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 speed 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

When is a lunch box not a lunch box?

I was most interested to see on Benjay’s Google+ page, he made use of items such as his daughters shape boxes to house amplifiers, speakers etc. It’s alway been a challenge to find non-expensive cases and cabinets for one’s projects and I think Benjay’s solutions are pretty innovative.  I’ll work back to that but I have to start the story differently.

When I worked in the Broadcasting industry in the 1960s and 70s, a lot of the programs we packaged for broadcast were still recorded on large, 12″, gramophone records. We even used to receive the ‘Goon Show’ by mail from the BBC on 12″ records. Tape recorders had been around for quite a few years but if you wanted a truly high quality recording, it was still done on disc. When the SABC wanted to broadcast a symphony concert from the city hall or wherever, a truck would be used to transfer the massive Neumann cutting lathe down to the venue. The machine would have to be manhandled into the building by two strong young technicians and set up with spirit levels to be truly horizontal on a solid base. A virgin wax disc was placed on the turntable and the concert would be cut on that. This precision, German-built Neumann record-cutting lathe was a really solid piece of equipment and the recordings it made were of a very high standard. For instance, the cutting head was not mounted on a swinging arm as we see with domestic record players. With a swinging arm, the cutting stylus would only run exactly true to the direction of the track at one point in its tracking arc. At all other angles of rotation, the cutting tip would be at a slight angle to the track and this would cause distortion. Later on, Hi-Fi record players in the studio and at home minimized this error by using the longest possible tone arm so that the total arc transcribed by the stylus was as flat as possible. But in the cutting lathe, no error at all was allowed and this was achieved by using a headstock that advanced the very solid and precise cutting head across the disc from edge to centre in a straight line as the recording took place. Neumann disc-cutting lathe

The delicate recording on the wax disc was then taken back to a laboratory where it was electroplated with a nickel alloy in a complex multi-stage process. When the nickel plating layer was then removed from the wax original, it was an accurate and hard, negative copy of the original. This could then be used as the die for stamping out playable records.  In the days before Vinyl records, the old commercial 78 speed discs were made of shellac, and if you dropped them they smashed into pieces much as a teacup would do today if you dropped it. But the records made specifically for high quality broadcasting of recordings such as the concert we have been talking about were pressed into shellac which was just the skin on an aluminium base disc. This made them more robust than commercial discs but of course they were still vulnerable and quickly developed surface noise etc. with usage. They were however only ever played a few times for broadcast purposes after which they were either archived or simply discarded.

Now this is where my story started. The shellac covered aluminium discs that were not archived were simply tossed out and we who worked in the technical department got to take boxes of them home with us for free. We would chuck such a disc into a basin of boiling water and this would cause the aluminium inside to expand and simply burst the shellac off it. We were left with a beautiful shiny aluminium disc you could see your face in, and this we could use for making equipment chassis, faceplates etc. How marvelous that was to those of us who wanted to build, amplifiers and radios and other gadgets. We would build those valve amplifiers etc. with great care so that they looked really good, The little glowing bottles would all line up like well trained soldiers and the wiring under the chassis was done painstakingly and attractively. Sometimes they looked so good that we used them just like that so everybody could see how neatly we had worked, but there were high voltages running round in those valve circuits and for this and other reasons we sometimes wanted to build our creations into outer boxes. This was however one of the trickiest tasks and we were always looking for readymade stuff to house things in just as Benjay does when he mounts his   “Zinky Smokey amp and Takamine acoustic preamps” in his daughter’s old shapes boxes.

But I have become lazy in my old age and these days if I need to knock up a device for some purpose, I don’t gCompleted unit - angle shoto to so much trouble. This electronic metronome I built recently using a 555 timer chip for instance was simply installed in a plastic lunch box. I’ll do another blog on the detail of building that thing but for now, here’s just a picture to show what it looks like. Nearly every part of it came from my spares box or from the kitchen cupboard.

Goto Home Page

 

The rise and fall of recording tape

Part 1 _ Wire recorders

When I was a kid back in about 1949 or so, we were all very excited because my Dad had acquired a Webster wire recorder. Up until then, the only recordings we ever heard were on gramophone records but that’s a whole ‘nother story. 

The Webster was an American machine in a box with a flat top. Against the back edge of the top but central between the sides was a record/playback head. Silver wire was drawn from a bobbin on the left of the deck, passed through a groove in the head and rolled up onto an initially empty spool on the right hand side. During recording, Webster wire recorder_1the sound from the microphone amplifier or radio created a magnetic field in the groove of the head and as the wire passed the head, it was magnetized to a varying degree in accordance with the sound.  Later, the same wire could be run past the same head and it would create an electrical signal in proportion to the magnetism in the wire, thereby theoretically reproducing the original sound. The wire would frequently break or stick, and the speed would vary depending on mechanical limitations in the system. The playback sounded pretty lousy. It was also true that the magnetism induced in the wire did not stay exactly where it was put because each particle of the wire slightly re-magnetized the adjoining particles. In other words, the magnetism spread in the wire to some extent and this caused a loss of fidelity when the wire was played back.

