TECHNICAL BULLETIN #3

The Technical Argument for Chromium Dioxide
Terence O’KeIIy
BASF has continued research and development ofchromium dioxide as a magnetic pigment for a numberof technical reasons. While some companies haveswitched to cobalt-modified ferric oxides for reasons ofcost, licensing, or politics, the fact remains that, in
terms of the physical uniformity of particles and in
overall magnetic properties, chromium dioxide is still
the best magnetic pigment available. It is for thisreason that BASF remains committed to chrome.

Tape noise is composed of bias noise and two kinds ofmodulation noise, AM and FM modulation noise.
Figure 1: Chromium Dioxide and Gamma Ferric
Oxide Crystals

The greatest single advantage of chromium dioxidelies in the shape of its particles. E.l. DuPont, an
American company, discovered the process by whichchromium trioxide decomposed in the presence ofwater at a temperature of 900° F and under a pressureof 30,000 psi to yield a synthetic mono-crystal ofchromium dioxide. These particles are very small, very
long and thin, and very similar to each other in sizeand shape. They are also totally free of the physicaldeformities that plague gamma ferric oxides. Gamma
ferric particles not only vary a great deal from eachother in both shape and size, they also suffer fromphysical flaws such as dendrites and holes. Dendrites
are branch-like imperfections that stick out from thetrunk of the ferric particle. The holes are created whengasses escape from the particle in refining stages.
Under a microscope the ferric particle looks like aspongy rod; a CrO2, particle looks like a smooth glassrod because it has no deformities (Fig. 1).
Freedom from physical imperfections and uniformity ofparticle shape and size provide chrome withelectromagnetic properties that are essential for goodaudio and video tape and that are unmatched by anyother formulation. The most important characteristics
are:
I. LOW NOISE
The most significant benefit of chromium dioxide is its
low noise. In audio tape, this means less backgroundhiss mixed with the recorded program and less“muddying” of the sound. In video terms it means less“snow” effect and truer colors on the picture screen.

A. Bias Noise — Bias noise is the noise added to atape as it passes under a rapidly alternating bias
signal from an erase or record head. The bias signal

makes the tape noise slightly greater than that on
virgin blank tape. The amount of bias noise added
depends chiefly on particle size: the smaller theparticle, the less the noise. Uniformity of particle sizes
and freedom from physical imperfections are also
contributing factors. Chromium dioxide particles are
not only more uniform in size and shape than cobaltmodified gamma ferric oxide particles, they are also
considerably smaller. The advantages of smaller sizeand greater physical uniformity result in lower levels ofbias noise. In fact, no other magnetic pigment canmatch the low noise levels of a good chromium dioxidetape. The difference is easily audible when a recordingwith no audio signal is made on a ferric-cobalt chrome“equivalent” tape and compared with that of a true
chrome tape; the chrome tape measures about 2.5
4db lower in tape hiss.
Video recordings do not use a bias signal for the video
portion of the recording (although the audio track isrecorded by a stationary head using bias); the very
high frequency of the luminance signal, the‘brightness” on the screen, acts as a pseudo-bias. Low
levels of audio bias noise are analogous to low levelsof luminance noise in video; so just as chrome audio
tapes have less hiss, chrome video tapes produce less“snow” on the video screen. Chromium dioxide video
tapes produce cleaner, clearer video pictures becauseof less luminance noise (video “bias noise”).

frequencies). These alterations will affect FM
modulation noise without changing the level of bias
noise. FM modulation noise causes a fuzziness insound reproduction that is visible as fuzziness on anoscilloscope display of a recorded signal. This type ofmodulation is a “muddying” of the sound caused by avery slight but continuous altering of tonal sound as
the tape moves imperfectly across audio heads. FM
modulation noise in combination with AM modulationnoise creates a noise that sounds as if it were always“behind” the signal. The noise rises and falls as thesignal rises and falls, and increased output merelyincreases this type of noise. On a video screen,
modulation known as “chroma noise” is discernible asslight shifts in the shades of colors. The chromeparticle provides extremely uniform dispersions andcoatings with smooth surfaces, and these superiorcoatings reduce the level of FM modulation noise and2
contribute to the unequaled low noise levels of Cr0tapes.
Figure 2: Comparison of Dynamic Range

