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Now, the biggest issue with miniature MP3 players tends to be usability. Combine small controls with a tiny screen (or no screen) and you have a recipe for fiddliness - a problem that affects both the T7 and Creative's Zen Stone series, though not Sansa's Clip. Luckily, the U4 gets things 90% right. The touch controls are a little sensitive for my tastes, but I found them more predictable than those on the U4's bigger brother, the Q1 Diamond.
Furthermore, to give you a level of feedback, you get short tones through the headphones as you flick through menus plus a bank of three LEDs submerged under the front of the case that light up when you touch a key. These LEDs also pulse sympathetically during music playback, and while this isn't the sort of thing that floats my boat, it is the kind of detail that might convince someone special that this is a more expensive, fully-featured player than it actually is.
The GUI is primarily text-based, but it's clear and well scaled to fit the 4-line screen, and you even get some nice touches like album art thumbnails or visualisations during music playback. Surprisingly, you don't control the volume via the rocker switch on top but by using the up and down on the D-pad while you're listening. Instead the rocker works as a play/pause button, a record button when using the voice recorder function, and a user-configurable function button that you can use to switch play modes or change the applied EQ and effects preset while you're listening.
An icon-based main menu gives you access to the player's functions, which include the voice recorder and FM radio features we've come to expect, plus a dedicated player for MP3 podcasts (or datacasts in Samsung terminology). The voice recorder does a decent job through the built-in microphone and radio quality is better than I'd usually expect from a player of this size and price point. The only mild annoyance is that you have to go into the setup menu to auto-tune the radio; I prefer it when you can do this from within the radio function itself.
In terms of format support, the U4 keeps up with the joneses by throwing in OGG VORBIS on top of the usual MP3 and WMA. It doesn't have FLAC like the Q1 Diamond but on a player with such a limited capacity, that is only to be expected. Battery life is also competitive, with approximately 16 hours of playback to the Sansa Clip's 15, though less than the Zen Stone Plus's 20.