AirPort Express AirTunes music streaming throughput limited to <160kbps?

Discussion in 'Digital Audio' started by ToTo Man, May 5, 2010.

  1. ToTo Man macrumors regular

    Joined:
    Oct 22, 2008
    #1
    I recently purchased an AirPort Express base station to take advantage of the AirTunes Wireless Music Streaming feature.

    Sound quality is essential, I have ripped my entire iTunes library as AIFF 1411kbps, and I was under the impression that AirTunes / AirPort Express could stream full lossless audio.

    I have it all setup and working, but I'm not convinced I'm getting full lossless audio. When streaming a track, I check the output speed on the iStat widget and it varies between 116kbps and 152kbps, which I assume indicates that the streaming quality I'm getting is equivalent to a 128kbps MP3/AAC track, far far away from the 1411kbps I was expecting. Is this normal or is there something amiss here?

    An A/B test between the Mac's 3.5mm LineOut and AirPort Express's LineOut corroborates the above, with the later sounding slightly more smeared / less focused than the former.
     
  2. Russell L macrumors regular

    Russell L

    Joined:
    Aug 10, 2006
    Location:
    San Francisco
  3. ChrisA macrumors G4

    Joined:
    Jan 5, 2006
    Location:
    Redondo Beach, California
    #3
    Why are you using AIFF? Apple lossless is the same audio quality and takes up half the space. Also Apple lossless allows tags ir ID fields to be placed in the file.

    I've not noticed what you describe. Possible AIFF can't be sent over the air but lossless can? Try encoding some of the music to Apple lossless and see what happens

    Finally a question. How did you measure the sound? Simply listening is subjective and not accurate unless you have several listeners and the test is blind. and in any case any difference may be do the the DAC and the internal line level amplifier

    One objective test might be to route the s/pdif data from the Apple Express back into the computer. Now you can compare the original digital ile to the one that was round tripped trough Airtunes
     
  4. jltnol macrumors newbie

    Joined:
    May 9, 2010
    #4
    First off, AIFF files can contain metadata just like .mp3 files can, and iTunes is capable of writing this metadata into the files.

    Second, I won't open the can of worms concerning compressed vs. non compressed, but after digitizing my CD collection at different MP3 bit rates, I decided to do the huge task one last time as un-compressed. Hard drive space is cheap, my time is not. Now, I don't have to worry about any future codec that may be better than what's out there now. If one comes along that I decide is ok, I still have the uncompressed files to use to create the new files in the new codec(but I doubt this will ever happen)

    I think the OP is right.... it would seem that iTunes is compressing the audio on the fly to send it to AirTunes.

    And truth be told, I'm going to guess that when you are using AirTunes, even the audio coming out of the computer running iTunes also goes thru the same compression/expansion stage for uncompressed files as well. From a technical standpoint, it would seem easier to get the timing right with compressed audio at all outputs, especially if you have several AirTune stations. As I add more stations, the network rate goes up, but seems to be about 128k/station.

    You can read this post as well.
     
  5. ChrisA macrumors G4

    Joined:
    Jan 5, 2006
    Location:
    Redondo Beach, California
    #5

    iTunes can store track data for AIFF but are you sure the data are stored INSIDE the files or is this data stored in a database? I read the spec for the AIFF file format and don't see fields defined for all the metadata that iTunes manages so I assume it it kept in a databse, outside of the file.

    There are two kinds of compression, lossless and lossy. Lossless compression is, just as the name says lossless. The content is bit per bit identical with what came off the CD.

    Yes disk SPACE is nearly free now but disk I/O rates and network bandwidth are not nearly free

    I know about having to re-rip many CDs. I did the same thing, ripped may CDs to MP3 that then later hade to go back to redo the process to Apple lossles
     
  6. ToTo Man thread starter macrumors regular

    Joined:
    Oct 22, 2008
    #6
  7. ToTo Man thread starter macrumors regular

    Joined:
    Oct 22, 2008
    #7
    Mainly to future-proof against codec developments, besides I have 2TB of HD space to waste. :D

    Having said that I plan to duplicate a large chunk of my library on my new MBP, so if it's true that AirTunes converts to ALC on the fly to the AirPort then I may convert the library on the MBP to ALC.
     
  8. ToTo Man thread starter macrumors regular

    Joined:
    Oct 22, 2008
    #8
    I think I've found the answer to my concern. A helpful member over on the Apple Support Discussion Forum noted that my network transfer rate calculation was out by a factor of 8. i.e. audio rate is measured in kbps (kilobits per second), but the readings on Activity Monitor / iStat are displayed in kB/s (kilobytes per second). 8-bits = 1-byte, so if I multiply my average kB/s reading of ~130 kB/s by 8, I get ~1000 kbps, which is approximately the rate of an ALC-encoded audio track.
     

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