B. Modulation Noise — Modulation noise is due toirregularities in the oxide coating, either on the tapesurface or within the coating. Modulation noise isdifferent from bias noise in that it is only apparent
when a signal is present (assuming the A.C. bias is nota signal). AM modulation noise is tested by recording adirect current signal (0 Hz; AM modulation is also
known as “D.C. noise” because of this test method) on
the tape and measuring playback. If a tape were ideal,
there would be no output; but slight irregularities in thetape will cause amplitude variations somewhat similar
to dust causing “crackle” noise on a vinyl record disc.
The noise occurs as amplitude spikes in either apositive or negative direction. The level of AM
modulation noise can be reduced somewhat bycarefully adjusting the bias current. Good tapecoatings do not seem to vary significantly in terms ofAM modulation.
FM modulation is due to varying tape/head contact
because of minute imperfections in the magnetic
coating or to varying frictional forces between tape andhead. FM modulation noise can be distinguished frombias noise by altering tape tension or by altering thefrequency of the recorded signal. (FM modulation ismuch less perceptible at low frequencies than at highII. DYNAMIC RANGE
Dynamic range, or signal-to-noise ratio, is defined as
the difference in decibels between the maximum
output level of a tape at 3% third harmonic distortion(or, in the case of high frequencies, the point ofsaturation) and the noise level, either weighted or
unweighted. The “signal” in signal-to-noise ratio isusually measured at low frequencies. It is important toremember that a signal-to- noise ratio or dynamic
range of 60 dB can mean either a given level of output
and a very low noise floor or a greater level of output
and an equally greater level of noise (Fig. 2). The
audible effect of different tapes with the same S/N ratio
may be different, however, depending on the S/N ratio
at higher frequencies, on the noise spectrum, and, to adegree, on the amount of modulation noise.
An examination of the dynamic range of chromium
dioxide tapes at high frequencies shows why so many
supporters of ferric-cobalt tapes state dynamic range
at low frequencies only (Fig. 3). Improvements in thehigh frequency sensitivity of ferric-cobalt tapes havehistorically been at the expense of increased noise.
Improvements in high frequency sensitivity ofchromium dioxide tapes, on the other hand, have notchanged the already low noise floor except, in somecases, to further reduce it slightly by reducing bothbias and modulation noise. An examination of thefrequency spectrum of the noise shows why thedifference is audibly significant (Fig. 4).

reduction systems will reduce many of the noiseproblems and some of the differences between
magnetic pigments. The argument that high output
levels will mask higher noise levels also has somevalidity, but both arguments overlook modulationnoise. FM modulation noise “smears” sound clarity inaudio and color sharpness in video. Noise reduction
systems and high output do not help alleviate this typeof noise; only a superior coating can. It is significant
that some critics of analogue and digital systems findthe most audible superiority of digital not to be infrequency response or dynamic range, but in the lackof FM modulation, both tape modulation and wow andflutter. Chrome tape is a step in the right direction.
Figure 4: Frequency Analysis of Tape Noise

Figure 3: The Dynamic Range of BASF
Professional II Chrome and Its Cobalt ModifiedCompetitors

III. MOL
Maximum output level is the first half of thesignal/noise ratio. The chief claim to superiority thatferric-cobalt and metal tapes make is that of greater
output, which, unfortunately, is accompanied byincreased noise. The net effect is that an increased
MOL at the expense of increased noise results in thesame -dynamic range figures as a low-noise chromethat has slightly less output. It is questionable whetheror not it is beneficial to sacrifice noise for increased
output when that noise also includes more modulationnoise. Increased output can mask increased bias
noise, but it will not mask modulation noise.
Modulation noise, in fact, will increase as output
increases!

