San Fernando Valley State College
NOISE REDUCTION TECHNIQUES
FOR AUDIO TAPE RECORDING
A project submitted in partial sat.isfaction of the
requirements for the degree of Master of Science in
Engineering
by
Thomas
c.
Bowles
June, 1972
The project of Thomas
c.
Bowles is approved:
San Fernando Valley State College
June, 1972
ABSTRACT
This paper examines noise associated
with professional audio tape recording and
reproduct.ion.
Noise sources inherent to
all phases of the recording process are
explained.,
Existing and future noise
reduction techniques are discussed and
evaluated.
TABLE OF COt?l'ENTS
SECTION
PAGE
1.
Introduction
2
2 ..
Tape Transport
4
3.
Magnetic Tape
6
4.
Filtering Techniques
9
s.
Dolby Techniques
12
6 ..
Digital Techniques
15
7.
Summary
17
8.,
Bibliography
18
-1-
INTRODUCTION
Since 1933 when Bell Labs successfully demonstrated
st.ereo t.ape reproduction, sound engineers and manufactureres
have been fighting noise problems.
Noise limits the
lower limit of a system 1 s dynamic range.
To obtain a
wide dynamic range, an equally wide signal-to-noise
ratio must be achieved.
The dynamic range required
to faithfully record a S]lmphomy orchestra is 75db.
Noise is introduced by all recording system components.
Research done by the telephone company< 12 ) shows that
most. of the audio spectral energy is concentrated
in the mid-frequency area around 1kHz, while the
objectionable noise is found at the low and
of the s.pectra.
h~gh
ends
The recording engineer must keep noise
at a minimum in the mast.er tape since it must. go through
mixdowns and dubbings before the recording reaches
the consumer..
Each time the tape in copied, noise
level increasesr signal-to-noise ratio will be 6db
lower in t.he third copy of a master.
tape~ 6 )
Among the various noise sources, electronic
device noise, which includes shot. noise, 1/f noise,
thermal noise and popcorn noise, contributes a minor
amount compared to other noise sources on the recording
(23)
system. -
For this reason, and because of the amount of
material on amplifier noise optimization available in
the
literature~ 9 )
This noise is not considered further
in this paper ..
-2-
The most. troublesome. noise is introduced by
the tape itself, and is heard as a hiss or
hum~B)
Every action in manufacture,, use, . and st.orag-e of the
tape affects its noise properties.
most critical period for a tape.
Manufacture is the
The t.ape transport
is responsible for a. variety of me:cha.nica.lly
caused noises including magnetically induced noise,
speed variations heard as wow and flutt.er, cross-talk
noise caused by misaligned t.ape heads, and tape
head noise.
Prehaps the most important noise reduction device
is the ear itself.
It has a natural frequency
response and response time that tend to eliminate or
integrate out most of the noise~l,l ) Noise reduction
2
techniques are based on these physical properties of the ear.
The following sections present numerous methods of
reducing the noise added during all phases of recording
in the professional studio.
Many of these methods apply
to corrunercial home: recording systems as well 1 b:trt the best
home equipment is limited by the quality of the source
material available..
The problems with the professional
master tape: must be solved before advances in home
recording may be obtained ..
-3-
TAPE TRANSPORT
The term tape transport is meant to include all of
the mechanical tape handling equipment of the recording
machine.
An imperfect tape transport can introduce noise
of several types: modulation noise, wow 1 fluttef, induction
noise 1 tape head noise, space losses.
Modulation noise arises when an unsupported length
of tape is placed under tens ion; the vibration that. ocurres
can lift the tape off the head periodically, causing a
fluctuation signal strength and the resultant varying
signal-to-noise ratio,.
The problem is cured by elimination of
long unsupported lengths of tape.
Quality studio recorders
keep unsupported lengths to less than
~
inch.
wow nad flutter are classical tape transport problems.
Flutter is an instantaneous t.ape speed vara:tion caused by
mechanical affects such as torsion, sticky and eccentric
rollers 1 power line transients, and tape surface frict.ion
(scrape
flutter)~ 12 ,lS)
wow is simply low frequency flutter.
WOvJ can be perceived as low as 0.,062%
.±
0.03%
rms~ 20 )
Careful tape transport design and cleaning can keep wow
and flutter at acceptable levels.,
Proper shielding must be provided in the transport
design to prevent stray electromagnetic fields. from
reaching the tape head or magnetic tape.
