How can an entire orchestra or pop
group be accommodated in your smart phone for you to listen to at your
convenience via earphones or a headset?
In
a previous presentation we noted that music is merely air molecules
moving in certain patterns. A vibrating violin string, for example,
triggers pressure waves in the air. These pressure waves are detected
by your eardrums, which vibrate in the same pattern as that triggered
by the violin string. The eardrum vibrations are converted by nerves
into electrical signals which are sent to your brain for interpretation. Click on the image to activate video.
Before
the invention of the phonograph (from the Greek words meaning
'sound-writing') by Thomas Edison in 1877 the only way to hear music
was at live performances.
Edison's
device incorporated a diaphragm to which was fixed a needle. The
pressure waves in the air impinged on the diaphragm, causing it, and
therefore the needle fixed to it, to vibrate. The vibrating needle cut
a groove on tinfoil wrapped round a rotating cylinder, the groove
pattern matching the pressure wave patterns.
When
the cylinder was subsequently rotated with the needle tracking the
groove, the diaphragm vibrated accordingly, setting up pressure waves
in the air. In other words, the phonograph could both record and play
back sounds.
To
improve the quality of the recordings various devices were invented.
Many of these devices made use of electromagnetism. For example,
instead of a needle, a coil of wire can be fixed to the diaphragm. If
the coil surrounds a magnet then its vibrations will generate electric
current in the coil, the voltage varying according to the diaphragm
movement. This is the principle of the microphone.
The
process is reversible, of course. If the electric current of varying
voltage from a microphone is passed to a coil-diaphragm assembly then
the coil and diaphragm will vibrate accordingly. This vibration can be
recorded on a master disc by means of a stylus for making vinyl records.
Alternatively,
the electric current can be passed through a coil surrounding a ferrous
metal annulus incorporating a gap, which sets up a corresponding
varying magnetic field at the gap. This in turn can magnetise a
suitable medium (such as coated tape) moving past it. Again, the
process is reversible - the varying magnetised pattern can be detected
and transformed into an electric current. This is the principle of the
tape recorder.
For
playback, the electric current from these recording devices can be sent
to a coil attached to a diaphragm, making it vibrate and thereby
generating pressure waves and hence sound. This is the principle of the
earphones to be found in headsets.
If
louder playback volume is required we must find a way of boosting the
signal from the recording devices. In an amplifier, the weak signal
from the recording device is used to control the voltage of a stronger
electric current. This stronger electric current can be sent to a
larger coil-diaphragm assembly. This is the principle of the
loudspeaker.
In
recent years new methods have been developed for recording the signals
from microphones. Here is a diagram showing the variable electrical
signal generated by a microphone recording the start of the first
movement of Beethoven's 'Eroica' Symphony. The two channels are for
stereo reproduction (left and right channels). Click on the image to activate the video.
Taking
one of the two channels, if we expand the diagram we can see that the
pattern can be approximated into a series of discrete signal values and
we can describe these values numerically.
Here
is the sample we've chosen. Signal values above the baseline are
positive and those below negative (representing reverse electric
current flow, when air vibration is moving the diaphragm in the
opposite direction).
The
memory chips in computers and smart phones are made from silicon. They
store data in binary ('digital') form. That is, the data is processed
and stored such that storage components in the chips are either charged
with electricity or uncharged. The storage components are called
capacitors and they are incredibly tiny - a chip the size of a
fingernail can incorporate millions of capacitors.
How
can we store our Beethoven music sample in binary form? We must convert
the numerical values of signal into binary form, which uses only 1s and
0s to represent numbers. In this system the number '2' is represented
as 10. Here are some examples.
So the numbers in the table above, including our Beethoven samples, will be stored thus in the memory chip.
As
with the other processes described above, the conversion of air
pressure waves to varying charging patterns in a phone's memory chip is
reversible - we can 'read' the binary data in the memory and convert
this signal into varying electrical voltage for sending to earphones
and loudspeakers. So that's how we fit an orchestra (or pop group) into
your smart phone!
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