360 Systems TCR Multi-Track - Manual (Page 140)

TCR Series User's Manual Appendix B. Maintenance 8-5
7. Select a track to calibrate:
•
(TCR4) Select item 1 to 4.
•
(TCR8) Select item 1 to 8.
8. Adjust the input gain trim level using the Jog Wheel for coarse adjustments and the ADJ+
and ADJ- buttons for fine adjustments. When the desired level is measured at the analog
output, you have two options:
•
Press ENTER to save the current input gain trim value.
•
Press EXIT to return the input gain trim value to the original setting.
9. Repeat steps 7 and 8 until all tracks have been calibrated.
Calibration Accuracy
The gain accuracy of a recording and playback system depends on more than the gain accuracy of
each unit connected to the TCR recorder. Although you can calibrate the TCR to within .01 dB as an
isolated device, it is necessary to take into account the load that each unit in the recording chain
places on other connected units.
Input Gain Accuracy
The input resistance of each leg of the TCR input amplifier is 21 k
Ω
. This value is typical of
bridging inputs, and is appropriate for low-noise circuit design. Note that when TCR is driven by a
source with a non-zero output impedance, a voltage divider is formed between the output resistance
of the source and the 21 k
Ω
input resistance of TCR. This relationship exists in almost all real-world
applications.
Assuming an output resistance of 51.1
Ω
for the source (instead of zero ohms), the resulting loss will
be -0.02 dB. Should the source resistance become 1,000
Ω
or more, as is often the case with
consumer equipment, larger gain errors will result. When good gain accuracy is required in an entire
system, it is advisable to perform calibrations with knowledge of the output impedance presented by
the signal source.
Output Gain Accuracy
The output resistance of each leg of the TCR output amplifier is 51.1
Ω
. As in the case described
above, this output resistance and the input resistance of the equipment being driven form a resistive
voltage divider. In most cases this will be a bridging input and the losses will be minimal. However,
when calibrating the TCR’s gain (and that of the entire system), these losses should be considered.
Note that in most studio systems, a good fraction of a dB of gain error will be created through
resistive losses between equipment.
If TCR’s 51.1
Ω
output is used to drive a 10k bridging input, the loss would be -.04 dB. However, if
it is used to drive a 600
Ω
load, the loss would rise to -.7 dB.
When calibrating TCR using test equipment, where it is not inserted into the studio system, remember
that impedance variations may occur. In the case of an Audio Precision
® test set, the generator output
impedance will usually be 50
Ω
, and the input amplifier impedance will usually be 100k
Ω
.