A high frequency
mag-netic field (>20kHz) or a magnetized ta.pe head will
partially attenuate or erase a signal recorded on a tape, or
it can attenuate and distort the output of the tape hea.dr a
-·4-
superi.mposE)d audible hum will result 1ii.Then a tape or tape' hea(I
is exposed· to an audio frequency mag·neti.c field..
Shielding
is not a problem in professional recording· equipment,.
Tape head eddy currents will reduce signal output ..
The standard technlque had been the use of larainat.ed heads
t.o keep eddy .currents below accept.able levels 1 but new
ferrit.El mat:erials have been found which allow solid t.ape
he~ads
to be used, resulting· iri superior
signal out.put wit.h
less head vJear ..
As mentioned above, cleaning reduces flutter1 a more
sig-nificant benefit is t.he eliminat.i.on of space losses..
Any
time t.he magnet.ic oxide of the tape is separated from the
tape head a signal loss is experienced.,
The• amount of this
(12,24)
dropout is determined by the space loss formula:
decreased level (db) =
54.3,
7'-
whElre d is separat.ion of tape, from head (inches) and /\
is recorded v1avelength (inches) ..
signal recorded at 15ips ( 1\
=
For example 1 a 7500Hz
2 mil recorded wavelength)
will be at.tenuated 6dbt or 50% 1 by a particle 0.,22 mil,.
Thi.s same loss can be caused by modula.tion noi.se or a rough
tape
surface~
Edge smear is caused by· bending of the tape as it
passes ove.r the tape guides and hE!ads.
the
effect~
Little is known about.
but i.t is t.hought to cause loss of presence in
the music,. possibly due to degradat.ion of att:ack transients
of some
instruments~
-5-
MAGNE'l'IC TAPE
Magnetic
system.,
oxidf:~
tapE:~
is the memory element in the recording
In the production of magnetic tape1
is adh.ered t.o an
means of a binder..
ac(~t:ate
,
a ferromagnetic
or polyest.er backing by
The binder and backing material are
nomna.gnet.ic and do not contribu:te any magnet.ic noiser
bindE:~r
The
material on the surface of t.he tape aannot be made
perfectly smoot:h1· small perturbations exist on the tape
surfacf:~ that~
passes over t.he record/playback heads..
lift. the tape off the t.ape head momEHlt.arily,
pe~rt.urbations
causing a slight drop j_n signal strength.
det.E:~rmined
drop i.s
pn::eviously.,
The
The amount of
by t.he space loss formula menti.oned
With the drop in sj_gnal is the accompanying
decrease in signal-to--noise ratio,.
Modern manufacturing
t.echniques have controlled surface smoothnesE; within ,
o.. ooooo1
inch.,
The actual mc:tgnet.ic signal is stored in magnetic
domain particles of gamma ferric
oxick~q
or cobalt-treat:ed ferric oxide.,
The opt.imum size and
chrorrd.um dioxj_de! 6
acircularity of these particles is approximate,ly 1 mi.cron
long by )4: micron
diameter~ 23 )
Unfortunately, all particles
c1o not obtain this si?.e and shape, causing nonunifor-m
magnetic propert.ies from pa.rt.i.cle to pa.rticle 1 and t:he
accompany.i.ng nonuniform output.
A more serious problem is
the d:tst.ribution of magnetic particles on the t.ape: surface 1
an area of tape wj_th a high concentration of pa.rti.cles will
have a stronger t.han normal output, and the out.put of a.n
-6-
area with a low concentration of particles will be weaker
than normal.
This condition is known as dropout or fading
and has been controlled well enough in the manufacturing
process so that it is not a problem in professional qualit_y
recording tape.
Particle orientation is a problem still
present in the highest quality tapes.
At the point in
manufacture when the oxide particles are suspended within
the fluid binder on the tape, a stromg magnetic field is
~pplied
to physically align the particles longitudinally
along the tape..
orientation..
All particles do not acheive the desired
This imperfection causes a calculat.ed loss
of 4db in signal-to-noise
ratio~ 23 )
Presently, thE!re is no
solution to this problem.
Hig·h output tapes have been developed that: allow a
stronger signal to be recoeded and reproduced, resulting
in an increased signal-to--noise rc:ttio at. low frequency.
This is accomplished by making the magnetic oxide coating
thicker.
