Airtunes streaming data rate... wtf?

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zozo
Posted:
in General Discussion edited January 2014
I can't really figure this one out.



I have Menu Meters installed so I can check out speed of upload and download.



Now that I got an Airport Express for net and music, I've noticed that when streaming music, its at constantly between 100-125KB/s. Thats 800kbs to about 1mbit.



Considering that most of my library is at 192kbs AAC... well... I dont get it. Shouldnt the stream be at about the same rate of the encoded file?



When it buffers a new song it hits about 200-250KB/s for a few seconds... and then just constantly stays at about 1mbit upload.



Is this normal? Whats the rationale behind it? Is it massive error correction or something?

Comments

  • Reply 1 of 9
    e1618978e1618978 Posts: 6,075member
    I have no real knowledge on this issue, but I can make some educated guesses.



    They are probably decoding the data into PCM and sending it after decoding it. The advantage of this is that you don't have to do a firmware upgrade on your Airport Express every time you add a new encoding format to iTunes. Also, the Airport Express may not have the CPU horsepower needed to decode the stuff.



    16-bit PCM at 44,100 samples per second per channel is



    2 bytes/sample *

    2 channels *

    44,100 samples/second *

    8 bits/byte = 1.411 mBits/second
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  • Reply 2 of 9
    nguyenhm16nguyenhm16 Posts: 203member
    Somebody did an analysis of the packet stream going to an Airport Express when it first came out, and apparently it is encrypted Apple Lossless. In other words, iTunes converts everything to Apple Lossless, encrypts it and sends it to the Aiport Express. Encrypted presumably to protect iTMS downloads on the way there. That's probably also why it's not a full 1.4mbps of raw uncompressed PCM.
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  • Reply 3 of 9
    hmurchisonhmurchison Posts: 12,464member
    Yup it is in fact Apple Lossless sent to the APE. I look forward to the day where AV Receivers can support AAC directly so that we can pipe right to the receiver without the need for conversion.



    That should happen in a few years. I'm already seeing AAC support in many new Sigma and Cirrus chipsets.
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  • Reply 3 of 9
    salmonstksalmonstk Posts: 568member
    Quote:

    Originally posted by nguyenhm16

    Somebody did an analysis of the packet stream going to an Airport Express when it first came out, and apparently it is encrypted Apple Lossless. In other words, iTunes converts everything to Apple Lossless, encrypts it and sends it to the Aiport Express. Encrypted presumably to protect iTMS downloads on the way there. That's probably also why it's not a full 1.4mbps of raw uncompressed PCM.



    Well I will tell you one thing. An old Lime 400mhz G3 can't keep up all that encryption and ecryption.



    I got an Airport Express for Christmas from my friend. But can't use it for Airtunes because it would skip all the time....



    Guess I need a new iTunes computer too!
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  • Reply 5 of 9
    mr. hmr. h Posts: 4,870member
    Quote:

    Originally posted by nguyenhm16

    Somebody did an analysis of the packet stream going to an Airport Express when it first came out, and apparently it is encrypted Apple Lossless.



    well, whoever did that wasted their time didn't they? Apple make no secret of this fact, check out the bottom of page 9 of this pdf (which is linked to from the Airport Express tech specs page)



    Quote:

    Originally posted by nguyenhm16

    Encrypted presumably to protect iTMS downloads on the way there.



    Indeed, it protects all your music from being "stolen" whilst you are streaming it.



    Personally, I do not like the AirPort Express/AirTunes approach to streaming music for two reasons:



    1. The high bandwidth useage and high CPU load for the serving machine.



    2. No decent remote control capability (there are third party solutions, but they don't provide a display)



    I think that these are a much better solution.
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  • Reply 6 of 9
    e1618978e1618978 Posts: 6,075member
    Quote:

    Originally posted by salmonstk

    Well I will tell you one thing. An old Lime 400mhz G3 can't keep up all that encryption and ecryption.



    I got an Airport Express for Christmas from my friend. But can't use it for Airtunes because it would skip all the time....



    Guess I need a new iTunes computer too!



    I don't think the skipping is due to the speed of your computer. My 1.4GHz mini also had the skipping.



    One thing to try - if the airport express is plugged into a power bar on the floor, surrounded by wires, them move it up into the free air.
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  • Reply 7 of 9
    amoryaamorya Posts: 1,103member
    Quote:

    Originally posted by salmonstk

    Well I will tell you one thing. An old Lime 400mhz G3 can't keep up all that encryption and ecryption.



    I got an Airport Express for Christmas from my friend. But can't use it for Airtunes because it would skip all the time....



    Guess I need a new iTunes computer too!



    my parents' indigo imac (also 400mhz) does ok with it...



    Sure there isn't another factor at work here?
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  • Reply 8 of 9
    salmonstksalmonstk Posts: 568member
    Quote:

    Originally posted by Amorya

    my parents' indigo imac (also 400mhz) does ok with it...



    Sure there isn't another factor at work here?



    Well I just assumed that is it. Maybe it is something els.e I have the aiport expressplugged into the acryual sterio plug.
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  • Reply 9 of 9
    zozo Posts: 3,117member
    hmm, interesting stuff, thanks for all the feedback.
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