DAC "clicks" with Airport Express

Question

Hi,
I was wondering if the folowing problem has an obvious solution (and what causes it).
Setup:
Hegel H70 integrated amplifier with onboard DAC <-> toslink/optical <-> airport express (AE) <-> iTunes/MacBook Pro or iPad/iPhone <-> LaCie Network Space
Most audio files are Apple Lossless. Midi setting is 16/44.
The problem:
Every time I start a song, forward a song or change a song the Dac makes a clicking sound (just the dac not over the speakers). This occurs only when done manualy; once an album or playlist is started, it does not occur untill I change something myself.
I've checked, but this only occurs when connected via AE. The Hegel has optical, coaxial and USB. With USB it clicks only once (at the start of the connection. Same with optical, when connected via the optical out / headphones directly from the MacBook. Coaxial I did not / cannot test. When tested with Airfoil this problem did not occur. When tested it occurs at any midi-setting.
I suspect this is caused by the way iTunes/AE/Airplay sends the signal; i.e. it interupts/stops the signal briefly when something changes by the users hand. This also given that it sounds like the dac makes two clicks in a very short time.
Any thoughts on this? I like the simple setup I have and sound is excellent. Late at night with low soundlevels the clicks are somewhat annoying though.

Answer 1

The AX works only with iTunes v4.6+ and is limited to music files that iTunes can read; ie, 16-bit data only. (An exception, as you know, is you can use a third-party product like Rogue Amoeba's AirFoil to stream other non-iTunes sources.) These data, though, can be in any file format that iTunes recognizes, from lossy MP3s at the low-quality end of the spectrum to Apple Lossless and lossless AIF or WAV files at the high end. It is also important to note that the AX functions only at a 44.1kHz sample rate. When you play 32kHz or 48kHz data, iTunes sample-rate-converts the data in real time before sending it to the AX.
iTunes uses a QuickTime CODEC to convert audio files to Apple Lossless, and then, uses AirTunes to send them to the AX. In turn, the AX uses built-in software that converts the Apple Lossless to an Encoded Digital Audio format. From there, digital audio is sent to a optical transceiver to convert the electrical signal to an optical one before sending it to the innermost part of the audio port. For analog, the AX has a built-in DAC to convert the Encoded Digital Audio to Analog which is sent to the same audio port.
One operational glitch is the fact that, as the AX doesn't have a local clock circuit, when the incoming data is interrupted, as it is when you change songs in iTunes, there is no longer a digital output to feed the DAC, which loses lock as a result. Some DACs can cope with its digital output switching off at the end of songs. Yours may not be one of them.