The signal increase at high frequencies is
limj.ted because the additional oxide of the thicker tape is
separated from t.he record head by the original oxide
thickness 1 the space loss formula shows that. the improvement
is minimal at. higher frequencies vvhere the most improvement
is needed.,
Low noise tapes use oxide particles more sensitive
(23)
to shorter wav..elengt.hs than conventional formulat.ions.,
Tapes with a signal.-to~noise ra_tio of 62db gave been produced~ 9 )
This new tape requires a simple modification of the bias
current supplied to the tape heads..
-7-
One of these new
forrnula.tions, chromium--dioxide, requires
an expensive
modification of pl.aybacJc equalization. This
tapE~
considera.bly more abrasj.ve than standard t.ape:S
i.s
r it is not
practical for studio applications.
Increasing the width of the tape increases the
signal-to-noise ra.t.io.,
Doubling the recorded track width
w1.11 gj.ve a 6db increase in signal w-ith only a 3db
increase in noise 1 resulting in a 3db gain in the signal.
( 23)
t o'"·no1.se
ra t.
.1.0.,
This would be an easy way to improve the
noise situat.ion except. for the change necessary in the
studio
(~
equipmE~nt.
inch,
~
Certain tape: widths ho.ve become standard
inch, 1 inch 1 35mm in motion picture sound rooms)
and the cost of changing these machines is more than t.he
gain in signal-to--noisEl ratio is wort.h.
The
consumE"~r
ma.rket
is in worse shape since the trend is towards thinner,
not wider, cassette tapes.
Print-through noise and demagnit.i:z.ation loss are
associated wit.h tape storage..
P:cin.t-through ocurres when the
magnetic signals on one layer of tape are induced on
ajoining layers, causing a sort of
pre-- and post.-echo.
Both thicker tape, like high output tape: 1 or loosEdy wound
tape solve this provlem&
Demagnitization loss is the natural
loss of magnetic strength7 it is not. considered a
serious problem ..
-8-
FIL'l'ERING TECHNIQUES
The prev•iously
mE~ntioned
methods of improving signal
strength or signal-to-noise ratio dealt w·ith design and
manufacture of the sound equipment.,
The following sections
recognize that. the opti.mum design and manufa.cture is only
a compromise of performance factors)' some syst;em t.apE) noise
will still exist and limit the signal-to-noise ratio and
dynamic range, even in the best professional systems.,
The
noise reduction methods presented in this section and the
next operate on the existing system \\7 j .. thout modifications
t.o t.he sound equipment ..
The first method used t.o rE!duce noise was simply
a high cut filter7 similar to ones on toclay•s commercial
equipmE:nt.
This
dra~:tic measurE~
did eliminat.e the most
object.ional noise, high frequency tape hiss, but i.t was
not. sele:ctive in its attenuati.on and eliminated hj_gh
frequEmcy prog-ram i.r1formation as 1;.1ell..
The trend towards
more realistic musical reproduct.ions make this technique
unsatisfactory for professional applications,
In
194~ H.F.Olsen(lg) suggested a system to
improve the lim:i.t.ed cha.ra.cteristics of high cut filtering.
The technique employs
a nonlinear filter w.:i..th character-
ist.i.cs that eliminat.e low level signals, but do not affect
medimum and high level sig-nals.
Four filters are used to
divi.de the audio spe:ctrum into four bands of operation ..
The basic principles presented by Olsen are
sound~.
but the
desig-n has not found application because of the hard limiting
-9-
of 10v7 level signals.,
As j_n the caE;e of hig·h cut filtering,
lov<T ·level sig·nals are lost 1 the change in program level
is abrupt...
The most sophisticate:d technique of th::"Ls type is
dynamic fj_ltElring..
This process takes a.dvantage of the
physiol.bgical phenomemom known as maski.ng~ l6 f 25 )
The ear
will not perceive noise when louder program mc'tt.erial of the
same frequency is present.
The program j_ntEmsit.y required
to mask noise is depemdent on the noise level and frequency.
Maski.ng effect is reduced when the program mat.erial is
separat.ed from t.he noise in frequency.
This is t:he reason
that noise reduction schemes operate in frequency bands ..
7 19
Experimentation has sho1,.;n ( 21 '
) tha.t four frequency bands
are required to use masking effectively in the audio spE:ctrum ..
Each dynamic filter has a soft limiting- characteristic 1
loud passages are not limited since adequate masking is
provided.
As t.he signal starts to decrease toward the
noise level, the masking effect is decreasedfanq at about.
