Metering Standards
This is a very confused and confusing area, based around a series of independently developed standards used for quite different purposes and for different technologies. In this short article I propose to put the standards into an historical and functional perspective ie when and for what purpose a standard was set.
0dBm
This standard was developed in the early
days of telephony and adopted by pro audio, when transformers were used as a
matter of course for balancing signals. 0dBm is the voltage developed by 1mW
passing through 600 Ohms. This was the characteristic impedance of an open
spaced telephone line. It is .775V RMS.
0dBU
This is .775V RMS regardless of
impedance. This was developed as the use of 600 Ohms termination of signal
lines, as a standard, was being used less and less, and significantly higher
impedances became the norm.
0VU
The VU meter was a meter for telephone
lines developed by Bell to standardise signals on telephone circuits. Originally defined as
0VU = 1mW into 600 ohms with 1000Hz Sine wave
See
http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf
0VU is now defined as 4dB above 0dBU for
nearly all audio purposes as +4dBU has also gradually become an industry
standard line up level.
0dBV
This is 1V RMS,which is the voltage
developed by passing 1mW through 1000 Ohms. -10dBV is used as a reference for
consumer equipment levels.
0dBfs (digital)
This refers to a digital audio signal at
full scale ie maximum amplitude. On its own it means absolutely nothing in the
analogue domain. There are a number of ‘standards' for the conversion between
the full scale digital and the analogue signal level that comes from the
Digital to Analogue converter. In addition some broadcasters require the
maximum digital level of recordings to be a certain amount below full scale.
See http://www.soundonsound.com/sos/jun00/articles/metring.htm
Commonly used meters and comparison of scales
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