C:\!MyFiles\Technical Papers\Master-Tape Eq\Master

Master-tape Equalization Revisited1
John G. (Jay) McKnight2 and Peter F. Hille
Ampex Corporation,
Redwood City, CA, USA
Optimum signal-minus-noise level of a commercial tape or disk-record requires the
signal- and noise-spectra of the studio master tape to be matched to those of the
commercial record. The use of the NAB 380 mm/s (15 in/s) equalization (3150 Hz
transition frequency) with modern tapes results in frequency- and noise-spectra which
have much higher levels at high frequencies than the final records. Other practical
equalizations are studied, and a 12 500 Hz reproducing transition frequency is suggested
for further evaluation.
The studio-master tape for most commercial magnetictape and mechanical-disk records is recorded on low-noise,
high-output tape at 380 mm/s (15 in/s), on 2 mm-wide
tracks, using the flux-frequency characteristic established
for broadcasters by the National Association of Broadcasters
(NAB) in 1953 [1].
It has long been realized that the high-frequency noisespectrum level of such a system is too high to reproduce the
full dynamic range of a symphony orchestra. When several
channels of a multi-channel master are “mixed down” to
form a 2-channel stereo recording, or if the program is
rerecorded in the production process, the noise level is even
further increased. Therefore, in many cases the noise level
is decreased by the use of a dynamic noise reduction system
[2], which in effect changes the equalization in accordance
with the instantaneous signal spectrum, in order to minimize
the effect of the noise added by the tape recorder.
There are, however, also many cases where the dynamic
noise reduction system is not used. This often occurs in
multi-track recorders, where the cost of the many noisereduction systems is considered by the studio to be
prohibitive. Unfortunately, these very multi-track recordings
are the ones which most greatly need the dynamic noise
reduction systems!
Thus, for the reasons of economy and also for simplicity
it is desirable to optimize the fixed equalization used in
studio mastering. The division of the equalization for tape
short-wavelength (high-frequency) losses, into pre- and
post-emphasis is based on the particular characteristics of
the tape in use. The recorded flux vs frequency is then
standardized – which in effect standardizes the postemphasis – and any change in the tape characteristics is
made up by adjusting the pre-emphasis, which is not
standardized. Thus when the short-wavelength losses are
sufficiently decreased by improvements in the tape
manufacturing art, a standard will outlive its usefulness, and
the division of equalization will need to be reevaluated and
a new standard written.
In this paper we consider the 1953 NAB equalization
which is still used in master recording, and propose a
different and more suitable equalization, based on the use of
modern-day tapes and applications.
Authors’ manuscript of a paper presented at the 42nd Audio Engineering Society Convention, Los Angeles, 1972 May 02...05. Corrected
text modified from AES Preprint 856.
J. McKnight is now with Magnetic Reference Laboratory, Mountain View, CA, USA, http://home.flash.net/~mrltapes
Audio Eng. Soc. Preprint 856, 1972 May
The usual approach to designing studio mastering
equalization is to consider the spectrum of the signal to be
recorded, and the weighted noise spectrum of the recording
and reproducing system. Then the equalization is divided
into pre- and post-equalization according to empirical tests,
in order to produce the best-sounding master tape [3].
These tests are often done with a single generation of
recording and reproduction, but the actual master tape for a
commercial record is more often the result of a multichannel mixdown, or of several rerecordings. Thus, an
equalization division which produces an adequately-low
noise level in an “A-B” comparison of “live” studio signal
with the recorded signal often produces a “brilliant” but
noisy final master tape after the several rerecordings,
mixdowns, etc.
Altho it is a noble goal to have a studio master whose
“brilliance” is indistinguishable from that of the original
signal, this approach neglects one very important fact: no
customer who buys a finished commercial record
(mechanical disk, or magnetic tape in any of the several
formats) ever gets to hear the studio master tape!
Thus the goal of the optimum master equalization should
be to produce the best commercial record, not to produce
the best studio master per se. We will show here that the
two criteria – best master per se, or best commercial record
– give very different results for the “optimum” division of
equalization, and that the present NAB equalization is not
appropriate for recording studio master tapes.
If our goal in designing the compromise between signal
level and noise level of the studio master tape is to best
match the signal level and the noise level of the commercial
record, we obviously first need information on the signal
level and noise level spectra of the several popular
commercial record media. Such data have been published
by Gravereaux et al [4]. The data in their Figure 10 shows
that, of the “commercially important” record formats –
mechanical disk, and the tape records in the cassette and 8
track cartridge packages – the disk has the best signalminus-noise level spectrum, so we will use that data as a
basis for further discussions here.
