Revolutions in sound recording
Revolutions in sound recording

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Revolutions in sound recording

2.8 Good times and bad

The music industry, like any other large industrial business, had good times and bad times. By 1924 the burgeoning of radio broadcasting in the United States caused a severe downturn in record and equipment sales, leading to amalgamations and bankruptcies of many of the record companies. Actually, radio broadcast studio technology proved of great importance to the record industry. The sensitive microphones and electronic amplifiers used in broadcast studios offered improved characteristics that were exploited in the record industry through the development of an electromagnetic cutting head by the American company Western Electric. Unfortunately, they agreed to sell electric recording technology (as it was known) only to American record companies. Not to be outdone, the far-sighted managing director of the British Columbia Record Company went to America and bought the then-ailing ‘States-Side’ company of the same name along with an agreement to use the new recording equipment, thereby securing the technology for use in Europe. To capitalise on the new technology, players with electromagnetic pickups, using an opposite technology to the cutter, were developed. Electric players rapidly replaced acoustic machines, as they were able to exploit the improved characteristics of the electric recordings. In particular, the new recordings were able to use electronic filtering or equalisation to improve the replayed sound quality, as described in Box 5.

Box 5: Electromagnetic pickups and equalisation

Audio signals are recorded as a lateral displacement or ‘wiggle’ in a linear spiral groove cut into the disc, as described earlier. A sinusoidal wave in the groove, as shown in Figure 15(a), will cause a voltage to be generated in the electromagnetic pickup coil, illustrated in Figure 15(b), due to the lateral motion of the stylus following the wiggles in the groove. (The stylus in the pickup moves very freely compared with the arm that is supporting it.)

Figure 15: (a) Sinusoidal signal in groove; (b) magnetic pickup

The level of the generated voltage is proportional to the frequency recorded in the groove, so as the frequency rises the output level rises. The maximum recorded level (the maximum amplitude of the groove ‘wiggle’) is fixed firstly by the groove spacing (if it is too high the groove wall breaks) and secondly by the ability of the stylus to follow the lateral movement in the groove (too large a ‘wiggle’ at too high a frequency and the stylus will be unable to follow the groove). The minimum recorded level is set by the noise in the system (mainly the noise generated from roughness in the disc material). To ensure that the amplitude of the groove ‘wiggle’ is reasonably constant over all audio frequencies, the lower frequencies (below 500 Hz) are reduced and the higher frequencies (above 2 kHz) are boosted when the groove is cut. This means that when the record is played back the opposite characteristics must be applied, i.e. the lower frequencies output from the pickup must be boosted and the higher frequencies reduced. In the late 1950s an international agreement was reached for the frequency characteristics of recording discs based on a specification from the Recording Industry Association of America (RIAA).

The ideal characteristic for such an RIAA replay amplifier is shown in Figure 16. The gain for frequencies above the low corner frequency is reduced until the middle corner frequency is reached. The gain then remains level until the high corner frequency is reached, when the gain is further reduced. Since it is not possible in practice to produce a response that has straight lines and sharp corners, Figure 16 also shows the actual response of a practical amplifier alongside this ideal response. This filtering should therefore restore or equalise the levels of the audio frequencies to those of the original performance.

Figure 16: Response of an RIAA replay amplifer

Prior to the RIAA agreement manufacturers specified their own equalisation, examples of which are included, along with the RIAA characteristics, in Table 1. The use of the RIAA characteristic also produces a useful improvement in signal-to-noise ratio. By attenuating the higher frequencies, ‘surface noise’ in the pickup output caused by dust and groove wear is reduced. However, boosting the lower frequencies means mechanical noise from the turntable and external electrical noise (e.g. mains hum) can be increased.

Table 1: Equalisation characteristics required for 78 rpm and LP disc media

Recording system Low corner frequency Middle corner frequency High corner frequency Level at 50 Hz* Level at 10 kHz*
HMV 78 50 Hz 250 Hz n/s +12 dB n/s
Columbia 78 n/s 300 Hz 1.6 kHz +14 dB −16 dB
RIAA LP 50 Hz 500 Hz 2.12 kHz +17 dB -13.6 dB

* These figures are relative to the amplifier gain at a frequency of 1 kHz.

† These figures are not specified.

Activity 15

Activity 3 should have reminded you that the frequency characteristics of an audio system should be substantially flat for frequencies between 20 Hz and 20 kHz. If a disc was replayed through an amplifier with a flat response, what would the sound be like?

Answer

Comment

The sound would be very tinny, thin and lacking in bass notes. This is because when the disc is replayed by an electromagnetic pickup, the voltage output at low frequencies is reduced in level but at high frequencies is boosted. So to play any record using an electromagnetic pickup it is first necessary to set the replay amplifier characteristics to match the RIAA equalisation.

The Great Depression of 1929 caused considerable losses, with sales dropping to a tenth of their previous value. One by one the record companies went bankrupt or were taken over. In the UK in 1931, The Gramophone Company and Columbia merged to become Electrical and Musical Industries (EMI). By the end of the 1930s the market had begun to rally, with the realisation that radio broadcasting could be used to advantage through record promotions. In America jukeboxes, similar to the one shown in Figure 17, flourished and by 1939 over 13 million discs were sold just to stock the nation's 225 000 jukeboxes!

Figure 17: A jukebox from 1939

By this time also, many recordings now considered historically (and musically) important had been made by composers such as Elgar, Stravinsky and Rachmaninov. Many of the world's finest performers had also made recordings.

