Ampex 351 - Page 40

Our major limiting factor today is the magnetic
Track Width
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Signal-To-Noise Ratio
Many factors complicate the signal-to-noise prob 
lem, some of them entirely beyond any control of the
manufacturer of magnetic tape recorders.
First is the tendency of both studios and "hi-fi ”
fans to reproduce music al a greater volume than that
of the original source. This, of course, also increases
the audible noise level.
Such a standard implies that the professional user
will have equipment to adjust his recorder to meet
these distortion specifications. It is rare that wave
analyzers or distortion meters are available, therefore
the calibration is usually made by using a standard
tape (see Basic Adjustments).
Distortion
Distortion in magnetic recording is a function of
both the bias adjustment and the recording level. We
have already seen the effect of the bias voltage near
the point of zero magnetization on the tape (see
Electronic Circuits) so in this we will cover only the
effect of the recording level.
of the middle frequencies, and the audible noise level
is increased. The noise coming from a small area is
also more noticeable than if it emanated from a large
source.
4
To achieve a maximum signal-to-noise ratio, we
wish to record at the highest possible signal level.
But as we increase our recording level we will even 
tually reach the point where any further increase
has little effect in magnetizing the tape. We have
“ saturated ” the medium, and any additional current
in the record head will simply give distortion
Where the maximum signal-to-noise ratio is nec 
essary, wide tracks are desirable, but there are certain
limitations. Economically, the amount of tape used,
and therefore the cost, increases roughly in propor-
In distortion caused by over-recording, the odd
harmonics will stand out, with the third harmonic
predominating. Our prevailing standards define the
normal recording level as the point where there is a
1% third harmonic content of the signal, and the
maximum recording level as the point where there is
a 3% third harmonic content.
the discussion of the tape transport, so we will treat
the first three in this portion and then follow with
additional factors encountered in stereophonic re 
cording.
tape. Our “ system noise ” (which includes the tape)
is from 8 to 10 db higher than our “ equipment noise ” .
A theoretical study has shown that an improvement
in the noise characteristic of the tape should be
possible by decreasing the size of the oxide particles,
and tape manufacturers are experimenting with this
theory.
Assuming a given tape noise, we are mainly con 
cerned with track width, track spacing (in multi 
channel equipment), tape speed, and equalization.
Then there is the fact that the average loud 
speaker is deficient in response, and directional at
high frequencies. The deficient response sometimes
results in the user increasing the high frequency
energy electrically (with an equalizing circuit)
during the recording process. This extra high fre 
quency energy increases the problems that exist in
high frequency overloading. The directional pattern
at high frequencies means that, if the average high
frequency energy throughout the room is to equal the
energy al lower frequencies, the high frequency
energy on the axis of the speaker is higher than that
But probably the major complication is that the
human ear is most sensitive to noise in the 1 to 6 kc
area, and the noise below 100 cps must be very great
before it is objectionable. The usual meter indication
consists largely of the low frequency component of
noise, which is inaudible; it is for this reason that a
recorder which tests quieter than another on our
normal measuring devices sometimes sounds noiser
when we actually listen to it. (Significant noise meas 
urements, therefore, can be achieved only by using a
weighting network with an inverse response to that of
the human ear. )
But these are things we cannot control. What
can we do to get the best signal-to-noise ratio?
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0066
Typical spectral noise density of the system (dash
line) and the equipment (solid line). Readings taken
on an Ampex full track Model 351 at 15 ips. Noise
spikes occur at 60, 120, 180, and 300 cps on both
curves (that at 60 cps rises to — 55 db and — 57.5 db
respectively). System noise taken with tape in motion,
equipment noise with tape stopped.