53 IRE 19 S1 ire standards on sound recording and reproducing methods for determining flutter content 1953

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PROCEEDINGS OF THE I-R-E
IRE Standards on Sound Recording and Reproducing:
Methods for Determining Flutter Content, j953*
COMMITTEE PERSONNEL
Standards Committee 1953-1954
A. G. JENSEN, Chairman
M. W. BALDWIN, JR., Vice Chairman L. G. CUMMING, Vice Chairman E. WEBER, Vice Chairman
K. E. Anspach
J. Avins
R. R. Batcher
W. R. Bennett
J. G. Brainerd
F. T. Budelman
H. F. Burkhard
C. A. Cady
P. S. Carter
A. G. Clavier
J. L. Dalke
A. W. Friend
F. J. Gaffney
W. D. Goodale, Jr.
W. G. Tuller
R. A. Hackbusch
P. J. Herbst
J. G. Kreer, Jr.
E. A. Laport
W. P. Mason
G. D. O'Neill
W. M. Pease
W. J. Poch
D. C. Ports
P. C. Sandretto
R. Serrell
R. F. Shea
R. E. Shelby
N. Smith
Measurements Co-ordinator
E. WEBER
Sound Recording and Reproducing Committee-1948-1954
A. W. FRIEND, Chairman, '48-'54
H. W. Roys, Past Chairman, '48-'541
L. THOMPSON, Vice Chairman, '48-'54 M. S. CORRINGTON, Vice Chairman, '50-'54
S. J. Begun, '48-'54
S. M. Fairchild, '53-'54
R. M. Fraser, '53-'54
G. Graham, '48-'51
G. P. Hixenbaugh, '48-'52
F. L. Hopper, '48-'51
E. W. Kellogg, '48-'53
C. J. Lebel, '51-'54
R. A. Lynn, '50-'51
J. H. McGuigan, '53-'54
J. Z. Menard, '48-'51
E. Miller, '48-'51
A. R. Morgan, '48-'51
C. B. Pear, '53-'54
A. P. G. Peterson, '50-'54
H. Schecter, '5O-'53
R. A. Schlegel, '48-'52
W. Shepard, Jr., '50-'51
C. F. West, '50-'54
R. E. Zenner, '50-'54
1. SCOPE
1.1 This standard specifies conditions which must be
met in measuring or reporting flutter in sound re-
corders, reproducers, or records.
2. DEFINITIONS
2.1 Flutter
In recording and reproducing, flutter is the deviations
in reproduced sounds from their original frequencies,
which result in general from irregular motion during
recording, duplication or reproduction.
Note 1: The colloquial term "wow" is defined in the
same way, but is commonly applied to relatively
slow variations (for example, one to five or six repe-
titions per second) which are recognized aurally as
pitch-fluctuations, in contradistinction to the rough-
ening of tones, which is the most noticeable effect of
rapid fluctuations.
Note 2: A constant difference in pitch such as results
from a difference in the average speeds during record-
ing and reproduction is not included in the meanings
of the terms "wow," "flutter," and "drift."
Note 3: By an extension of their meanings, the terms
"flutter" and "wow" are used to designate variations
in speed itself or variations in recorded wavelengths.
Note 4: Although most recorded sound comprises mul-
titudes of tones, it is convenient to refer to flutter as
variations in frequency, assuming the recorded sound
to have been a single steady tone.
* Reprints of this Standard, 53 I.R.E. 19.S2, may be purchased while available from the Institute of Radio Engineers, 1 East 79 Street,
New York 21, N. Y., at $.75 per copy. A 20 per cent discount will be allowed for 100 or more copies mailed to one address.
I Chairman, 50-52.
1954 537
3PROCEEDINGS OF THE I-R-E
2.2 Flutter Rate
Flutter rate is the number of frequency-excursions in
cycles per second, in a tone which is frequency-modu-
lated by flutter.
Note 1: Each cyclical variation is a complete cycle of
deviation, for example, from maximum-frequency to
minimum-frequency and back to maximum-frequency
at the rate indicated.
Note 2: If the over-all flutter is the resultant of several
components having different repetition rates, the
rates and magnitudes of the individual components
are of primary importance. (See Appendix 1.)
