High Frequency Pre-emphasis
The signal-to-noise ratio of the magnetic tape is perhaps the least satisfactory of its characteristics.
Great efforts have been made to improve this defect. It is possible to imagine, for example, a tape
recorder, which sends to the recording head a current, which is proportional to the input signal,
independent of the frequency (recording at constant current).
Experience shows that the tape becomes saturated for a given current in the recording head
irrespective of the frequency. At high frequencies the saturation takes on special characteristics.
The harmonics, which the saturation should produce, go out of the range of the spectrum, which the
playback head can reproduce. Therefore, a tape saturated in the high frequencies does not give a
distorted signal. Simply, an increase of the recording current does not produce an increase of the
recorded signal.
In effect, the tape becomes a limiter, which, in addition, alters the sonority of the recording.
A tape recorded under these conditions (constant current) should be played back on a head
followed by an amplifier fitted with frequency response correctors so that the ensemble will be
linear.
It can be seen that with the sounds, which are normally recorded, the level of the high frequencies
is noticeably lower than that of the middle frequencies. To be exact, the peaks of the high
frequencies can have a large amplitude but their duration is very short, and a limiting will pass
unnoticed.
From the idea of emphasizing the high frequencies during recording and to attenuate them during
playback, the noise level of the tape, which is annoying above all in the high frequencies, is
effectively reduced. This is known as pre-emphasis. It is used universally in disc recording and
frequency modulation radio transmission as well as in magnetic recording. This universality is very
important, for if there is a pre-emphasis in any link of a chain, it is useless not to have it in the other
links, because, in any case, the high frequency peaks will be limited in the link which has the
strongest pre-emphasis. On the other hand, the gain in the signal-to-noise ratio is preserved in each
link. As a summary, the recourse to pre-emphasis is universal, as it has been found that the
possible limiting of high frequency peaks is less annoying than the high noise level without pre-
emphasis.
How much pre-emphasis can be accepted? The question is complex, for it depends upon the type
of sound to record. The sound spectrum varies with different languages, and it is for this reason that
the standards for pre-emphasis vary from one country to another. For practical reasons, it is not the
pre-emphasis, which has been standardized in the case of magnetic recording, but the playback
chain. The recorder should be adjusted so that a tape produced on it and played back on a
standard playback chain should have a linear response, this is the same as standardizing the pre-
emphasis for a given type of tape. There are tapes whose capacity for recording high frequencies is
noticeably higher than that of standard tapes. To record on these tapes, according to the standard,
it is necessary to have a lower pre-emphasis.
Relation between Pre-emphasis and Bias
The American NAB standard at 7½ ips requires a greater pre-emphasis than the European CCIR
standard. In Europe, it is normal to slightly over-bias the tape. This gives a slightly better signal-to-
noise ratio, but reduces the recording level of the high frequencies. The final result is practically
identical to that obtained with the NAB standard without over-bias. The stronger pre-emphasis of
the NAB standard gives approximately the same improvement in signal-to-noise ratio and the tapes
become practically saturated at the same high frequency signal level. The NAB standard relies
upon a heavier pre-emphasis and the CCIR on the higher bias level.
