Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 39

Practical Conclusions 

The result is: 

a) 

It is possible to modify the pre-emphasis, within certain limits, by adjusting the bias 

level but still remaining within the limits of the standards. 

b) 

To record sounds particularly rich in high frequencies, it is possible that the use of 

tapes, which permit high recording level of the high frequencies, could give better 

results. 

c) 

It is necessary to determine which link in the chain gives the greatest pre-emphasis. 

If all links pre-emphasize to the same degree, this will produce the most rational 

chain. However, if one link becomes saturated, it is better for this to be the magnetic 

tape, for the saturation of the high  frequencies does not lead to audible distortion, 

which is not the case with a frequency modulation transmitter (or rather, the 

corresponding receiver). 

DETERMINATION OF THE BIAS LEVEL 

Tape Characteristics 

The tapes on the American market are very similar to one another from the point of view of the 

optimum bias level. This permits the adjustments to be made very close to the point of maximum 

efficiency. It would be undesirable to adjust directly on to this point, as an over-biasing is much less 

dangerous than an under-biasing. This working point close to the point of maximum efficiency is 

very convenient for the NAB standards, as shall be shown. 

In Europe there is a greater range of tape characteristics. In over-biasing the ordinary tapes, the 

working point remains correct for the tapes, the working point remains correct for the tapes at a high 

bias level. This ties in with the CCIR standards and gives good results. 

General Procedure 

It is necessary to use a reference tape whose characteristics are well known, above all in relation to 

other tapes on the market. The normal reference tape is CCIR PER 368 and NAB 3M 808. 

It is necessary to determine the bias level, which gives the greatest efficiency. The signal used will 

be of fairly low frequency (400 Hz). To locate the peak of the curve more easily, two points, E1 and 

E2, should be looked for. Point E1 has an underbias level which gives lowering of the playback level 

by 1 dB; E2 is an over-bias level, which gives a lowering of the playback signal by 0.5 dB. The 

asymmetry of the form of the curve justifies the difference in the playback levels of E1 and E2. It is 

evidently necessary to use a sufficiently regular tape so that variations of the sensitivity should not 

be confused with the loss of level due to under and over-biasing. 

The maximum efficiency bias level Em will be the geometric mean of E1 and E2. Multiplying E1 by 

E2 and taking the square root of the product find this. The working point will be Em

?

k where k is the 

coefficient of over-biasing. 

Variation of k 

The preceding rules take into account the dispersion of the characteristics of available tapes. If a 

recorder is only used with one specific type of tape it is possible to use a value of k, which will be 

optimum for the conditions. Values of k from 1 to 1.3 are possible.  

A small value of k can be used if the sounds to be recorded are rich in high frequencies, or if the 

tape is of a low quality for high frequencies. A value of k above average can be used in the opposite 

case. The value 1.2 is acceptable for the CCIR standard, and 1 to 1.1 for the NAB standard (50 ìms 

at 7.5"/second).