^^ FLAC is a type of lossless audio codec used by hardcore audiophiles. Unlike MP3 which compresses music only to the audiable spectrum (and even that is mostly compressed) FLAC (which includes filestypes such as .flac and .aac) uses more basic RAR compression on actual .wav files, therefore it does not lose any actual audio signal.
Mike, i dont know of any headunits that will decode those formats. If you're getting that serious about sound, run a carPC directly to an amp. You can't beat that kind of control/quality over audio quality.
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Hot: 2001 Estoril M Roadster
ZCP 19's michelin supersports, ZHP knob, JL 8W3
Cold: 2002 TiAg M3 6mt deoranged, dechromed, led tails, ZHP knob, UUC SS v3, GROM, OEM 18's w310's,
Fun: 2006 YZF-R6, black, ohlins, pazzo, cat delete, vance hines csone, YEC racing tune