Be sure, if you can, to compare the same mastering, else you're just comparing apples and oranges.
I'm not sure what you mean? Are you suggesting the sample rate Spotify streams at isn't consistent?
It's more to do with the work that's done at the CD production stage. You could have the same title of a CD, but not necessarily mastered byt he same person and that'll influence the resulting sound you hear. Probably moreso than the bitrate, or streamer, or streaming service that you use. Here's a brief explanation from Wikipedia:-
And here's an interview with the guy who's work I probably enjoy the most, Barry Diament. He used to be the mastering engineer at Atlantic and you've maybe got some of his CDs at home. He's usually, or at least, very often credited in the booklet:-
...and here's a list (sorry for all the links, but you get the idea by now maybe!):-
Onkyo TX-NR818 / Tannoy Revolution DC4 (bi-amped)
AVI Laboratory Series CD Player
Following the useful lesson above, can anyone confirm the setting I should choose on my Mac for a bit perfect output of Spotify Premium (via USB to async DAC)?
Would it be 44.1Khz?
Spotify premium streams at 320kbps (I think), no matter what you set your Macs output at it is going to be converted. There isn't a 'bit perfect' in this case. As the record spot has said, the quality of mastering is what's important regards sound quality. More important even then the kit you're listening to it through.
Synology NAS + ATV2 > ADM9RS
Spotify is not exactly forthcoming in explanation of the technology it uses to stream music.
It is generally considered to use the Ogg Vorbis codec which is arguably the best sounding, particularly at higher bitrates, ie 320 kbit/sec, Spotify's Premium service.
Lower bitrates, still Ogg Vorbis, are used as standard on mobile platforms but Premium users can select higher quality though apparantly many do not do so.
It is more complicated for those who use a web browser rather than a Desktop App, since all browsers do not support Ogg Vorbis it is believed that MP3 codecs are used in that instance, though Spotify seem reluctant to confirm this.
For what it is worth, I use Spotify a lot and the differencies between different 'releases' (of the same album) is sometimes quite apparent, obviously so in some cases.
We do so many shows in a row,
And these towns all look the same,
We just pass the time in our hotel room
And wander 'round backstage,
Till the lights come up, and we hear that crowd,
And we remember why we came.
Decoding is not quite the same as converting. A 320 kbps ogg vorbis stream will always decode to exactly the same PCM stream. That stream can or cannot be communicated "bitperfectly" to the digital output.
Of course, the lossy coding to 320 kbps has degraded the SQ much more than resampling would.
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