Each tape cassette deck has differences in frequency response
characteristics. Slight variations can often be found between identical models
due to manufacturing tolerances. These characteristics must be compensated for
during preparation of the synthesized random tape, to assure that a tape deck
with a 5 dB roll-off at 20 Hz will not result in an under test.
The procedure for testing a tape cassette deck to determine its frequency
characteristics is described in detail in Chapter 2, Tape Deck Requirements.
The average deviation in playback from the signal recorded in each band is
computed as shown in para 4-3.1.3. A compensation factor is then determined
for each band to offset this deviation. As an example, if the playback is down
-3 dB in the 20 to 31 Hz band, a +3 dB compensation factor would be applied to
that band. A sample compensation factor is tabulated in Figure 4-3, Column
The following paragraphs describe the procedure required to obtain a
graphical plot or tabulation of the tape deck's response characteristics
previously recorded on a tape cassette (as described in Chapter 2). Although
this procedure has been tailored to specific items utilized, similar type
equipment may be substituted and should provide equivalent results.
4-3.1.1 Equipment Description. The following equipment
described shall be used to recover the test data previously recorded (see Chapter 2):
a. Spectrum Analyzer: used to monitor the recorded output voltage as a
function of frequency (Nicolet Scientific Corporation, Model 440 or
equivalent). This analyzer provided data at a bandwidth of 5 Hz. Alternate
equipment with a bandwidth no greater than 25 Hz is acceptable.
b. X-Y Recorder: used, in conjunction with the analyzer to provide the
final plot of voltage versus frequency (Hewlett-Packard, Model 7047A or
a. Review the applicable portions of the Tape
Deck Data Sheet, Chapter 2, Figure 2-2 ,
to verify that the reference tape identification/serial is the correct one
for this application.
b. Connect the tape deck's left channel output to the analyzer
Verification of the right channel
output should be accomplished using the same procedure described herein.
Significant difference between channels should be noted in the remarks
section of Chapter 2, Figure 2-2.
c. Set the analyzer input
frequency range and voltage level to encompass the 2,000 Hz frequency range
and the voltage level listed in Chapter 2, Figure 2-2. (furnished
with the pre-recorded tape). Record this information, plus the filter
bandwidths in the Playback section of Figure 2-2.
d. Set the analyzer output mode to provide a frequency
scale and logarithmic amplitude (dB) scale.
e. Set the X-Y recorder to the correct sensitivities
to record the anticipated levels.
The Y axis (amplitude) sensitivity should be capable
of detecting a one dB variation.
f. Using the pre-recorded reference voltage (@ 1000
Hz), set the playback level control so that the output voltage level is at
the -3 dB value of the analyzer's full scale input range.
Once the playback level has been established at 1000
Hz, do not readjust the level control again for the balance of the
g. Playback the first (0 dB) curve
into the analyzer and record the actual dB level (@ 1000 Hz) on the Tape
Deck Data Sheet, (Chapter 2, Figure 2-2).
h. Using the output level (@ 1000 Hz), set the X-Y
plotter pen to a convenient level at the top of the graph paper during
playback of the curve.
i. Plot the 0 dB curve. which is the first recorded on
j. Repeat steps g through i for the - 10, - 20, - 30,
and - 40 dB curves. The output at 1000 Hz, for each curve, may be used as
the reference point for that curve and may also be used to locate the
plotter pen on the graph paper. Place each curve at a convenient point below
the proceeding curve (reference Figure 4-4).
4-3.1.3 Data Analysis. The output
voltage plots generated during tape playback (para. 4-3.1.2) are to be used to
determine the compensation (in dB) necessary to assure correct tape deck
a. Sub-divided the curve into frequency bands
corresponding to those for the random analyzer/equalizer system that is used
to shape the synthetic random spectrum. Chapter 4, para
b. Determine the dB level for each frequency band.
c. Determine the difference in dB level between each
curve data point (A) (see Figure 4-4) and its corresponding reference point
(B) (see Figure 4-4) at 1000 Hz, for each of the five curves.
d. Average all five difference values to determine the
overall average for each frequency band.
The overall average established in d, represents the
compensation require to account for any tape deck response