2.3 Drift
The term drift is used to designate random-frequency
variations which are continuously in one direction or the
other for periods of the order of a second or more.
2.4 Per Cent Flutter
The per cent flutter in a reproduced tone is the root-
mean-square deviation from the average frequency, ex-
pressed as a percentage of average frequency.
2.5 Per Cent Total Flutter
Per cent total flutter is the value of flutter indicated
by an instrument which responds uniformly to flutter of
all rates from 0.52 up to 200 cps.
Note: Except for the most critical tests, instruments
which respond uniformly to flutter of all rates up to
120 cps are adequate, and their indications may be ac-
cepted as showing per cent total flutter.
3. FLUTTER-MEASURING INSTRUMENT
3.1 The block diagram, Fig. 1, shows the principal ele-
ments of a typical flutter-measuring instrument.
Note: The limiter insures freedom from appreciable
amplitude fluctuations which can cause error. A low-
pass or bandpass filter is used after the limiter to re-
move the distortion products caused by the limiter.
Means are usually provided for checking that the
input signal is within proper operating limits as to
amplitude and frequency.
The discriminator converts frequency changes into
amplitude changes, and the output of the rectifier
fluctuates in identical manner with the input-fre-
quency.
The low-pass filter eliminates the "carrier" or orig-
inal tone.
Some flutter-measuring equipment is provided with
a continuously variable oscillator whose output beats
with the test tone to provide a sum or difference tone
whose fluctuations are measured. This makes it
possible to measure fluctuations in test tones whose
frequency differs slightly from normal value.
2 This would mean practically full response for once-around flutter
of a 33 rpm turntable. Only recording-type meters will satisfactorily
show drift.
For the purpose of analyzing the causes of flutter
in a recording or reproducing system, it is often desir-
able to make measurements in several ranges of
flutter-frequency. For such purposes bandpass filters
may be incorporated in the measuring instrument, as
shown at B, B, B, in Fig. 1, or an adjustable bandpass
filter or "analyzer" may be provided, so that flutter
components at specific rates can be measured.
IREPRODUCINGI LOW PASSI LOW PASS|
UNDER~ ~ ~ ANLYE
TEST|
BAND PASSv
FILTERIS |1 DMYLA
0~~~~~~~
Fig. 1-Block diagram of a typical flutter-measuring instrument.
Valuable information is given by a record of in-
stantaneous frequency deviations (flutter oscillogram,
or "wowgram") covering a period of several seconds.
The block diagram, Fig. 1, shows provision for such a
recording device, which can be used in addition to the
rms indicating instrument. (See 4.6.)
A simpler type of flutter-meter has had consider-
able use, particularly for servicing theater equipment.
It employs a circuit which gives a null point or zero
output at a certain frequency, which can be adjusted
to the mean-frequency of the reproduced tone. The
reading of the output-meter is an indication of flutter,
provided the null point has been adjusted to the ac-
tual mean-frequency.
3.2 Range of Flutter Rates Covered
It is essential for accurate measurement of flutter,
that the measuring equipment give full response for
fluctuations at least as slow as the lowest known cyclic
disturbance, such as the slowest rotating member in the
driving system of a disk turntable, or the natural fre-
quency of any mechanical filter which may be em-
ployed. It is equally important that the upper-frequency
limit be high enough to include all of the high-frequency
components of flutter which may occur in sufficient
magnitude to appreciably affect the resulting measure-
ments. As an example, in 35-mm motion picture work,
200 cycles for the upper limit has been recommended
so as to include the second harmonic of 35-mm film
sprocket hole modulation.
3.3 Type of Indicating Instrument
Instruments which respond to the average value of
frequency deviations or to a function intermediate be-
tween average and root-mean-square shall be considered
J711arcit538
I-R-E Standards on Sound Recording and Reproducing: Determining Flutter Content, 1953
satisfactory for all but the most critical tests, providedthe instrument has been calibrated to correctly indicate
the root-mean-square value for a sinusoidal frequency
variation at a single flutter rate.
