dbx 128 - Tape Noise Characteristics

SYSTEM PERFORMANCE ”
Close scrutiny of a dbx processed tape will demonstrate
that its frequency content is virtually identical to that of the
original source. However, the first impression of a dbx II
processed tape when compared with a non-dbx I! processed
tape, may seem to indicate that the non-processed tape has
better high frequency response. This apparent contradiction
can be explained by realizing that the ear has interpreted the
hiss of the non-processed tape as additional high frequency
content.
There can be some barely audible imperfections, even
after dbx tape noise reduction processing, which are due to
tape asperity noise with some types of very clear signals.”
Unfortunately, these noise components lie in the same fre-
quency region as the dominant signal energy, hence we must
look to improvements in tape technology for their reduc-
tion... . regardless of the noise reduction system in use. In-
cidentally, these asperity noise components can be masked,
almost completely, by a steady hiss. The required level of
this ‘‘noise perfume” is about —65 dBm. It is no coincidence
that noise reduction systems such as Dolby A, Dolby B, and
A.N.R.S., which claim “absolutely no audible effect’, have
this residual noise present in the output (dbx output noise is
below — 88 dBm).
By taking advantage of dbx noise reduction in live record-
ing, not only will there be no hiss, there also will be full
*Asperity noise is a random noise that is caused by minute imper-
fections in the magnetic coating of the tape. Due to surface
irregularities and magnetic particles that vary in size, tape becomes
more or less magnetized in a given field from the recording head,
causing a random noise to be superimposed on the recorded signal.
The noise due to this non-homogeneous coating cannot be sub-
tracted from the signal by compander processing. Improvements
in tape manufacturing processes could reduce asperity noise. To
draw a rough analogy, asperity noise is to a taped program as grain
is to a photograph.
dynamics in the live recording. Moreover, the distortion that 29
would otherwise have been introduced by tape saturation or
transient overload will not act to degrade the audio quality.
25
ao (TM TT
nt
ena
Conc
a
OT
CME CFT
Lae AUR | TTI
Sa
20 50 100 500 1K 10K 20K
Frequency in Hz
+10
-10
-20
-30
-40
Level in dB
-50
-60
-70
-80
-90
-100
Fig. 17 — Tape Noise Characteristics (description next page)