Everybody had lots of fun sending each other recorded messages etc but the wire recorder wasn’t much good for anything else. You couldn’t go down to the store and buy a piece of music recorded on wire. A better system was needed and sometime in the 50s or so, domestic tape recorders started to become available. Professional tape recording had been around since the 1930s but home equipment only happened about 20 years later. It was better than wire for various reasons. The tape was made of an acetate base of some sort with a layer of very fine metallic dust embedded into a varnish-like surface. The tape would be passed over a recording head and the magnetic field in the head would magnetize the separate metallic particles in the surface layer. Because these were not touching each other, the magnetism could not easily spread through the tape and the reproduction was very much better that wire recording. The system was however still fraught with problems and the quality was still not good.

But more about that in a few days time.

Goto Home Page

A haven to practice bagpipes

If this were Zen I would ask the question, If bagpipes are played in a concrete room with nobody there to hear them, are they played?

Most people know that the bagpipes are a very loud instrument and any piper who lives in a built-up area has the problem of finding a place to practice that will not disturb the neighbours. I live in an apartment block and practicing the pipes inside the building is definitely a no-no. My cat would report me anyway. But luckily for me, there is a concrete room on the basement level of our block with no connection to the main buildings.  The painters and maintenance men use as it a common room for their lockers, lunches etc.  It can best be described as a bomb shelter or concrete bunker and when the maintenance staff knock-off work at 5 PM, nobody uses it. I have fast-talked my way into getting a set of keys for this room and that is where I will practice making my pipes produce music.

Driveway to pipe room web

Here we are going down the driveway from the road to the under-building parking. Note the Red picket gate on left hand side away from building.

Pipe room outer gate web

We have turned left off the driveway and are now facing the gate we saw in the previous picture. Note that it is not under the building at all.

Pipe room yard web

 

 

 

Behind the gate is a small yard full of junk and the door of the piping room can be seen at the far end from the gate.

Pipe room_1 web

 

 

The room is a pigsty but who cares? It is large, 7m X 5 m and is full of sound       absorbing junk so it does not echo.

Pipe room_3 web

As you can see it is a veritable concrete bunker.  I will run there when the bombs start falling.

I have started increasing the lengths of my squawks on the bagpipes. From an initial, cat terrifying, three second skirl, I have managed to sustain a single note on the chanter, without drones, for almost a minute continuously. With the chanter corked and the three drones opened, I have been able to get all three drones to sound for quite a long time. I am learning that the piper does not so much squeeze the bag under his arm but rather holds the bag steadily under the arm and allows the expansion of his chest to compress the bag. This way, the pipe bag becomes a sort of extra lung. When the piper breathes in, his chest expands and compresses the bag making it play. Then as be exhales into the bag, the bag inflates again and his chest deflates. The theory is simple. Actually doing it is another thing altogether.

 

 

 

 

The bagpipes arrive

I am pleased to say that my search for a set of second hand Bagpipes in good condition has been fruitfully concluded. Gareth Rudolph, the Somerset West based supremo sourced a 30-something year old set of African Blackwood Hardies for me up in Johannesburg.

They have no cracks and have generally been well looked after. The bag and the reeds needed to be replaced so Gareth did that for me and he delivered the set to me on Wednesday evening. The challenge now is to learn how to blow them unwaveringly whilst actually playing a tune on the chanter. It is no  small step from practice chanter to full pipes.

As soon as it was not rudely early yesterday morning, I shut the doors and windows of my apartment, put corks into the tops of the drones as advised, hoisted the apparatus to my shoulder and made my first attempt. I had been to a bagpipe recital the night before so I was sure I knew exactly what position everything should be in etc. (Nothing like being cocky).  I stuck the blow stick into my mouth and inflated the bag. When it seemed to be fully pumped-up, I prodded down on it with my elbow and then tried to keep up a steady pressure. The chanter emitted a piercing F-note like a burglar alarm going off and then stopped as abruptly as it had started. My cat’s legs went into a wheel spin as she catapulted off the couch and skidded through the lounge and out through her cat hole into the big world of outside. When she came in much later she skirted the edge of the room giving me a wide berth. She shot me a look of pure hatred as she sidled past and to this moment we have not resumed full cordial communications. Well, after a while I tried again. I blew and pushed and on each occasion, I ended up getting a single clarion note from my bagpipes before they quit again. I have been warned though that this is how it goes so I will not give up just yet. If you find me suddenly deceased on the carpet with a spider like instrument sitting triumphantly on my chest, read the instructions in the plain brown envelope on the wall behind my desk.

