavelength, both halves of the cycle, positive and negative will be in the gap at the same time and the magnetic fields being produced will cancel each other out leaving no recording on the tape. So the gap needs to be very small compared to a wavelength so that it can leave a recorded pattern on the tape which truly represents the original signal. This is what defines the quality of a tape recording head. It is much harder and more expensive to make a head with a very small gap but the smaller the gap, the shorter the wavelength it can record or play back. If the wavelength gets shorter, the frequency (pitch) rises. The sound-wave travels past our ears at a more or less fixed rate so if the wavelength is shorter, more cycle will pass our ears and that means we hear more frequent peaks and valleys. We say the frequency increases as the wavelength shortens. Young people can hear frequencies of 15 000 Hz (vibrations per second) or even higher. That’s a very short wavelength and a head gap has to be really small to reproduce it. But then again it depends how fast the tape moves past the head. if we speed the tape up, each cycle of the recorded signal stretches out on the face of the tape. Or said differently, at high speeds, a wavelength representing a certain musical note covers a longer piece of tape. This means that a given head gap can record higher frequencies if the tape speed is higher.

So then, back to bias. We said that DC bias compresses the useable range of magnetic tape and therefore we get a poor signal to noise ratio and we hear noise on playback. if we use a high frequency signal to pre-magnetize (bias) the tape we have a different result. If the bias current vibrates at several times the highest frequency we need, it will lift the recording on its back onto the linear portion of the tape’s magnetic response but the gap cannot record this high frequency and the bias disappears as the tape leaves the head. This enables the tape to eventually record a much larger range of signal levels than DC bias does.

Its complicated to understand but it actually does work and high quality tape recording uses high speed biasing to improve the quality. This is dry stuff anyway so next time I write on this topic we can move onto video tape recording which is another thing altogether.

bar_magnet field