A
AAC Compressed audio format, to an extent popularised by Apple. Sounds better than MP3 for a given bitrate, though fewer products are able to handle it.
ACTIVE Speaker design in which the crossover, used to split frequencies between the drive-units, is located upstream of the amplification. Each drive-unit, or group of drivers (eg for bass or treble), then has its own dedicated amplifier. Not all 'active' speakers are fully active: some are merely conventional speakers with amplification built-in.
AIFF (Audio Interchange File Format) Applespeak for CD-quality files. Uncompressed 16-bit/44.1kHz music is stored on Apple computers as .aiff files
AIRPLAY Apple's solution for point-to-point audio and video streaming between its portable devices, computers running its iTunes software, and third-party components enabled for the service. Using it, music stored on an iPod can, for example, be streamed over a home network to a connected audio system or, via the Apple TV device, video and audio can be streamed directly to a television.
ANALOGUE The original way of storing audio data, as used by LPs and cassettes, and by VHS cassettes to store video and audio.
APP Short for 'application': small piece of software available for download to add functionality to a device, usually at relatively low cost. For example, various consumer electronics manufacturers now offer control apps, able to be downloaded from Apple's itunes Store to iPods, iPhones and iPads, to control items of equipment such as TVs, systems and Blu-ray players.
APPLE LOSSLESS (aka ALAC) audio compression format from Apple: has the advantage of (theoretically) identical sound to uncompressed audio, while reducing file size; has the disadvantage of being a proprietary Apple format, and thus not playable on some network devices.

B   
BASS Low frequency sounds, can be hard for smaller speakers to reproduce.
BASS REFLEX  Speaker design using airflow from a port in the cabinet to help deliver extended bass reproduction.
BIAMPLIFICATION (or biamping) Each drive unit of a speaker is driven by a separate amp channel, so a pair of two-way speakers needs two stereo amps and two runs of cable to each speaker.
BIWIRING Is thought by some to give some of the benefits of biamping, but at lower cost. You need suitable speakers (with two sets of inputs and a split crossover), then send twin runs of cable from the amp (again if suitable) to each loudspeaker.
BITRATE The measure of the quality of a digital file, in units of kilobits per second. However, simply having a higher bitrate doesn't guarantee better sound: the encoding format also plays a major part.
BLU-RAY Digital disc format aiming to provide a higher-quality successor to DVD. Offers a high-definition image and less-compressed sound

C   
CARTRIDGE The device that plays a record. It converts the wobbles in the record’s groove into electrical signals for an amplifier
COAXIAL (aka electrical digital) A digital audio connection, using 75ohm interconnect cable. Can be terminated with either conventional phono plugs or locking BNC connectors.
COMPRESSION (1) The process of squeezing data files to make them smaller, sometimes – as with MP3 – by discarding some of the musical information to make the files more practical. More correctly described as data-reduction.
COMPRESSION (2) Used by radio stations to reduce level differences between loud and soft parts of music.
CROSSOVER The circuit inside a speaker that splits the incoming signal, sending high frequencies to the tweeter, and low ones to the low-frequency woofer.

D   
DAB Digital Audio Broadcasting: digital radio.
DAC Digital-to-analogue converters (DACs ) turn strings of bits into audio.
DECIBEL (dB) Measures sound pressure. A change of 1dB is just about audible, +10dB is a doubling of level.
DECODING Blu-ray and DVD soundtracks usually include multichannel audio (see Dolby and DTS). You need a processor or AV amp to decode the data and pass it on to your speakers.
DIGITAL Digital data is stored in bits – as on/off signals (or pulses), able to be transferred with little degradation in quality.
DLNA The Digital Living Network Alliance, an organisation set-up by leading consumer electronics manufacturers to ensure interoperability between components, for example for the streaming of audio and video between computers and components such as TVs, Blu-ray players and systems (see also UPnP).
DOLBY A company whose technology is used to encode the multichannel soundtracks for films, which get transferred to Blu-ray and DVD. In the past, also known for its work in analogue noise-reduction.
DTS Like Dolby, DTS is a system for encoding audio on to movie soundtracks for playback on compatible AV receivers.
DYNAMICS The range, in decibels, between the largest and smallest audio signals reproduced by hi-fi, or stored on a recording.

