Lecture 3 Part 1 – Analog recording

     We use electro-acoustic transducers to capture the sound from the field of the recording. Then we use the opposite type of transducers to play the captured sound through the speakers. Thus we transform the acoustic signal into electricity to be able to operate with it. This is important because we can not capture the sound vibrations, so we need a medium. And this medium is the electricity.

     But the electricity preserves only the shape of the vibration curve, its form. So, what we transfer in this process, is not the energy itself, but the information about the signal. The sound is lost in the environment of the recording, it’s gone. Only the shape of the wave travels along different mediums.

     When it comes to recording, we need another way to store this information. Because the sound is a continuous process, the shape represents itself only in a continuous process. But if we store the voltage for example, we lose the information about the movement of the amplitude of the sound in the time. What we can do about this?

 THE ELECTRO MECHANICAL RECORDING SYSTEM

The first man to store the sound was Thomas Alva Edison: 

 He made a recorder, called phonograph. It consists of a cylinder of wax, attached to a rotating device. Above we can see a funnel, ending with a membrane and a needle:

When you rotate the phonograph, the needle is making a groove on the surface of the wax cylinder. The process can be reversed. In this case the needle will move according the groove and will produce vibrations, which are amplified by the funnel.

Later the wax was replaced with vinyl, and the cylinder- with a plate. After this the playback system was made electrical – with a piezo or electromagnetic transducer, attached to the needle. This is the modern electro mechanical recording system,

which looks first like this:

and later like this

The problems of this system lie in its characteristics. It has good frequency range – between 30 and 20 kHz. But also it has not so good S/N ratio – around 40dB, and because of this the dynamic range also is not good.

The next problem is with the high level of the harmonic distortions. And the last is the high coefficient of wow/flutter, which is because of the rotating elements of the gramophone system.

Also after listening of the recording too many times the medium – the plate – is damaged because of the force of the needle upon the surface of the plate. Because of this we are not able to preserve our recordings unless we don’t play them at all.

The next way of store the sound wave was the

ELECTRO MAGNETIC RECORDING SYSTEM

First the patent was for a system with a wire, not with a tape.

Later the wire was replaced with a plastic tape, on which surface there are glued miniature particles of metal, which became miniature magnets, when magnetised:

 The electric signal goes thru the wires to an electromagnet. On the top of this magnetic head a magnetic field is formed. When the particles of metal travel near the magnetic field, they are magnetised with the same strength and polarity like these in the electric signal. In this way we can transform the shape of the signal from electric to magnetic and later store it.

Here we can see the construction of the electromagnetic system:

First the tape goes to the erasing head. There the previous recording is erased. From there it is going to the recording head. The recording head receives its signal from the input of the magnetophone. There the incoming electricity creates a magnetic field, which in turn is transferred on the tape.  After this the tape goes to the playback head. There the process is reversed. The magnetised particles on the tape are inducting electric signal in the playback head. Thus the signal turns into electric again and can be reproduced on loudspeakers or transferred again.

Here is how looks a regular magnetophone:

A little improvement was made to make the construction of the magnetic recorder more easy to use. The manufacturers put the tape and part of the moving mechanical parts in one box. This is the cassette recording system:

Except from the construction it is the same system. It should not be confused with a different recording system!! It is the same, just in smaller size and with a bit worse parameters because the smaller tape and the smaller speed of the tape. This gives us less magnetic particles per second to come through the heads, which means less quality of the recording/playback.

The electro magnetic system has a good quality compared with the gramophone. The frequency range is a bit less – from 35 to 15-19 (depending from the heads and the tape)kHz. The S/N ratio is between 50 – for the cassette , and 60 – for the professional tape recorders. So it is better than the mechanical system.  The wow/flutter also are better, but still exist. The coefficient of distortions is somewhere about 1,5%, which also is better than the mechanical system.

What is great about this system however, is that with it you can make not only playback, but also and recording in domestic conditions. It is the big goal about this system and it made it very popular till now. Most of the greatest recordings if the past century were made by the magnetic recording system.

The next big improvement in the recording systems is connected with a total change of the method of capturing the sound. It is called:

DIGITAL