The lower noise level of chrome lies precisely in the“presence” region (1 kHz to 8 kHz) where the ear ismost sensitive to sound. Not only does chrome provide
a lower noise floor, but it also changes the “sound” ofthe noise to a “softer” or “less aggressive” disturbancethan that of chrome substitutes. The use of noise

Audio experts expect the digital revolution to besignificant in increasing dynamic range not by
increasing sound levels but by decreasing noiseinterference. A comparison of the practical applicationof dynamic levels is given in Fig. 5. The output levelsof various program sources were charted over a time
span. The 0-db level in the charts is equivalent to atape flux level of 200 nW/m on a cassette deck’sFigure 5: Dynamic Range of Musical Sources

meters. The output from the vinyl record discs was fed
through the electronics of a three-head cassette deckand fed to a Leader LFR-5600 frequency analyzer.
The pen was placed in its fastest writing mode, 0.1
sec., to measure the output levels from the disc. The
pen was not as fast as the meters in transient
response, of course, but it was fast enough to read + 6
db at the striking of the kettle drum at the end of
Telarc’s Firebird. What are more important are therelative differences of output levels over a span oftime.
The program with the least dynamic range was the FM
rock station with a mere 10-dB difference between
loud and quiet. The announcer’s voice was more
dynamic than the music. Next was the “beautiful”
music station with a 15-20 dB dynamic range,
including the two commercials. The Mobile Fidelity
Sound Lab Original Master Recording of the Pink
Floyd album averaged 20 dB, but the reliance on quiet
disc surfaces was obvious at several spots fordramatic effect: the opening heartbeat and the sudden

alarms at the beginning of “Time.” The digital recordingof the Firebird had the greatest dynamics. What issignificant is the number of times the output reachedwithin 1 dB of the 0 reference or exceeded it: 27 times
in 20 minutes. An increase in MOL was an advantage
for only thirty seconds (presuming each peak lasted afull second). That means that for the most demanding
music where dynamic range is most advantageous,
low bias noise was beneficial for 20 minutes or for
96.9% of the time, low modulation noise was important
for the full 20 minutes, 39 seconds, and the “big”
advantage of increased MOL was useful for 2.4% ofthe time.
The emphasis on MOL as an important criterion oftape must be considered in relation to noise; and, inthe case of the Firebird, MOL was relatively lessimportant. Some tape manufacturers continue toemphasize increased MOL as the major asset of theirtapes; but if a ferric-cobalt tape can receive a 1-dB
improvement in MOL, what guarantee is there that theend user can fully utilize it? The slow VU-type metersprobably will not allow the user to set the difference
precisely on his or her meters; neither will the popular
fluorescent-type meters that often move in 2-dB steps
in the critical range. It is far more likely that reduced
noise will benefit him or her more because no exact
settings are needed to take advantage of it. Increased
MOLs are often wasted by conservative settings onpoor meters; reduced noise is always beneficial to theuser.
Improved formulations of Cr02 have led to thedevelopment of Superchrome formulations withincreased sensitivity at both high and low frequencies.
Increases in sensitivity of Cr02 formulations result inequal or greater increases in MOL. Increasing
sensitivity in a ferric-cobalt will produce only an equal
or less increase in MOL. Double-coated, Superchrome
tapes are now equal or very close to the best ferriccobalt tapes in low frequency MOL, but their inherently
lower noise still produces greater dynamic range andmore practical benefits.
IV. GREATER WEAR LIFE
The binder system used for BASF chrome tapes is
especially durable because the binder surrounds each
perfectly shaped particle better. Chrome tapes
typically show less slitting debris than ferric tapes
because less oxide separates from the polyester
backing. Wear tests comparing rub-off of chrome andferric materials show that chrome tapes are cleaner
and more durable than ferric tapes. Greater wear lifeand less rub-off are especially important for video
tapes. The extended wear life will allow the tape to last
longer in still frame modes when the video head isscanning the stationary video tape. An internal quality
check at BASF requires that a tape show no increase
in dropouts or decrease in output after 60 minutes ofstill-frame scanning on the section of tape. Less
durable formulations will wear away because of thephysical contact. Less rub-off reduces the amount ofvideo noise caused by debris on the video heads andincreases the life of the heads. A cleaner tape meanscleaner heads for greater lengths of time.
V. GREATER PRESSURE STABILITY
Ferric-cobalt tapes were once notoriously subject tomagnetic losses caused by physical pressure on theoxide, a phenomenon known as the “magnetostrictiveeffect.” This problem has been reduced to insignificant
levels in audio tapes, but the pressures exerted by
rapidly rotating video heads do cause some signal
demagnetization on cobalt-activated ferric video tape.
Chrome video tape, on the other hand, seems to beimmune to this type of signal loss. A recording madeon chrome video tape played over and over again willretain the original picture quality that would havedeteriorated if the recording were made on a ferriccobalt tape.
Ironically, in spite of all its unsung technical benefits,
chromium dioxide has suffered from a charge stillwidely accepted, that chromium dioxide is very
abrasive to recorder heads. Particles of chromium
dioxide are harder than ferric oxides, and theirfrictional characteristics are also different. Tapes
coated with the magnetic powder of CrO2 particles
produce less wear on mu-metal heads, which are thesoftest type and most likely to suffer wear damage. On
these heads, ferric oxide tapes produce more wear.
On sendust and ferrite heads, which are much harderand very resistant to wear, the CrO2//ferric wear ratio
switches. Chrome tapes show more initial wear onthese types of heads as the hard, smooth surface ofthe tape polishes the grainy structure of the headmaterial and wears down the rough surface. In fact, inthe first one hundred hours of running virgin chrome
tape on ferrite heads, the amount of head material
worn away is shocking when compared to that of ferricor ferric-cobalt tape. However, for the next three
thousand hours, the rate of wear decreases
dramatically. More importantly, the integrity of the gap
edges remains intact, unlike the edges of ferrite heads
running ferric or ferric-cobalt tapes that erode.
This phenomenon is critical for video duplicators. As
the gap edges erode over time, high-frequency
luminance signals decrease in output. In those cases
where a ferric-cobalt video tape has extra lubrication
added to reduce head wear, the gaps become fouledwith lubricant and also gradually lose luminanceoutput. In order to bring luminance signals back totheir original levels, after 800 to 1,000 hours ofduplication time, technicians use lapping tapes topolish the video heads of machines running ferriccobalt tapes to clean and restore the integrity of thegaps. The lapping procedure reduces the protrusion of
the heads to the same wear level as that of heads thathave seen only chromium dioxide video tape and saw
the worst wear in only the first 100 hours. The chromeonly VCRs, however, do not need any down time
because the heads remain clean, the gaps are intact,
and the luminance stays the same. At two thousandhours, ferric-cobalt VCRs need relapping. Chrome
VCRs do not. The largest VHS duplicators havelearned that alternating ferric-cobalt and chromium
dioxide tapes on their industrial VCRs extends the lifeof their equipment and reduces down time formaintenance.
The great headwear scare that dogged chromium
dioxide tape is likely to have been based on
measurements over only fifty to one hundred hours

