THE MUSTS
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Nice looking pocket friendly design. (In my opinion RIOs mistake with the great Karma)
+ Good examples of this are the apple, sony, and iRiver players.
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Easy to use, and both file tree, and ID3 tag navi.
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Durability (RIOs other mistake)
+ A good example of a durable look is sony HD players.
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If targeter to people looking for good sound it should have the best sound ever in a DAP, and hopefully by far.
+ I should include as good a DAC as possible, and a really powerful amp. At least like Xin Supermicro
http://fixup.net/products/inlineamp/index.htm
+The headphone out and lineout should also be very good components.
+ For the video playback an option simulating surround sound should also be cool, which while not good for music is better for movies.
+ Customizable equalizer (8+ bands).
+ Great instrument separation, soundstage.
+ Many people would like an equalizer preset to make the music sound more upfront/dynamic. Default sound, however should be as neutral, clear, detailed, and full as possible (transparent sound).
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Gapless, in various codecs.
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Good battery life.
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It should have a good balance of all of the above. With no area lacking. An area lacking could be bad for the product.
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GOOD, though not completely nescessary.
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To attract also the consumer market, I believe some EQ presets would be nice, including one with a very dynamic sound, grado headphone like sound.
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Nice color screen, and GUI.
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FM, radio, and recording.
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Games, and other features.
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Bit perfect optical out, though not nescessary if the DAC and software is really good. Include an optical out only if it works like a real optical out and it is bit perfect.
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Hardly nessessary but nice.
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Crossfeed.
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If it had a touchscreen the equalising options could be nice. You could just draw the frequency line for equalization.
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Must Avoid.
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In my opinion, it is good to avoid including any feature that has no place in a DAP (cellphone features, and the like).