contents - History of Ericsson - History of Ericsson

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tional to the strength of what is recorded. The greatest width for music is 20
mm. The image phenomenon is a good aid to the study of the frequency ranges
of the amplifier and the engraving box. In the microphone intake of the
recording amplifier it is possible from a tone generator, which gives constant
tension over the whole frequency band, to feed in the frequencies it is desired
to measure. If, for example, one proceeds continuously from 50 c/s up to
10 000 c/s, there is obtained a fine picture, the Meyer picture, Fig. 6, which
gives tangible evidence of the quality of the apparatus employed and is of
enormous assistance in gramophone technics.
On account of the fact that the amplitude is in inverse ratio to the frequency
the low frequencies would, if one modulated according to a straight curve,
be so great that the groves would run into each other. Screening of the lower
register is therefore a necessity. A good engraving box which is strongly
attenuated would give a Meyer picture which rises slowly from 50 c/s to around
500 c/s. Between 500 and 2 000 c/s it is as near as possible straight and then
rises to appr. 6 000 c/s after which it falls steeply. A strongly attenuated
engraving box, however, demands access to great output from the amplifier,
15—20 W. A simpler engraving box may be operated with a 3—5 W amplifier,
but its upper limit frequency will generally lie as low as 4 000 c/s. The higher
the frequencies it is possible to get in, the richer in overtones will be the
speech and music; the i-sound in speech especially comes out very soft and
natural. In practice it is enough to stand at 5 000 c/s as the higher frequencies
make greater demands on the reproducing devices, so that possibly
one would never have the opportunity of listening to a dearly bought overregister.
A beginner with lacquer discs frequently finds these inferior to, c. g., the
gelatine discs formerly used. He has heard of the lacquer disc's freedom from
needle scratch, but finds to his vexation that the scratch is worse instead. The
natural thing is to accuse the factory of having delivered discs too dry. But
generally it is not as bad as that. When engraving on gelatine discs the slope
of the needle to the disc is about 75° and most of the recording apparatus
for amateurs have this angle unalterable. On account of the greater toughness
of the lacquer the angle must be as near as possible to 90°. If the shaving is
dull and brittle the angle is wrong. With proper inclination there is brought
up a tough brilliant shaving. The groove will then have its smoothest surface
and the needle scratch will be at a minimum. It is important that the engraving
needle is of good quality and correctly ground. Steel needles are
usually employed. A saphire or diamond needle is more expensive to buy but it
suffices for so many the more recordings and gives less needle scratch. Before
beginning to engrave with steel needle one should always cut a groove in the
centre of the disc, lay the ear against the engraving box and listen whether
it cuts without scratching.