The AirPort Express is a simplified and compact version of Apple's AirPort Extreme Residential gateway. The current model allows up to 50 networked users, and includes a feature called AirTunes (predecessor to AirPlay), allowing users to stream audio from a computer running iTunes or mobile device running iOS 5+ to a stereo system. The original version (M9470LL/A, model A1084) was introduced by Apple on 7 June 2004, and includes an analog–optical audio mini-jack output, a USB port for remote printing or charging the iPod (iPod shuffle only), and one Ethernet port. The model introduced in June 2012 includes two ethernet ports; one WAN, and one LAN.
The main processor of the 802.11g AirPort Express is a Broadcom BCM4712KFB wireless networking chipset, which has a 200 MHzMIPS processor built in. The audio is handled by a Texas Instruments Burr-Brown PCM2705 16-bit digital-to-analog converter.
An updated version (MB321LL/A, model A1264) supporting the faster 802.11 Draft-N draft specification and operation in either of the 2.4 GHz and 5 GHz bands, with almost all other features identical, was introduced by Apple in March 2008. The revised unit includes an 802.11a/n (5 GHz) mode, which allows adding Draft-N to an existing 802.11b/g network without disrupting existing connections, while preserving the increased throughput that Draft-N can provide. Up to 10 wireless units can connect to this AirPort Express.
The Airport Express functions as a wireless access point when connected to an Ethernet network. It can be used as an Ethernet-to-wireless bridge under certain wireless configurations. It can be used to extend the range of a network, or as a printer and audio server.
An often-overlooked feature of the AirPort Express is that it uses an audio connector that combines a 3.5 mm minijack socket and a mini-TOSLINK optical digital transmitter, allowing connection to an external digital-to-analog converter (DAC) or amplifier with internal DAC. Standard audio CDs ripped in iTunes into Apple Lossless format streamed to the Airport Express will output a bit-for-bit identical bitstream when compared to the original CD (provided any sound enhancement settings in iTunes are disabled). DTS-encoded CDs ripped to Apple Lossless audio files - which decode as digital white noise in iTunes - will play back correctly when the AirPort Express is connected via TOSLINK to a DTS-compatible amplifier–decoder.
The audio output feature of the AirPort Express on a system running Mac OS X Lion or earlier can only be used to wirelessly stream audio files from within iTunes to an attached stereo system. It cannot be used to output the soundtrack of iTunes video content to an attached stereo. OS X Mountain Lion introduced a feature to output system-wide audio directly to Airport Express. This allows output of the audio of protected video content within iTunes, and also correctly maintains the audio sync with the image displayed on-screen. System video is otherwise not synced to system video when playing system audio through an AirPort Express.
For Windows and Mac operating systems (before OS X 10.8 Mountain Lion) there are a few software options available for streaming system-wide audio to the Airport Express, such as Rogue Amoeba's Airfoil.