when the tape polishes hard heads the most.
Measurements taken over the next three thousand
hours indicate that total wear of head surfaces isnearly identical to that of ferric-cobalt tapes, and thegap wear is far less.
When particles are coated on a tape, however, otherfactors, such as the binder system, coating technique,surface finish, and environment are far moresignificant in determining the amount of abrasion likely
to occur than is the particle used. Several tapemanufacturers using exactly the same particle may
have greater differences in wear factor between theirtapes than one manufacturer may have in a number ofdifferent tapes using different particles. It is mainly up
to the manufacturer, not the particle. It is highly
unlikely that chrome tapes, while producing the least
physical modulation of all oxides, are at the same time
grinding down head surfaces at a rate that destroys
equipment.

The technical arguments for chromium dioxide would
remain merely theoretical if it were not for some hardpractical facts. When professional recording engineers
choose a tape for audiophile recordings, the choice
has usually been chromium dioxide. The basis forchoice is not only rigorous testing with sophisticatedmeasuring equipment, it is also extensive listening
tests. When the experts, such as those at theuncompromising Mobile Fidelity Sound Lab, A&M
Records, Deutsche Grammophon, American
Grammophone, CBS Records, Windham Hill,
Polygram, RCA, Vanguard Records, and Connoisseur
Society choose a tape for audiophile recording — inreal time or high speed — that choice has beenchromium dioxide. They, like BASE have chosen CrO2
for unarguably sound technical reasons. They, and we,
feel that any other reason is a compromise.