The Art of Sound Reproduction

John Watkinson

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136118531
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Acoustical Engineering
• Index: Technology & Engineering

Designed to make life a little easier by providing all the theoretical background necessary to understand sound reproduction, backed up with practical examples. Specialist terms - both musical and physical - are defined as they occur and plain English is used throughout. Analog and digital audio are considered as alternatives, and the advantages of both are stressed. Audio is only as good as the transducers employed, and consequently microphone and loudspeaker technology also feature heavily - making this the most comprehensive, up-to-date text currently available on all aspects of sound reproduction.

Chapter 1

Introduction

In this chapter sound reproduction is outlined beginning with a short history. There are many different types of audio with differing degrees of compatibility between them and these are compared here. It is important to bear in mind that audio has many applications and the criteria for each one may be quite different. Many of the subjects introduced in this chapter will be amplified elsewhere in the book.

1.1 A short history

Accurate sound reproduction is a very young subject because it depends heavily on other enabling technologies which are themselves recent. Of these the most obvious is electronic amplification. Prior to that prospects were limited, but many of the fundamentals which we regard as modern were in place before amplification.

The earliest technology of interest to us is the pipe organ. Large organs with many ranks of pipes posed a formidable control problem. The organist would select pipe ranks by the use of stops which would connect or couple his key presses to different pipes depending on the stop combination. Figure 1.1 shows that this is literally a combinational logic problem which a digital designer would instantly recognize. The organ solved the control problem initially by a combination of mechanical and pneumatic parts. Controlling airflow with a small valve was an early way of obtaining power gain in a control system, predating electronic amplifiers. There was little chance that mechanical systems of this kind could be made touch-sensitive or variable and to this day organ control is purely binary. The key is either pressed or it isn't and the pipe gets a fixed airflow or is cut off. In practical organs it is impossible to prevent a certain amount of air leakage and there is always a background hiss making the organ one of the first devices to have a signal-to-noise ratio.

When electricity was discovered organ control became electrical with switch contacts on the keys and solenoids to control the airflow. The coupling logic became electrical using relays, later electronic logic coupling was employed. The Solid State Logic audio manufacturer began business in this way. Electrical control made it possible to play the organ from a remote console. The wiring in between was a forerunner of MIDI (Musical Instrument Digital Interface).

Figure 1.1 (a) In the pipe organ one key can operate pipes in many different ranks according to which stops are selected. This is a combinational logic application which was initially solved mechanically. (b) Later organs used electronic logic.

The binary nature of organ playing made it possible to record the performance by punching holes in a moving card corresponding to the key pressed. The cards could be played by supplying air pressure to one side of the card and using the punched holes as valves. Figure 1.2 shows that with pneumatic bellows amplification a large organ could be controlled by a lightweight punched card. The cards were joined together with fabric hinges so that a long recording could be made and they were propelled through the reader by an air motor. City streets in the Netherlands are graced to this day by portable pipe organs working on this principle as they have for hundreds of years.

Punched cards designed for organ control were adopted for machine control during the industrial revolution and later for storing data in early digital computers. It is possible to see a direct lineage from the street organ to digital audio.

It is much more difficult to record arbitrary kinds of sound, rather than the commands required to control an instrument. The first major step was the invention of the telephone in 1876 by Alexander Graham Bell. The invention came about as part of his efforts to teach the deaf to speak. Figure 1.3 shows the main components of the telephone, the first sound reproduction system. These components are still present today. The sound falls upon a microphone causing it to modulate the flow of current from a battery in an electric circuit. The current flow is said to be an analog of the sound. The use of an electric current allows simple transmission over considerable distance. The modulated current then operates an earpiece which recreates the sound more or less. The microphone and the earpiece are called transducers from the Latin ‘to lead across’ meaning that real sound in the acoustic domain crosses to the electrical domain or vice versa.

Figure 1.2 Programmable organs used punched cards or rolls. Perforations admitted air to a bellows which operated the pipe valve.

Figure 1.3 Main components of the telephone. The microphone relies on vibrations altering compaction hence resistance of carbon granules. Varying current changes attraction of iron earpiece diaphragm.

Audio transducer design improved dramatically after the development of the valve or vacuum-tube amplifier. This meant that the transducer could be optimized for quality rather than to transfer as much power as possible. Using amplification, microphones having outputs in the order of millivolts were no longer a problem. Microphones based on metallic ribbons or coils moving in magnetic fields were developed, followed by microphones in which the diaphragm was one plate of a variable capacitor; possibly the ultimate in low moving mass.

Amplification also allowed breakthroughs in loudspeaker design. The first quality loudspeaker having a moving coil was developed by Rice and Kellog¹ in 1925 shortly after the valve amplifier. Electrostatic loudspeakers were also developed using the force experienced by a charge in an electric field.

The subsequent development of the transistor and then the integrated circuit made it possible to reduce the size of the audio circuitry. In audio quality terms the transistor was not necessarily a blessing as the small circuitry was often accompanied by small and inferior loudspeakers.

