Audio tests showed that the third-generation iPod has a weak bass response.[39][40] The combination of the undersized DC-blocking capacitors and the typical low impedance of most consumer headphones form a high-pass filter, which attenuates the low-frequency bass output. Similar capacitors were used in the fourth-generation iPods.[41] The problem is reduced when using high-impedance headphones and is completely masked when driving high-impedance (line level) loads, such as when using an external headphone amplifier. The first-generation iPod Shuffle uses a dual-transistor output stage,[39] rather than a single capacitor-coupled output, and does not exhibit reduced bass response for any load.
For all iPods released in 2006 and earlier, some equalizer (EQ) sound settings can easily distort the bass sound, even on undemanding tracks.[42][43] This occurs when using EQ settings such as R&B, Rock, Acoustic, and Bass Booster, because the equalizer amplifies the digital audio level beyond the software's limit, causing distortion (clipping) on bass instruments.
From the fifth-generation iPod on, Apple introduced a user-configurable volume limit in response to concerns about hearing loss.[44] Users report that in the sixth-generation iPod, the maximum volume output level is limited to 100 dB in EU markets. Apple previously had to remove iPods from shelves in France for exceeding this legal limit.[45] However, users who bought new sixth-generation iPods in late 2013 reported a new option that allowed them to disable the EU volume limit.[46] Some have attributed this change to a software update that shipped with these devices.[47] Older sixth-generation iPods, however, are unable to update to this software version.[48]
Originally, a FireWire connection to the host computer was used to update songs or recharge the battery. The battery could also be charged with a power adapter that was included with the first four generations.
The third generation began including a 30-pin dock connector, allowing for FireWire or USB connectivity. This provided better compatibility with non-Apple machines, as most of them did not have FireWire ports at the time. Eventually, Apple began shipping iPods with USB cables instead of FireWire, although the latter was available separately. As of the first-generation iPod Nano and the fifth-generation iPod Classic, Apple discontinued using FireWire for data transfer (while still allowing for use of FireWire to charge the device) in an attempt to reduce cost and form factor. As of the second-generation iPod Touch and the fourth-generation iPod Nano, FireWire charging ability has been removed. The second-generation, third-generation, and fourth-generation iPod Shuffle uses a single 3.5 mm mini jack phone connector which acts as both a headphone jack or a USB data and charging port for the dock/cable.
The dock connector also allowed the iPod to connect to accessories, which often supplement the iPod's music, video, and photo playback. Apple sold a few accessories, such as the now-discontinued iPod Hi-Fi, but most are manufactured by third parties such as Belkin and Griffin. Some peripherals use their own interface, while others use the iPod's own screen. Because the dock connector is a proprietary interface, the implementation of the interface requires paying royalties to Apple.[49]
Apple introduced a new 8-pin dock connector, named Lightning, on September 12, 2012, with their announcement of the iPhone 5, the fifth-generation iPod Touch, and the seventh-generation iPod Nano, which all feature it. The new connector replaces the older 30-pin dock connector used by older iPods, iPhones, and iPads. Apple Lightning cables have pins on both sides of the plug so it can be inserted with either side facing up.[50]
Bluetooth connectivity was added to the last model of the iPod Nano, and Wi-Fi to the iPod Touch.
The "Made for iPod" logo found on most classic iPod accessories. Many accessories have been made for the iPod line. A large number have been made by third-party companies, although many, such as the iPod Hi-Fi and iPod Socks, have been made by Apple. Some accessories added extra features that other music players have, such as sound recorders, FM radio tuners, wired remote controls, and composite video cables for TV connections. Other accessories offered unique features like the Nike iPod pedometer and the iPod Camera Connector. Other notable accessories included external speakers, wireless remote controls, protective case, screen films, and wireless earphones.[51] Among the first accessory manufacturers were Griffin Technology, Belkin, JBL, Bose, Monster Cable, and SendStation.
BMW released the first iPod automobile interface,[52] allowing drivers of newer BMW vehicles to control an iPod using either the built-in steering wheel controls or the radio head-unit buttons. Apple announced in 2005 that similar systems would be available for other vehicle brands, including Audi, Ferrari, Honda, Mercedes-Benz, Nissan, Toyota, and Volkswagen. Scion offered standard iPod connectivity on all their cars.
Some independent stereo manufacturers including JVC, Pioneer, Kenwood, Alpine, Sony, and Harman Kardon also had iPod-specific integration solutions. Alternative connection methods included adapter kits (that use the cassette deck or the CD changer port), audio input jacks, and FM transmitters such as the iTrip—although personal FM transmitters are illegal in some countries. Many car manufacturers have added audio input jacks as standard.[53]
Beginning in mid-2007, four major airlines, United, Continental, Delta, and Emirates, reached agreements to install iPod seat connections. The free service allowed passengers to power and charge an iPod, and view video and music libraries on individual seat-back displays.[54] Originally, KLM and Air France were reported to be part of the deal with Apple, but they later released statements explaining that they were only contemplating the possibility of incorporating such systems into their aircraft.[55]