2.4 Cutting the groove
The vertical (up-and-down) cutting method, which was nicknamed ‘hill-and-dale’, shown in Figure 9(a) was invented by Edison. The lateral (side-to-side) motion developed by Berliner is shown in Figure 9(b). In both cases the undulations in the groove are directly analogous to the sound vibrations.
The phonograph pickup was positioned by a feed-screw so if the groove disappeared, as it sometimes did, the position of the pickup was not affected. On the other hand, the gramophone pickup was simply guided by the groove, which in consequence had to be much deeper to avoid the pickup skating across the disc's surface. Figure 10 compares an actual vertical-cut phonograph cylinder in (a) with an old lateral-cut 78 shellac disc in (b) and a vinyl LP disc in (c).
Note the difference between the vertical and horizontal undulations and also the depth of the groove in (a) and (b).
When cutting the groove in the recording medium, the level of the captured sound directly affects the deflection of the cutter. A loud sound results in a large deflection causing a deep cut using vertical recording or a wide horizontal deflection for lateral recording. When recording on discs, the engineer had to ensure that loud passages did not cause the lateral cutter to make such a wide deflection that it broke through the groove wall of an earlier part of the recording. The groove spacing would be set sufficiently wide to ensure that this would not happen. However, this caused other problems because the wider spacing reduced the recording time. In the early days of disc recording, singers might be asked to turn their heads away from the horn or microphone during loud passages to ensure their voices kept the cutting stylus displacement to a reasonable level. This could be thought of as a very crude example of audio compression.
Cylinders had a standard groove pitch of either 100 or 200 tracks to the inch (40 or 80 per cm) depending on the playing time.
Because lateral-cut groove spacing depends upon the amplitude of the sounds to be recorded, as explained above, the spacing had to be individually set for each disc; but on average it was 70 tracks per inch (30 per cm).
Later record cutters offered automatic dynamically variable spacing (the groove spacing was varied within a disc as the sound got louder or softer). This allowed a closer groove pitch for quiet sounds whilst avoiding the likelihood of a loud sound breaking the wall of the groove, and therefore maximised the playing time. This can be seen in the spacing of the grooves of the LP shown in Figure 10 (c).
As the recording medium (i.e. the cylinder or disc) is turned a linear groove is cut. The maximum linear speed (surface speed) of the medium relative to the stationary cutting head determines the highest frequency that can be recorded without distortion. The cutter makes a wave with a length that is a function of the linear speed and sound frequency. The linear speed of a cylinder remains constant at 44 cm/s. However, because a groove around the outside of a disc is much longer than a groove towards its centre, the linear speed varies over the surface of a 78 rpm disc from 120 cm/s at the edge down to 44 cm/s at the middle (remember that the rotational speed is constant). The corresponding wavelength of a 1 kHz sine wave recorded on a 78 rpm disc thus varies from 1.2 mm down to 0.44 mm. The diameter of the pickup stylus or needle also determines the upper frequency response. This is because a blunt needle will not sit right at the bottom of the V-shaped disc groove, and will therefore not follow very high-frequency ‘wiggles’ in the groove, whereas a sharper needle will fit right into the bottom of the groove and so will be able to follow the fast lateral groove movements much better.
(a) From reading this section, what restrictions do you consider to have been placed on the ultimate sound quality of cylinder and disc recordings in terms of bandwidth and dynamic range?
(b) Why were these restrictions employed?
(a) The linear speed controls the upper bandwidth frequency. If the cylinder or record would have revolved faster, the linear speed would have increased and with it the upper frequency response. In the case of discs the track spacing is dependent on the dynamic range, as louder sounds require a greater deviation of the groove meaning an increased track spacing.
(b) In both cases the recording time (capacity) of the recording medium would be decreased, shortening the playing time. A trade-off between ultimate quality and sound quality had to be made.
Listen to the two audio tracks below. The first track contains an original recording by Emile Berliner and this is followed in the second track by a repeat of the recording of Edison speaking the nursery rhyme ‘Mary Had a Little Lamb’, made in 1927. How do you think Berliner's recording compares with that of Edison? Can you think why both men should have chosen to recite nursery rhymes?
Click below (17 seconds)
Click below (10 seconds)
The recording by Berliner was taken from an original 5-inch (13-cm) diameter vulcanite disc made in 1889. I think you will agree that the reproduction is poor compared with the recording of Edison. However, remember that Edison's recording was made in 1927 as a demonstration of his original invention. I wonder why both Edison and Berliner recited nursery rhymes? Perhaps neither could think of anything more propitious to say at the time, but bearing in mind the poor quality of the sound, it may have been that using well-known rhymes would make the recording more comprehensible. Maybe it was an early marketing ploy.
(a) Which cutting method, vertical or lateral, would give the recording engineer the opportunity to record the louder sound?
(b) Why was it advantageous to record sounds at the highest possible level in these early recordings?
(a) Edison's vertical (hill and dale) method used on phonograph cylinders was more suitable for recording loud sounds, as a greater depth of deflection of the diaphragm was possible. Berliner's lateral groove on the gramophone disc limited deflection of the diaphragm, otherwise the groove wall would be broken. So, in principle, vertical recording gave a better dynamic range.
(b) The larger the movement of the diaphragm in the phonograph pickup the louder the sound, and so the signal-to-noise ratio was increased thus making the system more suitable for mechanical reproduction.