# The D page

d
Lower case d when used as a prefix will be the short form of deci.

dB
Pronounced decibel or just dB. Gives an logarithmic relation between two powers. The scale follows the relation
```                P2
db = 10 * log   --
10 P1

where
P1 is the reference or input power
P2 is the output power
```
0 dB means that the powers of P1 and P2 is equal. 10 dB means that the power P2 is 10 times higher than P1. 20 dB means that the power P2 is 100 times higher than P1 and in the same way will -10 dB means that P2 is a tenth of the power of P1. Decibels is really an dimensionless scale, but it allways means the effective power ratio measured along a logarithmic scale.

dBm
An dB scale where the reference power is fixed at 1 mW. This scale is very common for use with signal strength of various kinds, both electrical and optical. The scale follows the relation
```                   P
dBm = 10 * log   -----
10 0.001
where
P is the power of the signal
```

dBr
An dB scale which gives the relative power relation between a measured power strength and a suitable reference signal's power strength. It is suitable to indicate difference as opposed to an actual power strength. The scale follows the relation
```                 Pm
dBr = 10 * log   --
10 Pr

Pm = power measured at the output of the system
Pr = power of a suitable input (reference) signal to the system
```

dBspl
An dB scale for acoustical sound pressure being referenced to 20 uPa (micro Pascal). It is usefull for acoustical sound pressure measurements.

dBu
An dB scale where the reference power is replaced with an reference voltage (appoximatly 0.775 V) which will give an effect of 1 mW over an 600 ohm resistance. This unit is used a lot in teletransmission and audio since it is easier to measure the voltage alone then measuring the true power.

dBV
An dB scale similar to dBu but differs in the sense that it will reference to exactly 1 V instead of an derived voltage (as in the dBu case).

Decay
Decay is a term used for envelopes. It is the period when the envelope falls from its peak level (as developed during the attack) down to the sustain level. The decay period is started when the attack has reached its peak value.

Deci
An d in most shorter notations. It is part of the SIS standard but is not to be preferred, never the less it is widely used and will probably not go away in forseeable future.

Decibel
An Bel scale with the prefix deci attached to it. The decibel scale has become a popular scale within teletransmission, telecommunication, audio and everywhere else where and relative power scale is of interest. It will give the ratio of a power compared to a reference power or the ratio between two powers converted into a logarithmic scale.

Delay
Delay is a phenomene by which a signal occurs at some later time than the causuallity would allow. An signal sent into some system is said to be delayed by the amount of time it takes for the signal to pass from the input of the system to the output of the system. A relative delay can exist between two diffrent outputs of the same or two diffrent systems given the same input signal. A system that intentionally provide some (possibly variable) delay to the applied signal is often called a delay or a delay unit. Such a unit will delay the signal with ideally no distorsion of the signal waveform.

(C) 1997, 1998, 1999
Magnus Danielson <cfmd at bredband dot net>