The heavy curves of Figure 1 show the saturation signal
level and the -octave-band noise level spectrum for a
four-channel mixdown, or for a fourth generation master
tape.3The light curves reproduce the corresponding data for
a mechanical disk record from Gravereaux’s Figure 7.4 It is
apparent that the maximum signal spectrum of the master
tape exceeds that which the disk will accept by 3- to 13-dB
at 8 kHz, and 5- to 20-dB at 16kHz. Therefore in the
transfer from tape to disk the large-signal response of the
system must be reduced by these amounts in order to avoid
distortion on the disk. If this reduction is done by a highfrequency limiter (of which several commercial models5 are
available), then the “punch” of the master is reduced, but
the high frequency noise of the master is transferred at full
The above-described combination, then, produces the
“pessimum” results: a drooping high-frequency response for
large signals, with all of the high frequency noise that might
accompany a system with flat response for high-level highfrequency signals.
Fig. 1 Saturation signal and -octave noise spectra. Heavy
curves: For a fourth-generation master tape with NAB
equalization. Light curves: For a 33 rev/min vinyl disk
pressing. See footnotes 3 and 4 for detailed conditions.
Conditions for the master tape: Tape: Ampex 406 or 3M-206.
Tape Speed: 380 mm/s (15 in/s). Equalization: according to the
NAB standard [1]. Track width: Approx. 2mm. Bias: overbiased
to reduce 10 kHz recording sensitivity by 2dB below maximum
sensitivity. Signal spectrum is the maximum possible
fundamental-frequency output voltage from the reproducer at the
stated frequency, without regard to distortion. Noise spectrum is
Audio Eng. Soc. Preprint 856, 1972 May
for a
,-octave band.
Conditions for disk: Disk: vinylite pressing. Rotational speed:
33 rev/min, Equalization: according to IEC Pub. 98 (also RIAA
and DIN standards). Signal spectrum is the maximum possible
fundamental-frequency output voltage from the reproducer at the
stated frequency, as determined by geometrical limitations given
by Gravereaux [4]. Noise spectrum is for a -octave band.
High-frequency limiters are manufactured by GRT, Ortofon,
Neumann, Fairchild (“Conax”), and CBS Labs (“Volumax”).
3. 1 Other Possible Divisions of Equalization
For practical equalizer design and specification, and for
good fitting of the total tape losses, 6dB per octave RC
equalizers are very satisfactory [3], and only this type will
be considered here. Figure 2 shows the signal-level and
noise-level spectra for three reproducing transition
frequencies: solid curve, 3150Hz (NAB Standard), dotted
curve 6300 Hz, and dashed curve 12 500Hz. The disk record
signal- and noise-spectra have been drawn in again (light
curves) for reference.
When we compare the various master tape noise spectra
with that of the disk record, it is apparent that the best
match (of the several transition frequencies considered here)
comes with the 12 500 Hz reproducing transition frequency.
This comparison is relatively simple, since the disk noise
spectrum is essentially unchanged from the outside diameter
to the inside diameter of the disk. The maximum signal
spectra comparisons are not so simple, due to the great
change from outside to inside diameter of the disk. The
Fig. 2 Saturation signal and -octave noise spectra. Heavy
curves: For a fourth-generation master tape with various
reproducing transition frequencies F = 3150 Hz (NAB), 6300
Hz, and 12 500 Hz. Light curves: For a 33 rev/min vinyl disk
pressing. See footnotes 3 and 4 for detailed conditions.
maximum signal spectrum from the master tape with the
12 500 Hz reproducing transition frequency falls some 2- to
4-dB below that of the disk at maximum diameter, but 2- to
12-dB above that of the disk at minimum diameter. It is our
judgement that this compromise of signal and noise spectra
Audio Eng. Soc. Preprint 856, 1972 May
(using the 12 500 Hz reproducing transition frequency) is
worth a field trial.
A further point of comparison is the amount of preemphasis needed for the disk record (relative to a constant
recorded velocity response), in comparison to the master
tape pre-emphasis for the reproducing transition frequencies
under consideration. These are shown in Figure 3: the disk
record pre-emphasis transition frequency is 3150 Hz (+10
dB at 10 kHz); and with the present NAB master tape (3150
Hz reproducer transition) the tape pre-emphasis is only 2 dB
at 10 kHz – certainly a poor match, The master with 12 500
Hz reproducer transition frequency, on the other hand,
requires the same pre-emphasis as the disk. As a further
reference, note that this 3150 Hz pre-emphasis transition
frequency (+10 dB at 10 kHz) is also that which is used
typically for 190 mm/s (7. 5 in/s) recording. The other
formats (cartridges and cassettes) – and even FM radio
broadcasting! – require even lower pre-emphasis transition
frequencies, and consequently greater amounts of highfrequency boost (13- to 15-dB at 10 kHz).
Thus altho a master tape with the proposed 12 500 Hz
post-emphasis transition frequency would appear to have a
slightly poorer large-signal high-frequency response than
the outside diameter of a disk, it would be equal to that of
a190mm/s tape, somewhat better than cartridges, cassettes,6
and FM radio, and much better than the disk at inner
The high-frequency noise of such a master tape,
however, would be reduced 5 dB at 5 kHz, and 10 dB at 16
kHz, relative to present NAB master tapes.