Note 1: When flutter is predominantly of a single rate,
meters which depend on peak, average or root-mean-
square deviations, would read alike, provided they
are appropriately calibrated. When, however, the
total flutter is made up of several components of dif-
ferent frequencies of rates, the additions would take
place in different manners. The root-mean-square
measurement would give more weight to the larger
resultant deviations than an average measurement,
but it would not carry this to the extreme of giving
relatively little weight to the lesser deviations, which
is the characteristic of a peak-reading system.
Note 2: Indicating instruments for flutter-measure-
ments should be unusually heavily damped, other-
wise excessive swings are likely to occur when there is
strung flutter at slow rates, making reading difficult.
When it becomes necessary to estimate the average
reading of the swinging needle of an rms instrument,
the mean position of the needle should be estimated
and the scale figure read at this point, rather than to
take the mean of the highest and lowest figures.
3.4 Test Tones
The standard frequency for flutter-measurements is
3,000 cycles.
Note: Satisfactory flutter-measurements can be made
using tones of different frequency, but interchange-
ability of records is highly desirable and use of 3,000
cycles is widespread. A number of flutter measuring
equipments for disk records employ 1,000 cycles.
3.5 Flutter-Test Records and Films
The test films used in testing reproducers shall com-
ply with the specifications for 3,000 cycles flutter-test
films published by the Society of Motion Picture and
Television Engineers, and Motion Picture Research
Council. Test records for determining the flutter content
of 33' rpm disk reproducers should conform to specifica-
tion published by the National Association of Broad-
casters. At the present time, there are no standard spec-
ifications for 78 or 45 rpm disk test records.
4. TEST METHODS
4. General
Measurement of flutter in one element of a sound re-
cording or reproducing system (recorder, re-recorder,
reproducer, or test record) shall be made under such
conditions that the flutter in the remaining portions in
the system is relatively negligible.
4.2 Testing Recorders
To determine the flutter of the recording device, a
tone of constant frequency and amplitude is recorded.
This test sound record is then reproduced with a ma-
chine having relatively negligible flutter, and the rms
frequency deviations of the resulting tone from the
average are measured with a suitable measuring instru-
ment.
4.3 Testing Reproducers
Similarly, to determine the flutter content of a sound
reproducing device, a test record having relatively
negligible flutter content is reproduced on the machine,
and the rms deviations of the frequency of the resulting
tone from the average are measured.
4.4 Testing Records
A test sound record is reproduced on a machine
having relatively negligible flutter content, and the rms
deviations of the resulting tone from the average are
measured.
4.5 Flutter Determination of Recorders Adapted to Both
Recording and Playback
In the case of recorders which can be used as repro-
ducers, the flutter content can be determined by repro-
ducing from a test record having relatively negligible
flutter content, and measuring the deviations of the re-
sulting tone from the average. If any factors are pres-
ent, however, which may make the flutter during re-
cording and reproducing differ, (for example, disk-cutter
load and reproducer-stylus drag) there should be sep-
arate determinations for each case.
Note: Alternate Method (Nonstandard). If a machine
under test is capable of operation both as a recorder
and a reproducer, and is itself superior with respect
to flutter to any other available machine, or record,
it may be used to test its own records.
When this procedure is followed, flutter com-
ponents due to a given cause, in recording and repro-
duction, add vectorially; but unless the relative phase
of the two vectors is exactly known, a single measure-
ment which shows the vector sum does not suffice
for determining the magnitude of either disturbance
alone. But the magnitude of one alone can be es-
timated from a series of measurements, with the range
of possible phase-angles covered in substantially uni-
form steps. If two equal vectors are added at n
equally spaced phase-angles covering the possible 360
degrees, and the squares of the n resultants are aver-
aged, the square-root of this average is 1.41 times the
length of one of the vectors. This holds independently
of the starting or reference point, and for n equal to
any whole number down to two.
Whence-In testing a machine with its own record,
the flutter for the combined operation is the square-
root of the average of the squares of the measure-
ments, with record in a series of positions relative to
driving system, such that range of possible phase-
relations between cyclic disturbances in recording and
reproduction is covered in substantially equal steps
1954 539
PROCEEDINGS OF THE I-R-E
If the measuring equipment reads rms flutter, and if
conditions are such that equal flutter is to be ex-
pected in recording and reproduction, the most prob-
able value of flutter for either operation alone is 0.707
of the measured combined flutter.