Following the Second World War (1939–1945), demand for records increased dramatically (supplies of shellac had been diverted to the war effort, so creating shortages of gramophone records – indeed, old records were recycled). An example of the upsurge is demonstrated by the figures for sales of an early recording of a popular piano concerto, which sold 102 copies in 1935 and 62 756 copies in 1946.

Record popularity was due in no small part to improvements in recording techniques. For example, an engineer at The Decca Company in England developed an extended frequency response as part of the war effort. The ‘full frequency-range recording’ (ffrr) technique gave a bandwidth from 30 Hz to 14 kHz; this ensured sounds of instruments included hitherto unheard harmonics, giving a much fuller sound.

Still, not everyone was happy with a technology that, apart from improvements in fidelity, had remained substantially unchanged since the early 1900s. Record consumers were no longer satisfied with excerpts of symphonies or musical works shortened to fit to one or two sides of a disc. Full-scale symphonies and choral works were available as sets. For example, Bach's St Matthew Passion (approximately three hours of music) came on eighteen double-sided 12-inch records, but listening to this work involved changing records 36 times, hardly convenient or indeed conducive to a fine musical experience!

In 1948 Peter Goldmark, head of research at Columbia Records in America, demonstrated a 12-inch (30-cm) non-breakable microgroove vinyl disc capable of playing 23 minutes each side. Columbia called it the LP (for long-playing) disc. It revolved at 33⅓ rpm with up to 300 tracks to the inch (120 per cm). The rival company RCA-Victor seemed not to be impressed with the LP. They responded with a 7-inch (18-cm) microgroove vinyl disc that revolved at 45 rpm, the so-called 45, which had a similar track pitch to the LP and played for up to 4 minutes. The ‘Battle of the Speeds’ commenced.

Activity 16

Can you suggest how the record-buying public responded to these two new ‘standards’?

Answer

Comment

The immediate response from the record-buying public was to stop purchasing records until the outcome of the battle was decided!

Fortunately for all the record companies a truce was declared by 1950, with the 78 rpm disc the loser. The LP was adopted for classical recordings and the 45 for popular music. In Europe the change took a little longer, but by the end of 1952 LPs were available from European manufacturers.

The LP is not quite the end of the story of the gramophone record. As far back as 1931, the British engineer Alan Blumlein designed and patented a stereophonic (from the Greek meaning ‘solid sound’) recording system that used two sound channels to create a virtual sound ‘stage’ where an individual sound source (instrument, voice, etc.) could be located at any point between two loudspeakers placed at the front left and front right of the listener. The location of the source is determined by the relative intensity in the two channels. The patent covered two possible ways of cutting the groove in the record to allow two separate channels to be recorded. The V-L (vertical-lateral) method combined hill-and-dale and lateral cutting systems, whereas the 45/45 technique was similar except the cutter was tilted at an angle of 45° to the surface of the disc, putting the stereo signal into the groove walls, as illustrated in Figure 18.

Figure 18: The V-L and 45/45 recording techniques compared

In 1958 the 45/45 standard was adopted by the industry, having been patented in the United States by Westrex/Bell as early as 1937.

Activity 17

Can you think of a problem that existing record users might find with the introduction of stereo discs?

Answer

Comment

Non-compatibility with existing monophonic (single-channel) systems meant the need to produce both mono and stereo discs. Retailers would have to stock both versions of the record. A stereo disc could be damaged if played on a mono pickup that was not designed to be compatible with stereo discs. This is because the stereo pickup needs to move in both the horizontal and vertical directions to cope with the 45/45 movement, whereas the monophonic pickup only moved horizontally; this could potentially cause damage to the stereo groove wall. Also not all the music information could be recovered by having a monophonic pickup. ‘Stereo-compatible’ monophonic pickups were eventually manufactured to overcome this problem, allowing the production of monophonic discs to be phased out.

This brings to a close the story of the record (cylinder and disc), the main source of recorded sound for nearly a hundred years. Apart from refining manufacturing techniques, little change to the technology took place. There is still a demand for vinyl discs from audiophiles, who believe the analogue sound cannot be surpassed. But it is DJs, who have made ‘turntablism’ an art form in its own right by creating new music by ‘scratching’ tracks from vinyl discs, that are keeping disc record playing alive. This demonstrates a use of the phonograph not even imagined by Edison!

Activity 18

Why are the following factors important in the quality of disc reproduction? What aspects of the quality of the reproduced sound do they affect?

  1. The hole is exactly in the centre of the disc.

  2. The disc lies flat on the turntable, not warped in any way.

  3. The disc is made from a smooth material.

Answer

  1. If the hole is not in the centre the disc will turn eccentrically, causing variations in the linear speed and consequently changes in the pitch of the replayed sound.

  2. If the disc is not flat there will be an audible noise every time the ‘bump’ is encountered. This will increase the signal-to-noise ratio.

  3. The smoother the material the better the signal-to-noise ratio, as any ‘roughness’ in the groove will be reproduced as noise. (This is apparent when comparing the signal-to-noise ratio of 78 rpm discs made of shellac and slate dust, which is quite rough against the smooth plastic material of the LP.)

Your answers to the first two parts of the above activity should have described an effect known as wow. Wow is a low-frequency pitch variation that, in discs, can be caused not only by the spindle hole in the disc not being exactly centred or by the disc being warped, but also by slow variations in the disc motor speed. There is a second related effect called flutter, which is a higher-frequency pitch variation caused mainly by faster variations in the speed of the disc motor.

TA212_3

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