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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