Glossary - alphabet order - "D"

DIP package

which are now also widely used. Epson's DIP products include the SG-51, SG-531, and real time clock modules with DIP-type leads. DIP packages are made both of metal and plastic.

Metallic DIP Plastic DIP
SG-51(left) and SG-531(right)

Deterministic Jitter

A type of jitter defined in specification. See "jitter".

Reference: Jitter

Dichroic filter

It is an optical filter that changes the amount of transmissivity of incident light in response to different wavelength of such light in central and peripheral areas. Since a single pickup supports two types of discs (CDs and DVDs), this optical filter is used to change a lens-focused beam's radius by changing the beam’s radius (aperture) in response to different wavelength when a light beam passed through the device.


Dichroic prism

It is a prism that changes the amount of transmissivity or reflectivity of incident light in response to different wavelength of such light. This is also used for a single pickup supporting two types of discs (CDs and DVDs.)


It is a phenomenon in which a light beam runs round the shade of an object when passing through the object. Light passes straight through when there are no obstacles, but generates this phenomenon when passing through an object because of it’s wave nature.

Diffraction efficiency

The ratio of exiting light intensity to incident light intensity. Also called the grating ratio. In a diffraction grating for pickups, a light beam splits into three beams: a zero-order beam in the middle and ±1-order beams above and below.

Disc AT crystal unit

This is an AT crystal unit that uses a disc-shaped crystal chip. In conventional standard metal packages (such as the HC-49U), disc AT crystal units used to be the main type of AT crystal unit, but now the bar AT crystal unit is the main type.

Reference: Bar AT crystal unit

Disc AT crystal unit

Divider Dividing circuit

This circuit is used to generate a low-frequency clock from a reference clock frequency. The division rate is usually 1/n (where n is an integer).

Drain resistor

This is an element in a crystal oscillation circuit. See "crystal oscillation circuit".

Reference: Crystal oscillation circuit

Drive level (Level of drive)

This refers to the power or oscillation output level required to operate (drive) a crystal unit. The drive level is expressed by the following formula. Drive level P = I2 × Re Where I is the current flowing to the crystal unit, Re is the crystal unit's effective resistance, and Re = R1 × (1+ C0/CL) (CL: load capacitance)

Crystal oscillation circuit Equivalent circuit of crystal unit

Duty (Symmetry)

The duty is the ratio of high-level period per clock cycle (tw/t in the drawing at right). In the case of a CMOS load, time is measured at the 1/2-VCC level, and for TTL load it is measured at the 1.4 V level.

CMOS load TTL load

D-type wave plate

It is a double-layered waveplate in which optical axes of each layer is orthogonal each other. This is also zero-order (N = 0) mode type, the first layer's phase difference is δ + N×360° and the second layer's phase difference is -N × 360°. This plate has a small variation in phase difference, since the incident angle change is provided only by the thickness of the zero-order thickness. This plate is useful for both divergent and convergent waves.

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