7
-40db; 21 ) the filte-r begins attenuat.ing- the signal and noise.
Additional a.tt:enuation is applied as signal level d.ecreasE":s
until 10 to 15db reduction is obtaj.ned.
Besides reducing
t.he noise level, the filtering increasE":s the effect of
masking by the program signal~ 25 )
A unique advantage of dynami.c filteri.ng over all
other techniques di.scussed is that it will reduce noise
inherent in the program source as v.rell as that introduced
by the recording process.
The cost of this advantag·e, is
t.he slig-ht loss of low level program
-10-
matE~ria.l.
For many
types of programs, those with high noise level a_nd litt.le
low lE!vel signal
contE~nt 1
this is an acceptable t.rade,;...off ..
This noise reduction method does modify the program material
somewhat, but t.he soft limiti.ng characteristic of the program
controlled filters makE! it an acceptable: tool in the recording
studio.
The techniqt:ie has been extended to some products
in the comn1E!rC ial market.,
Design of the fdlters is simplified
b~cause
of
a physical effect symilar to masking called desensitiz_ation
of the
ear~ 16 )
This prov:ides a residual masking effect for
approximctt.ely 100 miliseconds after a loud passage nas
e:nded~ 7 )
This allows the time necessary for the filters to
readjust and settle wit.hout a S1i'7iShing SOUnd being heard
full attenuat.ion is applied to the unmasked
-11-
noL~e ..
as
DOLBY TECHNIQUE
In 196~ Ray M.. Dolby documented( 7 ) a revolutionary
new idea in noj_se reduction.
His system deals not v.rith the
out.put. signal alone, but v.rit.h t.he input and output signals
as a closed system..
The input signal to the recorder is
compressed in dynamic renge..
recorded and systE!m noises are
'I'his compressed signal is
adde:~d.
When the program
is reproduced, the sj,gnal is expanded to orj_ginal levels.
leavi.ng the program material unchanged, but the t.ape noise
is decreased by the amount of the expansion..
The technique
is designed to t.ake maximum advantage of physiological
masking effect.
Again, four frequency bands g·ive the mqst
successful results..
Wh.en a high level signal is present in
a frequency band, no at.t.enuation of the noise takes place
since the noise will not be perceived through-the louder
signal.
As the signal level drops, increasing attenuation
is applied to the noise, leaving enough difference in
levels for the signal to adequat.ely mask the
noise~ 25 )
Compression of a. signal in the fourth fi·equency
band is obtained, starting around 79db program level;s)
by adding a differential component to the main sig·nal.
A larger component is added as t.he sig·nal level decreases.
until a maximum 15db component is S1frrunE.!d at program levels
of 40db and lov.rer..
In playback exactly the same network
is used, the differential component being subtract.ed from
the signal this time, producing an unchanged signal overall ..
When a low level signal is played back, the same amount of
-12-
signal that. was added during recording is now subtracted,
and with it the same amount of noise.
This is t.he key to
t.he Dolby systE:m; an amplified signal is recoTded so that.
it may be a:ttenuat.ed when played back.
When t.he signal is
attenuated, so is the noise that was added in the recording
process.
The noise level, including cross-talk! print-
through 1 and other ta.pe
noises~
is reduced so that a 15db
smaller signal can effectively mask t:he noisee
The signal-
to-noise rat.io and dynamic rang·e of the recording have been
extended l5db in this case.
The discussion above is of t.he fourth frequency band,
9kHz and above i the other three bands function exact.ly the
same way 1 except. a maximum of lOdb improvement in signal-to-·
noi.se ratio is obtained.
80Hz low pass,
hlgh }:":lass.
used, some
The f·ou.r bands are as follow.s:
80Hz to 3kHz band pass, 3kHz high pass, 9kHz
S:tnce conventional 12db/octave filters a.re
ovE~rlap
of bands is experienced.,
For this
reason, band tv.ro operate:s only when bands one and three are
are providing little or no compression-expansion.
As with
the dynamic filtering technique, this system taRes advantage
of residual masking effects following loud passages, arid
ha£; filter timE! constant.s consj_stant with the ear.
Closing
times of the filters, about 100 milisec.Onds, are a
compromise between swish a.nd reverberation
effects~ 21 )
The Dolby system is t.he only noise reduction system
to find wide acceptance in recording st.udios throughout the
Free world and Communist contries..