Discussion of the application of this principle to sev-
eral cases appears in Appendix II.
4.6 Instantaneous Flutter-Records
Flutter-tests are sometimes made by making records
(wowgrams) of instantaneous frequency deviations.
Such records give valuable information but do not afford
a readily determined figure for the per cent flutter except
that the peak-high and -low excursions of the frequency
may be determined. It is desirable that rms measure-
ments be made as part of the same test. (See 5.3.)
5. REPORTING ON MEASUREMENTS
5.1 Statements of per cent flutter shall always be ac-
companied by a statement as to the range of flutter-
rates wherein the measuring instrument has substan-
tially uniform response. For this purpose it is recom-
mended that the range limits be taken as the frequencies
at which the response is 6 db below that at mid-range.
5.2 Statements of flutter should, when possible, include
information as to predominant rates, or of the distribu-
tion with respect to flutter rate, as for example, amounts
of flutter in different flutter-rate bands or ranges.
5.3 It is inevitable that in many cases it will be neces-
sary to transmit information shown by an instantaneous
flutter record or wowgram, of which no simultaneous
rms flutter-reading will have been taken. When a figure
for per cent flutter is reported on the basis of visual
inspection of a flutter record or wowgram, the figure
shall be one-half the extreme excursion between highest
and lowest frequency and designated as "peak-flutter."
It should be recognized that such peak-flutter will, in
practically all cases, exceed the rms value by consider-
ably more than the well-known 1.41 factor which ap-
plies to sine waves.
APPENDIX I
Definition
Flutter Index is a measure of the perceptibility of fre-
quency modulation of a single tone.
Note 1: Based on data presented in an article entitled
"Analysis of Sound Film Drives," by W. J. Alber-
scheim and D. MacKenzie, Jour. Soc. Mot. Pict. Eng.,
vol. XXXVII, p. 453; Nov., 1941. An approximate
formula for flutter index for continuous tones in a
moderately live room is as follows:
Afx kfx
r lOOr
where
Af= rms deviation of frequency from mean in cycles
f= frequency of tone
k = per cent rms flutter
I= flutter index
r =flutter rate
X6 for rates greater than 5 cps.
Note 2: For flutterrates 1 to 5 cps, the following rela-
tions are suggested:
r
30
from which
when f = 3,000 cps, I = k.
Note 3: For flutter rates less than 1 cps, the following
relations are suggested:
r2
30
from which
when f = 3,000 cps, I = kr.
Note 4: For the general case per Note 2, wherein
X6= , the flutter index, when multiplied by 6AV2, is the
argument of the Bessel functions of the first kind and
the coefficients of the various orders of the Bessel
functions have been shown to represent the ampli-
tudes of the corresponding orders of the side-frequen-
cies present in a frequency-modulated tone.
Note 5: For flutter rates above 5 cps, the ear appar-
ently hears the side-frequencies as extraneous effects,
and therefore will perceive approximately the mini-
mum-flutter at the same value of flutter index over
a wide range of signal-frequencies, percentages of
flutter, and rates (assuming relatively constant acous-
tic conditions).
Note 6: For flutter rates less than 5 cps, the ear appar-
ently distinguishes the time-frequency variation
rather than the discrete side-frequencies, and so the
expression which describes the phenomena becomes
more complicated.
Note 7: The flutter index of any given device having
a constant per cent flutter will vary with the signal-
frequency, so that the test-frequency should always
be stated with flutter index in such cases. Unless
otherwise stated, the flutter index will be assumed to
refer to the standard test-frequency per section 3.4.
APPENDIX I I
Applications of principle of testing with record in several
positions as called for in 4.5 Note 1
The requirement that a record be tested in a series of
positions with respect to the member which drives it, if
no other cyclic disturbance is to be considered, requires
no further explanation, beyond stating that the 360
degrees of possible relative positions should be covered
in a number of equal steps. While two positions 180
degrees apart would give the correct relations so far as a
pure sinusoidal, or once-per-revolution disturbance is
concerned, a larger number is desirable, as better
averaging the effects of harmonics of the fundamental
March540
I-R-E Standards on Sound Recording and Reproducing: Determining Flutter Content, 1953
rate, and for more nearly meeting the objective with
regard to random variations. The number of different
positions used should be in no case less than five.
