This is a subject that can cause a lot of confusion it seems.
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 (approximately 1.26) or root-power ratio of 101/20 (approximately 1.12).
dBV
The dB unit expresses a relative change in amplitude, but may also be used to express an absolute value as the ratio of a variable value to a fixed reference value. When used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value.
For example, when using reference value of 1 V, a common suffix is “V” (e.g., “20 dBV”).
dB Scaling.
Two principal types of scaling of the decibel are in common use.
When expressing a power ratio, it is defined as ten times the logarithm with base 10. That is, a change in power by a factor of x 10 corresponds to a 10 dB change in signal level.
When expressing root-power quantities, a change in amplitude by a factor of x 10 corresponds to a 20 dB change in level. These decibel scales differ by a factor of two, so that the relative power and root-power levels change by the same value in linear systems, where power is proportional to the square of the signal amplitude.
The origin of the dB system of signal level measurement.
The definition of the decibel originated in the measurement of transmission loss and power in telephony of the early 20th century in the Bell System in the United States. The bel was named in honour of Alexander Graham Bell, but the bel is seldom used. Instead, the decibel is used for a wide variety of measurements in science and engineering, most notably for sound power in acoustics, in electronics, and in control theory.
In electronics, the gain of an amplifier, the attenuation of signals, and signal-to-noise ratios are frequently expressed in decibels.
How dB are used in SynthEdit.
In Synthedit we are using the dBV measurement, and the reference level of 0 dB is taken as 10 volts which is our maximum audio amplitude, after which audio signal clipping will occur. So if 10 V is our 0dB level, 5 V would be -6.0206 dB lower, and if we allow the signal to rise to 12 V we would get an increase of 1.583625 dB. But don’t forget these are the ratio between two signal levels. So we always need to define what voltage/wattage/sound pressure we are going to refer to.
There is a handy online online dB calculator which can be used to work out dB signal ratios.
Fortunately to save us doing a lot of messing around with math modules and formulae there are two third party conversion modules.
Important Note: These modules are not intended to convert audio signals, they will merely introduce distortion to the audio signal. They are purely for use with control voltages.
TD_dB2Volt:
To convert from dB back to the standard 0 to 10 volts scale, where 0 dB = 10 volts.
TD_Volts2dB
To convert the linear 0 to 10 volt scaling to a dB scale where 10 volts = 0 dB.
Stock Volts to Float dB conversion.
The stock Volts to Float2 module also gives us a Volts to dB conversion which can be selected from the Response plug.
An example of it’s use can be found in the stock Prefab VU meter which converts the 0 to 10 volts audio signal range to a dB scale and displays it.