Amplification also revolutionized radio communications and made sound broadcasting practical. The first sound broadcasts used amplitude modulation (AM) of the carrier wave in the MF and HF bands. This is shown in Figure 1.4(a). AM broadcasts are prone to interference and do not reproduce the whole frequency range. Shortly after the Second World War, frequency modulation (FM) was developed as shown in (b). This is less susceptible to interference and has a wider frequency range, but needs a wider spectrum requiring a move to the VHF band which is only operative within line-of-sight of the transmitter. Adding a further subcarrier made it possible to transmit stereo in VHF-FM.

Terrestrial television sound channels were mostly delivered using a mono FM subcarrier, but in some countries a digital stereo system known as NICAM was introduced in the late 1980s. Recently, trial broadcasts of digital radio, also called DAB (digital audio broadcasting), have begun.

Telephones and radio broadcasts only reproduce sound in another place. Recording allows sound to be reproduced at another time. The first sound recorders were purely mechano-acoustical as the technology of the day did not permit anything else. The cylinder recorder is shown in Figure 1.5(a). This has a diaphragm which moves according to the incident sound. The diaphragm is connected to a stylus which makes a groove in the surface of a rotating cylinder. The sound modulates the depth of the groove which is therefore an analog of the sound. The cylinder might be covered with a soft metal foil or a layer of wax. The cutter is driven along the cylinder by a leadscrew. On replay the groove modulations are transferred to the stylus and actuate the diaphragm.

Figure 1.4 (a) In amplitude modulation (AM), the carrier signal is multiplied by the audio signal. (b) In frequency modulation (FM), the audio signal changes the carrier frequency leaving the amplitude unchanged.

Figure 1.5 (a) The cylinder recorder stores the vibration waveform of the diaphragm in the varying depth of a helical groove formed as the cutter is driven along the cylinder axis. (b) Vertical or hill-and-dale modulation is prone to the stylus being thrown from the groove by transients. (c) Lateral modulation improves tracking. (d) Orthogonal modulation of groove walls allows stereo recording with reasonable channel separation.

The cylinder recorder was a limited success because the cylinders could not be duplicated. Applying the modulated groove principle to the surface of a disk resulted in a recording which could be duplicated by pressing. Figure 1.5(b) shows that the first disks used vertical stylus motion, the so-called hill-and-dale recording. This soon changed to lateral recording (c) which tracked better. Stereo was possible with independent orthogonal (at 90^°) modulation of the two groove walls shown in (d).

The first disks were also purely mechano-acoustic, and recording and replay required a large horn which would couple as much energy as possible between the air and the stylus. Figure 1.6 shows that once amplification became available it was then possible to use a microphone and an amplifier to drive an electromechanical transducer in the disk cutter. In the player another transducer known as the pickup produced a low level electrical signal which was amplified to drive an electro-acoustic transducer.

Figure 1.6 The invention of electronic amplification transformed sound recording because it was then only necessary to reproduce information rather than power. Amplifiers between microphone and cutter and between pickup and speaker allow both microphone and pickup to be optimized for signal accuracy not output.

The first disks were purely mechanical and significant power was obtained from the groove wall. In order to limit the pressure exerted by the stylus this had to be quite large. Figure 1.7 shows that the short wavelength response limit of a stylus is reached when the stylus is too big to follow fine detail in the groove. To reproduce high frequencies, the groove speed has to be high. Consequently early disks turned at 78 rpm and playing times were short. Once electrical pickups and amplifiers were developed, there was no need to transfer so much power from the groove and a much smaller stylus could be used. A slower speed could then be used and 33.33 rpm² became the standard for 12-inch microgroove records, with 45 rpm being used for 7-inch records. A further ingredient was the switch from shellac to vinyl which had a finer structure and produced less noise. The microgroove vinyl disk was very popular until it was eclipsed by the compact disc.

Early experiments were made by Poulsen in the 1890s with magnetic recording using soft iron wire driven between two spools. Again this development was waiting for the valve amplifier to make it practicable. Magnetism inherently suffers hysteresis which distorts the sound, and high-frequency bias was developed to overcome it.

Soft iron wire with the right magnetic characteristics was flimsy and prone to breakage. The solution was to make a composite recording medium where the weak magnetically optimal material is supported by a mechanically optimal substrate. This is the advantage of tape. In the first tapes paper was the substrate but this soon gave way to cellulose acetate and then polyester. The analog tape recorder as it is known today was essentially developed during the Second World War in Germany. The story of its development despite the difficulties caused by hostilities is quite remarkable.

Since the Second World War the analog recorder has been refined and its sound quality improved almost to the physical limits. The work of Dolby had a great impact on analog tape because suitable complementary signal processors before and after the tape recorder gave a significant reduction in tape hiss. The open reel tape recorder was not a great success with the consumer as it was too fiddly to operate. Open reel recorders remained popular in professional circles, but the consumer took to the tape cassette instantly. The Compact Cassette introduced by Philips in 1963 was originally designed for dictating machines, but continuous development has improved the sound quality considerably.

Figure 1.7 Disk reproduction reaches an upper frequency response limit whe...