Fig. 3 Recording pre-emphasis for master tape with different postemphasis transition frequencies F = 3150 Hz (NAB), 6300 Hz, and
12 500 Hz. Pre-emphasis used for disk recording and for 190 mm/s
(7.5 in/s) open reel commercial tape records shown for
comparison. See footnotes 3 and 4 for detailed conditions.
The wavelength-losses of chromium-dioxide tapes are less than
those of traditional iron-oxide tapes. But since most cassette
recorders use a special post-emphasis with chromium-dioxide
tapes, with a 5 dB higher flux at high frequencies, the preemphasis used is about the same for both tapes!
3.2 Comments on Practicality of the 12 500 Hz
Reproducer Transition Frequency
Field tests of this proposed equalization change are
obviously needed to evaluate its practical advantages and
disadvantages. We do have some guiding experience,
however: First, the pre-emphasis originally used at 380
mm/s with the NAB standard and tapes and recorders
available in 1953 had a 6300 Hz transition frequency (+6 dB
at 10 kHz) – not the +2 dB at 10 kHz now used.
Second, all of the 190 mm/s open-reel stereo tapes
produced by Ampex Music Division (AMD, formerly
Ampex Stereo Tapes, AST) from masters made since 1968
– and this includes in particular the “EX +” series – have
been made from duplicator masters recorded at 380 mm/s
using the proposed equalization.7 The high-frequency, highlevel response of these 190 mm/s “open-reel” tapes is
generally agreed to be the best of all commercial records –
tape or disk.
We have shown that the presently used NAB
equalization used for 380 mm/s studio masters is
inappropriate – it gives a large-signal 10 kHz response
which is 4- to 14-dB greater than any commercial record
format (mechanical disk, or cartridge or cassette tape
record) is able to utilize. The price paid is not only the
difficulty of modifying the signal on the master in order to
“squeeze it thru” any commercial record format, but also a
high-frequency noise level which is obtrusive, especially
when multi-channel mixdowns and/or several rerecordings
are necessary.
Altho the noise may be reduced by dynamic noise
reduction systems now available, a much simpler and less
expensive alternative is to reoptimize the mastering
equalization. We propose field tests of a reproducing
transition frequency of 12 500 Hz, in place of the NAB
value of 3150 Hz. This would reduce the noise at 8 kHz by
6 dB, and at 16 kHz by 10 dB.
The increased pre-emphasis necessary with this proposed
mastering equalization will undoubtedly cause highfrequency tape compression which will be heard as the
session is in progress. Realizing that this compression would
have occurred in the transfer to the commercial disk or tape
record, we feel it actually to be advantageous for the
producer and recording engineer to be aware of the problem
during the studio mastering, so that the appropriate change
of microphone placement or gain can be made with the
greatest artistic control.
A previous paper [5] has discussed the disadvantages of
the low-frequency pre-emphasis prescribed in the NAB
standard, and the presently proposed change in highfrequency equalization should be accompanied by a flat
(nonboosted) low-frequency recording response.
C:\!MyFiles\Technical Papers\Master-Tape Eq\Master-tape
Equalization Revisited 6.wpd
2004-05-10 18:55
In the transfers from the inter-masters sent by the various studios
to AMD for making the duplicator masters, an equalized peakreading level indicator with dynamics identical to those
standardized in the German Standard DIN 45 506 is used. This
indicator in itself helps in optimizing the recorded level, and in
preventing high-frequency overloading. Its use may not be
necessary in recording studio masters, but this can only be
determined by field tests.
Audio Eng. Soc. Preprint 856, 1972 May
[1] NARTB Recording and Reproducing Standards, See, 2, Magnetic Recording (1953 June). (US) National Association of (Radio and
Television) Broadcasters, Washington DC, US. Note: this Standard was replaced in 1965, but the equalization for 380 mm/s tape recording
was in effect re-affirmed. The 1965 standard is summarized in detail by George Bartlett in “The New NAB Tape Standards”, J. Audio Eng
Soc. vol. 13 Nr 3, pp 248...252 (1965 Jun).
[2] R.M. Dolby, “An Audio Noise Reduction System”, J. Audio Eng. Soc. vol. 15, pp 383...388 (1967 Oct).
[3] J, G. McKnight, “Flux and Flux-Frequency Measurements and Standardization in Magnetic Recording,” J. Soc. Mot. Pict, & TV Engrs.
vol. 78, pp 457...472 (1969 June).
[4] D.W. Gravereaux, A.J. Gust, and B. B. Bauer, “The Dynamic Range of Disc and Tape Records.” J. Audio Eng. Soc.vol. 18, pp 530...535
(1970 Oct. )
[5] J. G, McKnight, “The Case Against Low-Frequency Pre-Emphasis in Magnetic Recording,” J. Audio Eng. Soc.vol. 10, pp 106...107
(1962 April).
Audio Eng. Soc. Preprint 856, 1972 May
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