Ot.l1er systems using
pt.·e- and post-processing have been suggested,
-13-
5
One system ( )
would record a signal proportional to the cube root of
the signal amplitude, then cube the sig·nal upon playback
t.o achieve noise reduction..
3
Another system ( ) would
superimpose an inaudible high frequency pilot tone to
control record-playback gain..
As Dolby points out,
11
the
net.works can have almost any characteri.st.ics what. ever,
with the proviso that. they are the so.me.
11
(B)
The important
advantage t.hat the Dolby system has· is standardization1
the recording indust.ry has accepted Dolby l·evel as an
operat~ing
st.andard.,
The commercial market has successfully used a
single high frequency band Dolby network t.o i.mprove
performance of the inherently noiser cassette recordj.ng
mc:tchines.,
-14-
DIGITAL TECHNIQUES
The most revolutionary approach to noise reduction
comes from
~:;xpe:rience
with tape recorders used in
instrumentation configurations.
The technique is simple ..
Convert the analog program to a digital sd,gnal and record
the digital pulses 1 then playback t.he digi tctl sig-nal and
convert to an analog program identical to the original ..
The dig·ital approach offers unique advantages over
the conventional analog· me:t.hods.
No bias signal is added
t.o t.he prograrnr t.his is necessary in analog recording t.o
operate: the magnetic tape in the linear portion of the
hyst.eresis curve..
The noise and distortion associated wit-h
bias current and symetry do not exist..
Since the data pulses
are applied in sat.urat.ion on the tape 1 magnetic nonlinearities of the oxide do not E:ffect the reproduced program.
However, a suitable high frequency data recording tape
must be used.
Present day digital technology is capable
of recording music with a.··signal-to--noise ratio of 120db 1
the dynamic range of the ear itself..
cara
(6)
has demonstrated
a dig-ital audio recorder with 80db dynamic range, more
than is requj.red for an exact facsimile recording of a
sy-mphony orchest.ra.
The problem of ·tape copying is overcome by digital
recording.
By the third generation, analog copies have lost
as much as 6db signal-to--noise ratio.,
The fifth direct
dj_gital copy is a perfect reproduct:ion of t.he original,
maintaining- all significant musical properties.
-15-
This is
more cop1es than is necessary for the prerecorded t.ape
to reach the consumer.,
Tape noises such as print-through,
cross-talk, and storagE, demagnetizat.ion do not exist since
t.hey cannot change the
digit~al
level f:rom 1 to 0 or 0 to 1 ..
Interestingly, other problems not dealing with noise are
also cured..
Harmonic distortion and intermodulat.ion distortion
are ke,pt. below· percept.ible levels, about 0.5% 1 at. all
frequencies and signal levels.,
Phase shift. distortion is
mc-d.ntai.ned at less than 50° t.broughout the audio spe:ctrum.,
The most severe audio transients of 13 microseconds are
reproduced v-1ithout ringing and overshoot...
A problem not corrected by dig·ital recording is
v10w and flutter 'i this is strictly a transport and tape
design problem.
However 1 the digital technique pos,?.s no
more stringent requirements on the tape transport than
does analog recording.
Additional noise problems are
created by use of digitc:tl conversion 1 such as quantitiza.tion
noisel nit crovJding 1 and conversion linearity errors ..
These have been reduced below a percept.ible
leve1~ 6 )
The cost. and availability of digital recording equipment
keeps it out. of the recording
studio~
The technique has
been perfectedr it only remaines for the LSI circuit
designers and tape transport manufactures to t.ake advantage
of this new technology.
Digital recording is pa.rticuJ.arly well suited to
live recording in the field.,
The equipment is less
cumbersome 1 not requiring the extra devices presently used,
includi.ng limiting amps, pre- and post--signal processors 1
and automat.ic gain controls.
-16-
SUIVTMARY
Dramatic advances i.n noise reduct.ion have been made
in the pa.st five years.
These improvements have been
facilit.ated by technical advances in several fields;
development of new magnetic oxides 1 high spe:ed electronic
components including LSI circuitry 1
mot~
sophisticat.ed
measurement instruments a.nd techniques, wider applications
of digj_tal
technology~
and new research into the
physiological effects of hearing ..
The professional recording engineer now has t.he
means to produce t.hird generation copies of 7Qdb dynamic
range with overall system noise level at 43db.,
This is
done using t.he best recording tape, transport,
electronic amplifiers and the Dolby noise reducti.on systeme
In two or three years digital recor·ding equipment
should be available allowing the engineer an 80db dynamic
range wj:th a 35db noi·se level.