E
ETHERNET Shortand for the IEEE802.3 standard for wired Local Area Network (LAN) connections between devices, using simple twisted-pair cabling – often referred to as Cat5 or Cat6 – terminated in standard RJ45 plugs. Although replaced by Wi-Fi wireless networking in many homes, Ethernet can give much more reliable connectivity for the streaming of music, especially in locations prone to radio interference, either from other domestic devices or neighbours' Wi-Fi networks!

F
FIREWIRE Ultra-fast data connection used by some MP3 players, computers, and high-end home cinema equipment.
FLAC Free Lossless Audio Codec. An open-source (ie free) audio coding and decoding system, using lossless compression to reduce file sizes, theoretically without loss of quality (though some feel they can hear a difference between music stored in FLAC and uncompressed CD-quality WAV files).
FLOORSTANDER A taller type of speaker that stands on the floor (sometimes on built-in spikes).
FRAME RATE Blu-ray pictures are produced at 24 frames/ second. Your TV must handle this to keep motion natural.
FREEVIEW 40-plus TV channels available for free using a normal aerial with a set-top receiver. One of the main platforms for the digital TV switchover in the UK.
FREESAT The satellite equivalent of Freeview, offering free HD channels via satellite.
FULL HD Whereas HD uses 720 horizontal lines of picture, Full HD uses 1080. It has the potential for even more detailed TV images.

H   
HARD DISK A device for storing large quantities of data. Hard disks first appeared in computers, and are now found in MP3 players (for audio and photos), set-top boxes, DVD recorders (for video and audio), and network music servers.
HDMI (High-definition multimedia interface) High-quality digital video/audio connection, used between Blu-ray and DVD players and AV amplifiers and displays. Able to carry multichannel high-resolution audio as well as digital video.
HD READY Label on any TV that shows the set will handle 720p and 1080i high-definition pictures. These days most TV are 'Full HD', with only some budget sets merely HD Ready.
HERTZ (Hz) Unit of frequency. One Hz means a signal has one cycle per second.
HIGH-RESOLUTION AUDIO Usually understood to mean any audio format beyond the 16-bit/44.1.kHz CD standard. Common high-resolution formats, which should offer better sound quality, include 24-bit/88.2kHz and -/96kHz, while some recordings are also available as 24-bit/192kHz. However, these require special players to handle them, including some streaming client devices, and even a few Blu-ray players. Matters can be confused by the BBC's suggestion that its Radio 3 website is offering 'HD Radio': this service, at 320kbps, isn't anywhere near CD quality, but is merely 'higher definition' than its existing web-streamed services (and indeed DAB).

I  
IMPEDANCE A low impedance draws a high current from the source, while a high value impedance draws a little. Loudspeakers or headphones with low impedance are likely to be more difficult to drive. Bear in mind, however, that the impedance figure given by speaker manufacturers is usually a nominal one, and that impedance can vary with frequency, to a greater or lesser degree, depending on the speaker design.
IP ADDRESS A unique identifier for a device on a home network, enabling other devices to 'find' it. On many home networks IP addresses are assigned dynamically according to the order in which components are connected, using something called Dynamic Host Configuration Protocol, or DHCP: this can cause problems if an item is turned off and then on again, or disconnected and reconnected. For example, in a streaming music system, the client player may find that the network music storage is no longer where it 'expects it to be' – ie its IP address has been reassigned by DHCP – and so music can't be played. The solution is to reserve, or assign, a static IP address to crucial components on such a network, so they can always 'find' each other.

K   
KBPS (or Kb/s) Kilobits per second. This is either a transfer rate – how fast data can be piped down a cable – or, more commonly, a compression rate: how much data has been retained when a music track (eg an MP3) has been compressed. A higher figure means larger files, but usually better sound.

L   
LCD Along with plasma, the main flatscreen TV technology. LCD is the dominant type for sets with screens up to 40in, though larger sets are becoming available. Picture quality is improving, too. Similar technology is used for many projectors.
LINE LEVEL Inputs that don’t need additional amplification/equalisation before a preamplifier can handle them. For example, the analogue output from CD players, radio tuners, etc.

M   
MEDIA CENTER Microsoft software: when installed on a PC, this allows you to run it as a home-entertainment system – with TV tuning, hard-disk recording, DVD (and sometimes Blu-ray) playback, and more.
MEMORY CARD A small card that fits in devices, from portable players to TVs, DVD recorders and video camera. It’s possible to record audio, video or images and transfer data to or play it back on another device.
MIDRANGE The frequency range where most instruments and voices are heard. Must be reproduced to a high standard if singers, strings, woodwind, etc., are to sound natural.
MP3 Audio compression for music files. Can reduce file sizes up to a factor of 10 compared with CDs, but can also reduce quality.

