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Now looking at definitions starting with letter "i"

IAC : Interference Absorption Circuit developed by Phillips to absorb interference on FM (i.e. from car’s engine). Used under license by other makers.

IDE : Integrated Disk Electronics. A standard interface bus in PCs, most commonly used for hard disks. EIDE is Enhanced IDE, somewhat faster than the original IDE specification. This later evolved into ATA (Advanced Technology (AT) Attachment) and UltraATA. This evolution is fairly parallel to the SCSI bus technology used by Macs.

IEC characteristic : The European pre-emphasis and de-emphasis, equalization standard for magnetic tape recording.

IEEE The Institute of Electrical and Electronics Engineers, or “eye triple-ee” as its called, is the AES of all things electronic. According to its website, the IEEE is a standards-setting body responsible for making “engineering decisions consistent with the safety, health and welfare of the public, and to disclose promptly factors that might endanger the public or the environment.” The IEEE is very aggressive in its standardization efforts, covering a broad range of technologies impacting every aspect of daily life.

IEEE 1394 : Institute of Electrical and Electronics Engineers standard 1394(a.k.a. firewire or serial SCSI). High-speed standard for attaching external devices. It supports the daisy chaining of multiple devices, making it ideal for external audio disk drives.

IEEE 802.nx : The 802 working group of the IEEE sets the standards for data networking. There are many individual committees addressing specific applications.

IEM : In-Ear Monitor. Earphones used by musicians when recording to hear a special cue mix, overdubbing, or during a performance to better hear other musicians. Sometimes used instead of stage monitors to reduce problems of feedback or to provide each musician with a separate monitor mix. See earwig.

IFPI : International Fédération Phonographique Industrie. The European equivalent of the RIAA.

IIR : Infinite Impulse Response. See FIR."

IMA : The Interactive Multimedia Association.

image : (1) The apparent relative placement of individual sound sources, as imagined by a listener of recorded audio, created during the recording and mixing processes, as well as by the final format of the media, e.g., stereo, surround-sound. See imaging(1), Haas effect. (2) See imaging(2).

image shift : In multichannelstereo sound reproduction, a change in the apparent left-to-right position from which a particular soundvoice seems to emanate.

imaging : (1) The ability to localize the individual instruments, voices, or other sound sources when listening to a stereophonic recording is called imaging. Accurate imaging with two channels is almost impossible, requiring both channels to have identical gain and frequency response, the two loudspeakers to be within 1dB of each other in frequency response and the phase must be identical. In addition, the listener must be precisely between the two speakers. The lack of accurate imaging with traditional, two-channel stereo has lead to three-channel (LCR) and higher-channel audio recording and reproduction in an attempt to improve the listening experience. Contrast with stereo spread. (2) The resulting output of a D/A converter is a stair-step waveform which contains a great deal of high-frequency distortion. To reconstruct a smooth replica of the original signal, the stair-step is passed through a steep lowpass filter called an anti-imaging, or reconstruction filter. See quantization error.

impedance (Z) : The opposition (measured in ohms) of circuit elements to an alternating current. The impedance includes both resistance and reactance. In an AC circuit, resistance presented to the current is a function of frequency, called impedance and is also measured in ohms. Ohm’s law can be used for AC if the reactances are zero, that is, if there are no capacitors or inductors in the circuit. See Appendix B.

source output impedance: the actual impedance of the output circuit. The output impedance of a device determines just how much current it can provide to a load.

source load impedance: the impedance that the source circuit is designed to serve as a load. This is usually the figure given in the specifications, e.g., the impedance value that the equipment was designed to be connected to.

load input impedance: the impedance that the load is designed to be fed from. As long as this value is higher than the source output impedance, the rest doesn’t matter. For example, if a tape recorder is connected to a mixer, this value is the impedance that the recorder wants to serve at its input.

load termination impedance: the actual impedance of the input circuit.

impedance-matching : If the absolute maximum power is to be transferred from a source to a load, the impedances must match exactly. The source impedance output, the cable impedance, and the load impedance input have to be equal. This is called a matched circuit. However, when a circuit is matched, the voltage swing is reduced 6dB, lowering the best possible S/N ratio by that amount. Because of low-noise preamplifier circuits, there is a better audio trade-off of power transfer for a larger voltage swing. Therefore, the idea that circuits should be impedance-matched is fallacious, and instead modern audio equipment should simply be connected via balanced input and output lines. Systems that use high impedance mics are limited to cable lengths of around 10’ or so, but there is no reason to worry about the actual impedance value; it is either high or low. See bridging circuit.

impulse response : Impulse response is the manner in which a device (usually a transducer of some sort) behaves after the initial energy input (sound) has stopped. An impulse is a signal or sound that has a very short, in fact, a vanishingly small, duration. A true mathematical impulse has zero duration and infinite amplitude, but still a finite amount of energy. The energy in an impulse is spread evenly over a wide frequency band, and this means that it can be used as a test signal to measure the characteristics of an audio device. The impulse response convolved with an input signal gives the output response.

in the mud : Slang term for a signal level or level on tape that is too low, i.e., hardly moving the meters.

in the red : Slang term for a signal level or level on tape that is too high, i.e., above 0dBVU, and thus likely to produce distortion.

in-band gain : The standard for adjusting subwoofer response such that the subwoofer SPL, within the operating range of the loudspeaker, is louder than a full-range screen speaker in the same frequency range. All modern digital film sound formats use 10dB of in-band gain.

in-line : A configuration for input channels on a mixing desk, where the tape returns are connected via a parallel and additional line input to the channel. This obviates the need for a separate tape monitoring section on the desk, hence the tape returns are said to be in- line. The advantage is that, during remix, the input channels are all available as inputs, allowing all available tape tracks to be mixed from all available tracks of audio from synthesizers. Thus, in- line operation effectively doubles the number of inputs on a mixer. The opposite of a split console-type mixer.

in-position sampling : A technique for miking drum kits which uses two matched microphones in an X-Y arrangement, typically using large-diaphragm, condenser-type microphones. The main focus of in-position sampling is preserving the stereo spread: Most two-channstereo spread of the kit and producing a punchy drum sound.

inches-to-millimeters : Useful when trying to deal with European tape widths.

incident sound : The first (audio) source event to reach the sensor. Also called the incident wave, direct wave, primary wave, first arrival. See also reverberation.

incoherent : Two complex waveforms that are partially or completely out-of-phase most of the time. This happens when a sound source is recorded with microphones placed with different distances between the microphone and the sound source. Coincident pair and spaced pair microphone placement is designed to produce a coherent stereo sound field.

inductance (L) : The capability of a coil to store energy in a magnetic field surrounding it. It produces an impedance to an AC current. Inductance is measured in Henrys, although milliHenry is more common.

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