AirPort Express sound format, frequency & bitrate

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AirPort Express sound format, frequency & bitrate

The tech. specs of the Airport Express aren't very explicit about the supported sound formats or how sound is streamed from iTunes to the AE and from there on to the amplifier. I'm currently using foobar2000 under MSWin to drive an AV amp over HDMI, which means that the amplifier receives the sound data as it's encoded, without resampling, bitrate conversions or any of Windows' bells and whistles. How does this work with Airplay streaming? I've read somewhere that it'll stream 44.1kHz without conversion, but what about the 48kHz or 92kHz recordings I have?

MPB 13"/i7@2.7 8Mb-OTHER, Mac OS X (10.6.8), 12" PillowBook 1.0Ghz 0.7/ext-80Gb; PB15" RIP; dual G5@1.8Ghz

Posted on May 6, 2012 3:16 PM

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Answer 1

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Tesserax

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148,251 points

May 6, 2012 8:44 PM in response to RJV Bertin

iTunes does most of the work. When iTunes plays back standard audio content (AAC, MP3, audiobooks, Internet music streams, etc., it decompresses those file formats and creates what's essentially a raw, uncompressed audio stream. That stream is compressed using Apple's Lossless Compression (via a QuickTime codec), encrypted, and then, streamed (via AirTunes/AirPlay) to the AX. The AX decrypts the stream, decodes it, and outputs it in either analog format or as a digital PCM stream.

For analog audio, the AirPort Express Base Station (AX) uses a 16-bit Texas Instruments PCM2705 DAC. The AX only functions at a 44.1kHz sample rate. When you play 48 or 92 kHz data, iTunes sample rate converts the data in real time (to 44.1 kHz) prior to sending it to the AX.

If iTunes is playing back a digital multichannel file format like AC3 (Dolby Digital) or DTS, those bitstreams (like the lossy formats) are wrapped in Apple's Lossless compression, encrypted, and then, streamed to the AX. In these cases, the AX would first decode the stream, and then, output the raw AC3 or DTS stream via its optical digital audio port.

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Answer 2

RJV Bertin Author

Level 3

955 points

May 7, 2012 2:26 AM in response to Tesserax

Thanks, that helps.

Do I understand correctly that "standard" (read, mono or stereo?) content always gets resampled to 44.1kHz, even if the AX is connected via its optical output only? If so, is there any reason to expect that this would give better quality than when I let MSWin take care of it in its output path?

I didn't mention it, but the main reason for using foobar2000 is that the sound quality is audible less good when I play back from iTunes - and I have all MSWin bells and whistles I could find turned off (and the frequency set to 192kHz so as to preserve the original waveform and not lose any information).

BTW, not the appropriate forum, but is the situation any different on the (current) Apple TV?

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Answer 3

Tesserax

Level 10

148,251 points

May 7, 2012 8:21 AM in response to RJV Bertin

Do I understand correctly that "standard" (read, mono or stereo?) content always gets resampled to 44.1kHz, even if the AX is connected via its optical output only?

The resampling happens with iTunes to be able to stream via AirPlay. iTunes digital audio (AC3 or DTS) is output in its raw format out the AX's optical digital port at whatever sample rate it was recorded at.

If so, is there any reason to expect that this would give better quality than when I let MSWin take care of it in its output path?

I assume by "MSWin" you are referring to the sound card used in your PC or built-in the PC's motherboard. That will depend. Most sound cards offer better overall audio quality, especially for analog, over the AX.

BTW, not the appropriate forum, but is the situation any different on the (current) Apple TV?

Straight from Apple Support - Specifications (for the 3G Apple TV):

HE-AAC (V1), AAC (16 to 320 Kbps), protected AAC (from iTunes Store), MP3 (16 to 320 Kbps), MP3 VBR, Audible (formats 2, 3, and 4), Apple Lossless, AIFF, and WAV; Dolby Digital 5.1 surround sound pass-through

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Answer 4

RJV Bertin Author

Level 3

955 points

May 7, 2012 9:40 AM in response to RJV Bertin

Yes, I read that the 44.1kHz 'limitation' actually stems from the Airplay protocol. Curious in that light that the ATV has a fixed output frequency of 48kHz ... that suggests that there might be 2 transcoding/resampling steps applied!

No, I did NOT refer to the PC's sound card. As I said, I drive the amp over HDMI, which means the audio path of the graphics subsystem is being used (an ATI chipset). This doesn't change - the audible quality difference is between foobar2000 playing back over HDMI (in 'passthrough' mode) vs. iTunes playing back over the exact same hardware (but taking whatever path the OS imposes).

In fact, from what I understand it's quite likely that foobar2000 does a better job even than iTunes on my own MBP (which imposes a fixed output setting via Audio Midi Setup) ...

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Answer 5

Receiver

Level 1

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Apr 22, 2013 10:29 PM in response to RJV Bertin

If iTunes is playing back a digital multichannel file format like AC3 (Dolby Digital) or DTS, those bitstreams (like the lossy formats) are wrapped in Apple's Lossless compression, encrypted, and then, streamed to the AX. In these cases, the AX would first decode the stream, and then, output the raw AC3 or DTS stream via its optical digital audio port.

From http://forums.macrumors.com/archive/index.php/t-1174140.html

Q. can it do dolby/dts whenever i play dvd/hd movies on my iMac?

A. when playing back videos in iTunes, remote speakers are not an option.

Total Newb to this forum so please go easy on me.

Has DTS or DD5.1 an option added to Lion? I am currently running Snow Leopard on a Mac Mini.

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