The Apple Airport Express (AE) is a beautiful device. One of its functions is the ability to act as audio Airplay receiver. Airplay is a streaming protocol, based on a lossless 44.1KHz/16 bit stereo audio stream. It’s part of iTunes since a long time. It uses Bonjour for service advertisement and as such is zero conf.
My Airport Express is the 2nd gen model with 802.11n (update: The A1264 model). All models have a combined analog optical output. Many years ago I used it with an Apogee Minidac and the results were very good. One year ago I connected everything again and was disappointed with the results. Of course, your first thoughts are something like: “I forgot that it sounded so bad”, or “this optical cable is bad” or whatever. So I dismantled everything..
A few days ago I hooked it up again, now connected to my brand new SMSL M8 DAC. What a surprise: dropouts, dropouts! WTF, this is not happening (why me..) Time to Google. Of course this has nothing to do with: my WiFi network, the optical cable, my neighbors etc. etc. Then I found this post about dropouts starting after a certain firmware upgrade. I never thought about that one. (this is always a sign that you are on the right track in solving a problem). Time to try it out.
Downgrading firmware on the AE is surprisingly simple. Use the magic ALT button in the Airport Utility program to show hidden functions and select some older firmware version, in this case 7.6.1. Reboot, listen. WTF again: not only are the dropouts gone (completely!!) but the good old sound quality is back. Time to reflect.
The dropout are caused by very bad clock (SP/DIF) behavior. So bad, that the M8 DAC cannot keep track. Why this is caused by the new firmware, only Apple knows. My guess is that the addition of more features in newer firmware pushed the CPU to its limits. The Apogee Minidac was able the handle the bad clock without dropouts, but bad timing makes bad audio…