Magnetic Field, Recording Head, Bandwidth, Wire, Magnetic Tape, Helical Scanning, VCR, and Tracking

Posted by PITHOCRATES - December 12th, 2012

Technology 101

By passing a Long Thin Wire across a Fluctuating Magnetic Field we could Impart a Signal onto the Wire

A magnet has a north and south pole.  Between these poles is a magnetic field.  If a piece of metal comes into contact with this magnetic field the magnetic field will draw the piece of metal towards it.  We can make a magnet by running a current through a wire wrapped around a metal bar.  When a current flows through the wire it will create a magnetic north pole at one end of the bar.  And a south pole at the other.  With a magnetic field in between.  When we stop this current flow the magnetic field goes away.  We can see this in a junk yard when a crane-mounted electromagnet picks up a junk car.  Moves it over where they want it.  And then they stop the current flow.  When they do the magnetic field goes away and the car falls.

We can’t see a magnetic field but it is there.  And it can do work for us.  Not just moving cars.  But a lot of other things.  Such as recording an audio signal on a wire.  Like a spoken voice.  Or musical broadcast on the radio.  By passing a long, thin wire across a fluctuating magnetic field we could impart a signal onto the wire.  Leaving a magnetic footprint, if you will.  Pulling this same wire across a de-energized electromagnet the magnetic field from the magnetic footprint in the wire will reproduce the audio signal that originally imprinted it on the wire.  Once a recording head picks up this signal it is amplified and sent to a speaker, reproducing the sound in the air where we can hear it.

Thus began recording on a magnetic medium.  Which was superior to recording on wax phonograph records.  Greatly increasing the length of audio recordings.  And simplifying the editing process.  As you can stop the recording process and resume it right where you left off.  And you could cut and splice a wire.   Which you just couldn’t do with wax phonograph records.  And a wire had a pretty good sound quality for the day.  These recorders pulled a very long wire across the recording head at a pretty good speed.  Recording a lot of information.  Giving it a pretty good bandwidth.  For the day.

A Magnetic Tape is a Plastic Film with a Thin Coating of Material that can hold a Magnetic Footprint

Bandwidth.  You hear a lot about it.  Especially with the Internet.  People know they need a lot of bandwidth for live-streaming video.  Which is why they have broadband Internet access.  Like a cable modem.  But what is bandwidth?  A mixture of frequencies makes up sound.  Take a 4-piece rock band.  Lead guitar, rhythm guitar, bass guitar and drums.  The bass guitar plays low-frequency notes.  And the lead guitar plays high-frequency notes.  The frequency ‘distance’ between the lowest frequency and the highest frequency is bandwidth.  If this band put out a recording with only the bassist playing one note for 3 minutes this would need very little bandwidth.  Because there would be only one frequency.  Whereas a recording with all four musicians playing every note possible with their instruments would require a lot more bandwidth.  Because this recording will have a great many different frequencies coming together to make music.

This is why recording on wire worked so well.  There is a direct relationship between bandwidth and recording speed.  The greater the recording speed the greater the bandwidth.  Picture a lie detector machine with those needles moving back and forth across a moving strip of paper.  Consider the needles moving back and forth slowly as low frequencies.  And the needles moving back and forth quickly as high frequencies.  You can see information in the resulting patterns.  The person administering the test makes notes on the paper identifying the questions that caused the needle movements.  If that paper moved at one fourth the speed, though, instead of getting useful information you would just see scribbles on top of scribbles.  The ‘high frequencies’ would be especially lost.  Providing no useful information.  Just noise on paper.  This would be an example of insufficient bandwidth for the amount of information in the source signal.  So the paper moves at a sufficient speed to provide the necessary bandwidth.  Just like the wire recorder.  Of course, the greater the recording speed the faster the recording medium will pass across the recording heads.  Which means if the recording medium isn’t long enough it may run out before the recording is complete.  Wire worked well because it is very thin.  Allowing a very long piece of it to wrap around a spool.  But it wasn’t very user friendly.  Like a fishing line off of a fishing reel it knotted and tangled easily.  And despite having a pretty big bandwidth the sound quality was not very good.

An improvement in recording medium was magnetic tape.  Which was a plastic film with a thin coating of material that could hold a magnetic footprint from a changing magnetic field.  Produced by a magnetic recording head housed in a drum.  Which pressed the tape flat under the recording head.   Audio recorders have fixed heads.  Much like the wire recorders.  The tape mechanism carefully pulls this coated plastic film quickly over the magnetic head.  To provide the necessary bandwidth for quality audio reproduction.  While the tape speed isn’t so fast that it results in short recording times.  During the height of the audio cassette era the typical audio cassette held 90 minutes of music.  Which could hold most double LPs.  For those too young to remember, that was two long-play vinyl records we once played on phonographs.  Spinning at 33 ⅓ revolutions per minute.

The Video Cassette Recorder (VCR) is an Engineering Marvel thanks to Helical Scanning

That was audio.  It was a different story with video.  Because of bandwidth.  There was a lot more information in a video signal.  Which meant a lot more frequencies to imprint on a magnetic tape.  And a lot of high frequencies.  For both video and audio.  Requiring a fast tape speed.  But increasing the tape speed meant a longer tape.  But a tape long enough to record a movie with fixed recording heads required a lot of tape.  Requiring very large reels.  Or cassettes.  Too large to be practical.  For unlike a wire we can spool onto a reel like a fishing line (going from one side to the other as we wound the wire onto the reel and then back the other way, placing coils of wire side by side as well as on top of each other) a flat magnetic tape only wound neatly on top of itself.  Creating a conflict.  Either having the length you needed but not the speed.  Or having the speed you needed but not the length.  They needed a way to solve this conflict.  And they did that with the helical scan.

Instead of moving the tape fast they moved the recording head fast.  The tape moved at a manageable speed that provided suitable recording times.  The heads, though, instead of being fixed were housed in a rotating drum.  That spun very fast.  And at an angle to the tape.  Instead of the heads recording continuously along the tape in the same direction the tape traveled these heads recorded in diagonal swaths across the tape.  Which is why a video tape is wider than an audio tape.  To be wide enough to record a series of strips of the video signal across it.  When recording signal processing circuits break down the source signal into these short pieces and imprints them on the magnetic tape.  When playing back signal processing circuits reassemble these strips into the continuous movie they once were.

The video cassette recorder (VCR) is an engineering marvel.  Or was.  You just put a video cassette into a VCR and pressed play and it did the rest.  It pulled enough tape out of the cassette to wrap half way around the spinning drum.  A bunch of rollers and guides routed the tape over the erase head, the video head drum and the audio head.  A capstan (a precision rotating spindle) spun at a constant speed to regulate the speed of the tape over the heads.  A rubber wheel (or two) pressed the tape against the capstan.  And if the VCR didn’t reassemble these strips quite right there was something you could do to help.  Adjust the tracking.  Which helped to fine tune the reassembling of those diagonal strips from the video tape.  Once you did that you only needed one other thing.  Popcorn.  Then you could enjoy a night at the movies in your own home.  Thanks to helical scanning.  And the VCR.


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