So a new chapter in the life of Neal the would-be piper opens. My teacher, Peter Odendal says he will help me get started next time I have a lesson scheduled with him. Watch this space.

For those reading this who are pipers or have an interest in pipes, here are a couple of photos and the specifications of the quintopus I have procured.

Pipes day_1 web
The full set of pipes – What I have called the Quintopus
Bag & Bottle snip
Composite bag – Zipped open to show moisture trap bottle

Here’s what my pipes currently consist of

Bag: New Bannatyne hybrid bag (i.e. cowhide exterior bonded to an interior membrane). Side zip giving access to inside of bag and moisture control system.

 Moisture control system: At this stage only a single moisture control bottle is fitted. It appears to have a moisture absorbing membrane inside it and apparently, this swells up as it absorbs moisture and needs to be dried out from time to time.) Optional crystal systems are available for this bag but were not supplied to me.)  (See photo)

Stocks and drones: R.G. Hardie of Glasgow. Combed and beaded African Blackwood with synthetic ivory projection mounts. The mounts are a yellowish colour. I am told that R.G. Hardie is a major supplier to the military and the yellow mounts are standard military issue. Well Mil-spec can’t be bad.  I do have an RG Hardie practice chanter and it seems to work fine. The stocks and drones are 30 years old but have no cracks or damage. The ferules are silvered and the silvering has become a bit tarnished. This I do not perceive as a train smash.

Blow stick: African Blackwood and matches the stocks and drones. But the mouthpiece appears to be Polypenco (plastic).

Chanter: There is no manufacturers name on the chanter and unfortunately, it also appears to be Polypenco. (plastic). I’m sure it will be quite adequate for now anyway.

Drone reeds: The Bass drone is fitted with a Canning fiberglass reed and the tenor drones with standard Canning reeds .

Chanter reed: The chanter is fitted with a molded Shepherd  reed

Tone enhancers: Tone enhancers have been fitted to the drones

 

 

Let’s make Celtic music

The exciting news for me is that I will be receiving my new (to me) set of bagpipes on Wednesday. But wait, that’s the end of the story, or of this installment of it anyhow. Let’s go back to the beginning.

I was born and grew up in South Africa but I had English and Scottish ancestors. Somehow I have always leaned more towards the Celtic culture. My parents used to bring home gramophone records like “Words and music of Scotland” by the likes of John Laurie & Kenneth McKellar. We played them a lot and as young people do, we learned all the words and the tunes and grew to love them. My father and his two brothers, (my uncles) all played the bagpipes and there was nothing we kids loved more than to sit on a moonlit beach and listen to the strains of the pipes carrying across the sea to us from where the brothers were playing. Later in life I visited Scotland many times. I would always just rent a car in Edinburgh and drive on over the Firth of Fourth bridge, up the East coast and then climb left into the highlands before following some other route back down past the lochs to Glasgow. I’ll talk about the West coast and the Hebrides om another occasion.

But despite this love of the pipes and Scottish culture, I never learned to play the instrument. I worked hard my whole life and I had a lot of interests, some of which may yet emerge on this blog. The years passed by and somehow I never found the time to actually start playing. I think I lived in awe of people who could actually play the pipes because I believed it to be a very difficult thing to do. Well, to cut a long story short, when I reached the tender age of 69 I was retired and I joined the Caledonian Society here in Cape Town. The honorary piper, Peter Odendal, was running a course for people who wanted to play the chanter and they asked if I would like to join the group. I told them that it would be impossible for a 70-year old with increasingly arthritic hands and a shrinking brain to even consider it but they insisted and I did join the group. It was such fun that I continued to go to the lessons every week. I never learned much in that group though because we were all beginners and everyone was squawking and battling along and collectively getting nowhere. After about a year though, the minister of the church we were practicing in, retired and the hall was no longer available. So our teacher who lives 60 km away from me in Somerset West made himself available for those who wanted to continue to come for individual lessons a couple of times each month. This worked very well for me and so now at age 72, I feel that I understand enough about the instrument to graduate to the next level of learning. I’m not getting younger so I have raised the money and bought a 30 year old set of Glasgow made, Hardie black-wood pipes through a contact. The pipes themselves are apparently in excellent nick although the bag and all the reeds need replacing. Well I swallowed hard, took out a loan and placed the order. Tonight I heard that my pipes have been fully refurbished and that they will b e handed to me when I attend a bagpipe recityal on Wednesday evening.