N
NAS A Network Attached Storage device: a hard-disk storage unit connected to the router for a home network, allowing its content (music, video, pictures, other files) to be accessed by any computer or other device on that network
NETWORK SERVER Stores tracks from a large number of CDs (sometimes as MP3 files), so they can be piped around your home.
NETWORK PLAYER Connects to a router or a network server via Wi-Fi or Ethernet, and allows you to access music or video files stored elsewhere.
NTSC The North American system of analogue TV broadcasting.

O   
OHM The unit of electrical resistance to current. The lower the figure, the harder a speaker is to drive; so the higher, the easier the speaker is to drive.
OPTICAL A digital audio connector, using light to transmit data down a transparent fibre, sometimes made of glass, but more usually plastic. (See Coaxial)

P   
PAL The European system of TV broadcasting. (See NTSC)
PCM Pulse Code Modulation. Digitally stored, uncompressed audio, as used by CDs. Also available as a soundtrack option on some Blu-ray discs.
PHONO STAGE A turntable’s cartridge outputs signals at a much lower level than a CD player, so it needs extra amplification from a phono stage. Many amps have them built-in.
PLASMA A type of flatscreen TV – these sets are available up to 152in. Sizes go down to 37in.
POWER AMPLIFIER Increases the level of audio signals from a preamplifier to a sufficient level to drive loudspeakers.
POWER-HANDLING The highest safe power for speakers. Note: it’s easier to damage speakers with an amp of too-low power driven hard, than with too much.
PREAMPLIFIER The control part of an amp. Built into integrated amps, but can be separate and used with a power amp or active speakers.
PROGRESSIVE SCAN Creates DVD pictures in one hit, writing all the lines of picture information at once, rather than writing alternate lines and relying on persistence of vision to build up the whole picture, as in interlaced TV scanning. Gives clearer, flicker-free images. Blu-ray is capable of delivering 1080p (1080 lines, progressive scan), while HD TV via satellite or terrestrial platforms is 1080i (interlaced) at best.

R   
RDS Radio Data System. Data piggybacked on FM, allows RDS tuners to display station names and perform a range of functions.

S   
SCART Still a commonly used AV connection, carrying analogue audio and video on a 21-pin connector. Now being supplanted by HDMI, which uses digital connection on most new components.
SD CARD Memory card able to hold audio, video or images
STANDMOUNT A compact speaker, sometimes also referred to as a 'bookshelf' design. Sits on a dedicated support for the best sound.

T
THX A set of standards for home cinema that should ensure a certain quality of sound. Equipment and software (ie DVDs/Blu-rays) are submitted to THX for testing to ensure they adhere to the standards the company sets, but that's not to say products without the THX certification are any less capable.
TRANSIENTS Short and sudden events in music such as a cymbal crash or the crack of a stick on a snare drum. Difficult for hi-fi to reproduce.
TREBLE High frequencies.
TWEETER Small speaker driver that handles the treble.

U   
UPnP Universal Plug and Play is a set of networking standards designed to ensure interoperatbility between computer devices and suitable audio/video components. As an example, UPnP server software loaded onto a home computer or network storage device will allow it to be accessed by streaming music clients on the network.
USB A type of data connection originally used on computers, but now  commonly used with MP3 players and other hi-fi and home cinema products.
USB 2.0 Faster connection replacing the slower 1.1, and in turn being replaced (eventually) by USB 3.0.

V   
VALVE (aka 'tube' in the USA) Amplifying device using electrodes in a vacuum tube. Felt by some enthusiasts to give a warm, seductive sound.

W
WATT More Watts can equal more power, but the levels achievable depend on speaker sensitivity and room size
WIDESCREEN The 16:9 (1.78:1) screen shape used on most modern TVs and DVDs. Many cinema films are made in an even-wider 21:9 (2.35:1) shape, so your picture will still have black bars on a widescreen TV
WI-FI Shorthand for the IEEE802.11 standard, wirelessly connects data and AV devices, especially computers. Available in various versions – IEEE802.11b (or 'Wireless b'), -g and -n – in ascending order of speed and thus data throughput. The standard requires that faster versions should be back-compatible with the slower ones, but bear in mind that using an 802.11b device on an 802.11n network, for example, will slow everything down to the lower speed.
WMA Windows Media Audio is Microsoft's rival to MP3. Claimed to offer similar sound with more effective audio compression.

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