Half-Duplex
The inability to send and receive data simultaneously which, in digital audio terms, translates to the inability to record and play audio at the same time. Many older sound cards are half-duplex. Most modern sound cards are capable of recording and playing audio simultaneously. This capability is called "full-duplex".

hard disk recording
A computer-based form of tapeless recording in which incoming audio is converted into digital data and stored on a hard disk.

Harmonic
A frequency that is a whole-number multiple of the fundamental frequency. For example, if the fundamental frequency of a sound is 440 Hz, the first two harmonics are 880 Hz and 1,320 Hz (1.32 kHz). > overtone; > inharmonic.

Headroom
The amount of additional signal above the nominal input level that can be sent into a module before clipping distortion occurs.

Hertz
A unit of frequency equal to one cycle per second. Named for Heinrich R. Hertz who did the original scientific investigations.

high-pass filter
A filter that attenuates the frequencies below its cutoff frequency.

Hz
Abbreviation for Hertz