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this question is just out of pure curiosity.
I noticed, that when I sync my phone with my Spotify playlists, music is transfered really fast, I haven`t benchmarked or something, but it just feels times faster than just downloading 20 or so mp3s.
How they do it ? Is it some kind of special audio format ? Or maybe they compress the data somehow when transfering ?
This isn't really a programming question, but I'll bite.
Spotify uses OGG audio for its mobile clients. Depending on the quality you choose for offline syncing, it's either 96, 160 or 320 kbit/sec, and not really much different in size to the equivalent MP3 bitrate. However, the Spotify client and backend do have a very optimised network stack, so it's possible you're seeing this, but I'd be surprised if the latency benefits the network stack provides is enough to have a visible effect considering the vast majority of the time is spent actually downloading the track rather than requesting it.
The funny thing is that Spotify uses OGG to stream music, but it can't play local OGG files.
http://community.spotify.com/t5/Spotify-Ideas/Local-file-support-OGG-Vorbis/idi-p/5451