User manual

B-2 Appendix B: Audio Considerations
Moseley SL9003Q 602-12016 Revision G
B.1.4 Old Habits Die Hard
Unfortunately, the “dBm” terminology has hung on long after its use is generally appropriate.
Even some of the most competent manufactures of high-technology digital and analog
professional audio equipment still use the dBm unit in their setup instructions. Users are told to
apply an input signal of “+4 dBm” and then to adjust trim pots for an exact 0 VU indication on a
24-track digital audio tape recorder, for example. Yet, the line input impedances of that tape
recorder are 10 kilohms. What the manufacturer clearly wants is a +4 dBu input level (1.22
Volts). If we truly applied +4 dBm to that 10,000 Ohm input, the resulting 5.0 Volts would
probably not even be within the trim pot adjustment range for 0 VU. So, a good general rule
when working with modern audio equipment unless you know it to be terminated in 600 Ohms is
to read the manufacturer’s “dBm” as “dBu”.
(Reprinted from the ATS-1 User’s Manual, published in July 1994, with permission from Audio
Precision, Inc., located in Beaverton, Oregon)