If in addition to a disturbance at a fundamental rate,
such as once-per-revolution of a disk turntable, there
are other more rapid cyclic disturbances, it is possible
generally to choose a series of record positions which will
suitably distribute the angles of vector addition, for
both the fundamental and the faster disturbance, pro-
vided these are in fixed relation, as in a geared driving
system, and if their rates are in simple relation.
Let m stand for the speed or rate ratio of the two
sources of disturbance. If m is a whole number, dividing
the fundamental cycle into n equal steps (provided m is
not divisible by n) will be found to also divide the faster
cycle into n equal steps.
If m is not a whole number, the cycle of repetition
will cover a number of revolutions of the slowest rotating
member, and the reference point must be taken at some
definite point in the repetition cycle (i.e., gears in cer-
tain relative positions). For example, if a turntable
makes 3 revolutions while a driving gear makes 16,
and five record positions are to be used, the record
would be shifted - of a turntable revolution between
each test and the next. Assuming the record to have
been made as one continuous recording, if no movement
of the driving system takes place during the operation
of shifting the record, it is not necessary to check gear
positions to obtain the desired shift of the record with
reference to the driving system, but only to keep track
of the total angle from the first position. In the case
just mentioned, with n = 5 and m = 16/3, the angles
would be
Angle A (revolutions)
repetition cycle 0 1/5 2/5 3/5 4/5
Angle on turntable 0 3/5 6/5 9/5 12/5
Angle at gear 0 16/5 32/5 48/5 64/5
To judge the distribution over the cycle, subtract
from each fraction the largest possible whole number.
Then the angles (in revolutions) are
at turntable 0 3/5 11/5 4/5 2/5
at gear 0 1/5 2/5 3/5 4/5
sprocket rotation are in question, it may not be possible
to choose a value of n which makes m/n other than a
whole number. For example if the sound sprocket has
32 teeth, and cycles of 4 teeth and 16 teeth are suspected,
the following positions, 0, 2, 8, 10, 16, 18, 24, 26 teeth,
will be found to give one or more pairs of opposite phase
relations for each cycle, and if each test can be paired
with one in opposite phase, the sinusoidal part of the
disturbance is properly averaged.
The endeavor to achieve a distribution of phase-
angles for several rates of cyclic disturbance may be
unimportant if the more rapid disturbances contribute
a negligible part of the total flutter, as is often the case
with machines which, as mentioned at the beginning of
this note, are themselves superior to other available test-
ing equipment or records.
Under these conditions, and in the case of nonsyn-
chronous driving systems (and some synchronous sys-
tems) where it is not possible to predict or control the
relative phases of possible sources of cyclic disturbance,
the best that can be done is to shift the record to an
appropriate series of positions to provide distribution
for the fundamental cycle, and depend on an adequate
number of tests to reduce the probable error with re-
spect to all faster cycles and random disturbances. Repe-
titions in the same record positions in nonsynchronous
systems, are in effect additional tests, with respect to
flutter in random-phase relations. This applies to mag-
netic-tape recorder-reproducer machines employing
pure friction drive, where definite positioning on the
capstan can scarcely be accomplished, but random
shifts on the capstan, between measurements, are better
than depending entirely on creep or speed difference
between recording and reproduction, to afford random-
phase relations.
Although magnetic-tape machines are more fre-
quently used in service to play their own records than
other recorders or reproducers, their flutter rating is to
be considered to be that for one operation alone, unless
a measurement is stated to be for the combination of
recording and reproduction.
It is recommended that the total number of tests be
not less than five (which figure has been named ar-
bitrarily as a compromise between inadequacy on the
one hand and burdensome testing on the other).
When this alternate method is used, results reported
should be accompanied by a statement of the procedure
followed.
When shifting a film record with respect to a sound
sprocket, there are only certain possible angles of shift,
and if possible disturbances at higher rates than the
1954 541