He will have the
capability of producing a virtually perfect facsimile
recordi.ng of ct live performance.
In a relatively short
time, prehaps five years 1 the consumer will have a digital
open reel recorder in his horne 1 followed in time by digital
tape cassette:s a
-17-
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Acoust.ics HandbooJ<.~ u Hewlett-Packard Company 1 Palo Alto 1
California, Application Note AN-100, Nov. 1970.,
1.
11
2..
uAudio Denoiser 1 11 Kenwood Company, Gardena, California,
Technical Specification 710420s.
3 .. L., H.. Bedford, Himproving the dynamic range of t.ape
recorders, 11 Wireless world 1 vol., 66, p .. 104 1 March 1966.,
4., J .. D .. Brice# HDigital signal processing concepts," IEEE
Trans. on Audio and Electroacoustics, vol.,AU-18:4,
p .. 344, Dec .. 1964.,
5., Re s .. Burwen, uDesign of a noise eliminator system,u
J. of the Audio Engineering Society, vol.. 12:4,
p. 280 1 oct., 1964.,
6,.
w.
Cara, 11 Digital technology applied to audio recording 1 11
Recording Engineer/Producer, val. 12:3, p .. 9, May 1971 ..
7. R. M. Dolbyo IJAn audio noise reduct.ion system, II J a of the
Audio Engr. Soc., vo1 .. 15:4, P .. 383 1 Oct .. 1967.
8., R. M. Dolby, 11 Audio noise reduction: some practical aspects,
Audio, vol. 52:6, June 1968.
9 .. N. Doule 1 "A low noise tape preamplifier,u Fairchild
Company 1 Mountain Vie·w,, California, Application
Note APP~l80 1 Sept. 1969,.
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IEEE Trans,. on Audio, vol., Au~l2:5 1 p., 100 1 Sept... 1964.,
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Audio Engr. soc.,, vol .. 17:5, p .. 532 1 Oct. 1969.
12. J. A. Howard, JIMagne:tic tape recording handbook 1 JI
Hewlett-Packard, Palo Alto 1 · California, Application
NOtE~ AN-100 1 Nov. 1968,.
13. D. H. Howling 1 )!Noise in magnetic recordi.ng t.apes, u J. of
the Audio Engr .. soc., vol .. 28:5 1 p .. 977 1 Sept.~ 1956.,
1
14.. "Isoloop foils f 1utter~ 3M Company 1 J.. of the Audio
Engr. Soc .. , vol. 15;4, p. 423 1 Oct .. 1967~
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orchest.ral instrumEmts 1 " J .. of the Audio Engr .. Soc.,,
vol. 13:3 1 P. 194 1 July 1965.,
-18-
11
16.
E. Luscher, 11 Adaptation of the ear to sound stimuli,u
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17..
11
Magnetic recording t.ape data:.,ll Memorex
Santa Clara, California, 1971.
18.
11
Magnetic tape system inst.rumentation users handbook, u
Bell and Howell Company, Pasadena, California 1
Technical Document 1330/0571, 1970.
Corporation~,
19.,
H. F. Olsen, 11 Audio noise reduction circuits 1
Electronics, vol., 20:12, Deca 1947.
20 ..
H. Sakai, "Perceptibility of wow and flutter 1 11 J. of
the Audio Engr. Soc., vol .. 18:3 1 p. 290 1 June 1970.
21 ..
H ..
22 ..
E. P. Skov, 11 Noise limitations in tape recorders,ll
J. of the Audio Engr. Soc .. « vol., 12:4 1 p .. 280, Oct. 1964.
23 ..
c.
24.,
R. L .. Wallace, Jr., 1 "The reproduction of magnetically
recorded signals, 11 Bell System Tech. J., p. 1145 1 Oct. 1951.
25..
I., M.. Young, "Masking of white noise by pure tone,
frequency modulated tone 1 and narrow band noise 1
J .. of the Acoust.ical Soc. of America, vol .. 41 ~ 3 1
p. 700 1 March 1967.
·
11
H~ Scottq "Dynamic noise suppressors," Electronics,
vol. 20:12 1 p .. 96 1 Dec. 1947.,
Stark:, 11 Quieter tape," Srereo Review, vol .. 28:3,
p. 65 1 March 1972.
-19-
11