iPod Touch Audio Performance
Article By Alvin Foster And Stephen H. Owades
BAS Volume 31 No. 4 December 2009
Apple's iPod Touch is a portable media player, personal digital assistant, and Wi-Fi mobile platform launched in September 2007. It has a multi-touch screen and a graphical user interface. My 8GB iPod Touch model MB528LL v2.2.1 ($240) has a tiny internal piezo-electric speaker, whose sound is essentially unintelligible at a distance of more than 12 inches.
I have no plans to use the unit's audio output to listen to music although it boasts an easily accessible headphone plug. I use it almost exclusively as a remote control, selecting my music — no audio signal from the iPod enters my listening chain. However, because millions of people use iPods as a source of music and video entertainment, I was curious about its technical audio performance.
The iPod remotely controls my computer-based hard-drive (HDD) and CD/DVD recorder/player. I have loaded about 30 albums onto my computer's HDD in the Apple lossless format. The Touch can operate my computer's iTunes application from almost anywhere in my house via Wi-Fi. The "Remote" application, a free download from the iTunes store, controls album selection, volume control, rapid advance within a selected track, etc.
CD-based music is imported into the iTunes library by inserting an audio CD into the computer's drive and selecting "Import CD." [Between inserting the disc into the drive and selecting "import CD" I edit the track data shown on the import screen. It isn't required, but I find it makes sorting and finding my tracks easier when I then make playlists. DJW] If you have an Internet connection, iTunes will (usually) automatically retrieve the song's title, artist, and other information. The tracks can be loaded onto the iPod Touch, once you make a playlist in iTunes. The iPod's color screen displays a list of the tracks available in the computer's iTunes library; if the music has also been loaded onto the iPod Touch, the album cover art is also shown. The track listing, artist(s) name(s), and much more are available with a few easily memorized steps. The Touch also permits music selection by searching for a song title, artist, etc.
The Touch is a nice addition to my computer-based playback system. I now use iTunes in lieu of Microsoft's Media Center (MMC), because the latter was problematic and didn't include some features I wanted. The MMC remote, for example, cannot fast-forward within a CD track, and it employs infrared to communicate with the MMC audio software. Infrared generally requires line of sight for consistent connectivity. The Media Center displays album artwork only on the computer's monitor and not on its companion remote. The information on the computer's monitor, however, was consistently more usable and the print size larger than that displayed on the monitor with iTunes.
The Touch's icons are brightly displayed on the smallish screen and provide the primary access to the Touch's many applications and iTunes. For example, there is a volume up/down button associated with an icon used to control the iTunes audio level.
The photo illustrates the Touch's CD/DVD album display with Apple's Cover Flow (available only for media that has been loaded onto the Touch). You can select an album by touching the desired album cover and the play options for the album are displayed.
It takes time to learn where the buttons are located and how to touch only one at a time. Even after months of use it often takes me several tries to select the desired feature or application. I get the sense that the iPod was designed for people with much smaller fingers than mine. Although the current iPod Touch models are much the same as the originals except for memory capacity, the top two current models have faster processors and a few additional features.
Connectivity, Size, And Battery
I have found the Touch's Wi-Fi unpredictable and inferior for connecting to the Internet when compared with the built-in Wi-Fi connectivity of my Toshiba Qosmio laptop. On several occasions I have placed the Touch and the Toshiba side by side. The laptop easily links to my ISP and web site while the Touch remains unconnected to the hub. A trip to the Apple store determined my Touch was working fine; however, the unpredictable ISP connectivity continues. When out of town it sometimes fails to connect to the hotel's router even though others using laptops are surfing the Web.
The iPod Touch, 4.3 x2.4 x0.33 (HxWxD in inches), weighs 4.05oz. The high-quality 3.5" (diagonal) widescreen, multi-touch display has a 480x320 pixel resolution at 163 pixels/inch.
The unit has a built-in rechargeable lithium ion battery. Apple claims up to 30 hours of music playback from a single full charge. I could get no more than two hours of continuous use [It is possible that using the RF output drains the battery much more rapidly than driving earphones from the Touch's internally stored music. DJW]. The battery is not user replaceable, and Apple charges $79 to swap your Touch for a new (or refurbished) one. If your model is not in stock, a comparable Touch will be sent.
The iPod Touch has a 3.5mm stereo headphone jack and a unique Apple dock connector. An adapter cable plugged into the connector terminates in a USB plug for the computer. The computer's USB port provides power and updates to the Touch, plus the ability to synchronize the music, photos, videos and data between the iPod and the computer. Memory and costs.
The iPod Touch memory size is available now in 8, 32 and 64GB (64GB Touch: $400srp). For my use 8GB was sufficient. It holds about 23 of the 30 albums in my iTunes. It will not display all the albums if the playlist CDs fill the internal memory. To get around the memory limitation, I use a custom playlist containing only the songs I want to hear. With Remote I can access all the content in the computer's iTunes library, albeit without the fancy CoverFlow display on the Touch. Furthermore, I could not justify the huge price difference for additional memory, which incrementally costs Apple so little.
I connected the Touch's headphone jack to my E-MU 1616M PCI outboard converter, and from it to my PC to use my SpectraPlus audio analysis software (a PHS product; $300-$1300srp depending on options) that provided a complete spectral analysis of the iPod's audio signal. A 30kHz lowpass filter was used for the tests. All tests were run on a fully charged Touch battery, and the Touch's volume control was set to maximum. To my surprise, the iPod Touch measurements were very good. Maximum output and distortion
The maximum output and distortion with no load at 1kHz was 1.04dBV with 0.009%THD. The numbers did not change for 300Ω to 600Ω loads. The unit is easily able to cope with most headphone impedances because the output impedance is around 0.9Ω. The S/N ratio was 79.5dB. At a more typical 0.5dBV output level the distortion was 0.014% and the S/N was 76dB. The 2nd- through 5th-harmonic distortion components never exceeded 75dB. The IMD was 0.008% using the IM standard 60/7000Hz tones in a 4:1 amplitude ratio.
[When Apple began developing the iPod line, Steve Jobs mandated that the headphone output would have enough output power to drive earphones loud and clean enough for his taste. I have found that my 30GB iPod Video (5th-generation) has more than enough power to adequately drive my Etymotic ER-4Ps, even when I add the iPod's bass boost feature. DJW.]
Frequency response at 0.75dBV output, both
channels into 300Ω
The tiny buttons on the iPod Touch take time to get used to. I'm finding it almost impossible to select a button without touching an adjacent key. The short-lived internal battery is only good for about two hours of manipulating iTunes, because keeping Wi-Fi active uses a lot of power. The iPod must be connected to the computer's USB port or an external AC adapter for recharging. The unit will operate with the connection cable plugged in.
The home screen has a list of icons for the available applications that are too numerous to list and for me go generally unused. Lest anyone forget, it plays music, too.
The Touch includes Apple's Cover Flow, a handy way of browsing albums by cover art.
My iPod Touch serves as my iTunes remote control for my media center until something with a more reliable RF connection and with larger print comes along.
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