Watch this space because when I get my pipes I will surely post pictures of them here. I suspect it will be quite a while before I can actually blow the bag up and keep it blown up whist squeezing all that air out through the chanter and drones and then somehow still trying to remember the fingerings at the same time. But keep Chanter nealTaoraluath neallistening out, I may dump a sound recording onto this site at some time in the not too distant future.

 

 

Growing up with car radios

I spent a large part of my life as a technologist with a major broadcaster. But long before that I was an unruly kid who built electronic gadgets like radios and things and often caused little explosions and minor household disasters. My parents were very long suffering.

During my early years I installed quite a few car radios. 12F8 valveIn those pre-transistor days they were valve sets, and those valves needed a high DC voltage on their anodes to make them work. To achieve that we had to install a vibrator pack in the engine compartment. It made a loud clattering sound as the vibrating contact blade switched the voltage from the car battery fast between positive and negative to produce an alternating current. The voltage could then be stepped up by a transformer to 90 volts or more for the valve anodes. The high voltage then had to be reconverted to DC (continuous current) because that’s what the valves needed. The vibrator pack was bulky and noisy and interfered electrically with the radio, so all sorts of screening and silencing systems had to be used. In the end, all we could receive was a pretty ropy AM (medium-wave) radio signal. Some radios had shortwave as well but interference from the vibrator & the engine usually made that unlistenable. To buy a set of car spark-plug leads today is a pretty expensive exercise because instead of using normal wire they are made of special carbon-based composites. This allows the high voltage for the spark plugs to pass whilst at the same time preventing the leads from causing radio interference.

When FM (Frequency Modulation) came on the scene in the 60s, it was  by its nature far less sensitive to interference from electrical pulses given off by the various parts of a car engine. That’s because spark interference has the same nature as an AM (Amplitude Modulated) radio signal. The interference would mix with the radio transmission and the AM radios of the time couldn’t discriminate between the interference and the program. FM on the other hand ignores the amplitude variations of the radio carrier and detects instead variations in the incoming radio wave frequency i.e the rate at which the signal changes polarity. The frequency is varied by the transmitter in a manner that represents the music or whatever. Electrical sparks in cars don’t affect the frequency of the FM wave so, if strong enough, the program is heard without noise.

Today we are moving a step further with technology by introducing digital radio. The signal at the studio is turned into a pattern of pulses which digitally define the program signal, and the receiver knows what the code should look like. It therefore totally rejects any interference that is not digital code and the result is a perfectly clear signal. It also works much better than FM in weak reception areas. Digital transmissions are also not confused when ghost signals reach the receiver slightly delayed after reflecting off buildings or mountains.  This was a problem with earlier technologies.

Neals quill icon

 

 

 

 

 

 

 

Bagpiping_1

Somewhere, sometime in history someone realized that if you tie the flat surfaces of two thin bits of reed together, put these blades between your lips, and blow, the reeds will vibrate and make a squawking sound.

At school we did the same thing with tissue paper over a comb. This principal was later embodied in many musical instruments to provide them with a voice. Clarinets and the like use just one flappy reed against a fixed lip-piece and other instruments use two flaps of reed which vibrate against each other. But the bagpipes are rather special. With other instruments you have to stop the music periodically to allow the musician to take a quick breath so he can go on playing without expiring. The bagpipes have a lung of their own. The bag is just that. It is an external lung which the player props in under his arm and squeezes steadily to provide the airflow needed to maintain the tune even whilst the music maker is taking in fresh air. He then blows more air into the bag through his blowpipe to keep it inflated. This is tricky to do in practice but when perfected it works very well. Actually, they discovered that it works so well that they could attach three more reeded pipes to the bag and get them all to sing in harmony. The piper can of course only play one tune at a time. So the reed instrument taht is being given voice by air from the bag, has a number of holes down its length to enable the piper to play different notes. This tube is called the chanter. The other three reeded pipes each simply sound off on a single monotonous note. They are called drones for obvious reasons. Two of them are tenor drones and they each produce a monotone which is exactly an octave below the ‘low-A’ note of the chanter. The third and longer, bass drone produces a note that is yet another octave lower. For those not musically informed, two notes an octave apart will be in perfect harmony. For each flap of the reed in the lower tuned pipe, the reed in the shorter pipe will flap twice. The combined tone we then hear is acoustically interesting because we can hear that the two notes are very different but the combination pleases us because they beat in time with each other. To understand a similar thing in the visual world, imagine the following. Two soldiers march side by side. ThePiper 2 nealy move forward at the same speed but one takes two short paces for each long pace that the other takes. Their movements are patently different but they are clearly still in step. They can be said to be synchronized, or in harmony.  More on bagpipes at another time.