Handbook for Sound Engineers

Analog Disc Playback 1035

tion from the record. The first two beams aim at the left
and right shoulders of the groove for tracking. The next
two read the stereo sound at 10 microns below the
shoulder (the standard position). The final beam main-
tains the height between the laser head and the surface
of record to manage thicker or warped records. The LT
eliminates acoustic feedback and sound alteration and
will play warped and rippled records (up to 5 mm devia-
tion). For convenience, the LT comes with a remote
control and can be paused and scanned much like a CD
player.

The same audio information on records is engraved
from the shoulder to the bottom of a record groove. The
laser reads audio information that is 10 microns below
the shoulder, Fig. 27-29, therefore, the laser picks up
audio information which has not been touched or
damaged by a pickup. It plays the virgin audio informa-
tion on the groove without digitization.
The incident area of the laser beam on the groove is
one-fourth the contact area of the best stereo needle and
twenty-six times smaller than a mono needle, Fig.
27-30. The laser beam travels to the wall of the groove
and back. The reflection angle is transferred to the audio
signal. Therefore, the LT maintains analog sound
through the entire process, without any digitization. As
a result, the LT cannot differentiate between an audio
signal or dirt on the record, so the vinyl record must be
absolutely clean and free of debris.

The laser beams must reflect from an opaque surface
in order to be read. Clear or colored records are trans-
parent, or translucent, and will not reflect light to the
sensors. Other types of records that may have difficulty
include:

• Vertical cut records like the early Edison “Diamond
  Cut” series. The modulation is up and down rather
  than lateral.


• Rounded groove shoulder.


• A groove with a rounded bottom produces
  distortion.-

No Acoustic Feedback or Sound Alteration. Feed-

back is typically caused by sound from the loudspeakers

(or from elsewhere) reaching the turntable and mechani-

cally picking up the vibrations, to be amplified again.

There is no needle singing. The LP is safely in a drawer

and the laser reads only the undulations of the groove,

therefore there is no need for elaborate vibration isola-

tion pads. The LT will not hear outside noises such as

Figure 27-28. ELP Laser Turntable. Courtesy ELP
Corporation.

Figure 27-29. The laser beam picks up the signal closer to

the shoulder where the standard needle does not touch.

Figure 27-30. The incident area of the laser beam is one-

fourth the size of a stereo needle and twenty-six times

smaller than a monaural needle.

Monaural needle

Laser beam

Oscillating circle

Bottom

0.7 M

Shoulder

Groove width 51–58 M

F

10

M

F

2 M

Laser

Wavelength of 1 kHz (internal groove) = 200 M

33.3 rpm LP

Reference level 1 kHz 5 cm/s lateral

3.54 cm/s left or right

Amplitude 5.6 M

Can’t read

Can’t read

10 kHz wavelength 20 M

20 kHz wavelength 10 M

Laser High-end stereo needle Mono needle

F= 2 M F= 4 M F= 10 M