Can I alter the bit rate being streamed to the Airport Express.
Basically, no. AirPlay is just a transport protocol suite. As you can imagine streaming audio will differ somewhat between analog and digital formats ... and primarily relies on the ALAC audio codec.
For streaming analog (AAC, MP3, audiobooks, Internet music streams, etc.) from a Mac to an AirPort Express base station (AX), iTunes does most of the work:
- iTunes decompresses those file formats and creates what's essentially a raw, uncompressed audio stream.
- That stream is then compressed using Apple's Lossless Compression (ALAC) via a QuickTime codec, encrypted with 128-bit AES1, and then, streamed (via AirPlay) to the AX. ALAC is used over the UDP transport layer protocol to stream two audio channels.
- AirPlay uses the Remote Audio Output Protocol (ROAP) to establish the streaming connection type. By default, as you know it is set to 44100Hz @ 16 bits. at 44.1kHz. This cannot be changed as this is what the AX is expecting.
- The AX decrypts the stream, decodes the ALAC stream to an encoded digital audio (PCM) format at the same quality level of the original audio source format, and then, buffers it for up to 2 seconds before it outputs it to either:
- An optical transceiver to convert the electrical signal to an optical one before sending it to the innermost part of the AX's audio port, OR
- As input to the built-it Texas Instruments DAC to be converted to an analog audio format before sending it to the outermost part of the AX's audio port.
For a digital multichannel file format like AC-3 (Dolby Digital) or DTS, iTunes:
- Compresses the bit stream with ALAC, encrypts it, and then, streams it to the AX.
- Again, AirPlay uses the Remote Audio Output Protocol (ROAP) to establish the streaming connection type.
- The AX decrypts the stream, then decodes it, buffers it, and finally, outputs the raw AC-3 or DTS stream via its optical-digital portion of its audio port.
- So, for example, if you stream an AIFF file, it will first be compressed to ALAC, and then, decompressed to PCM with the same quality as the original AIFF file. On the other hand if you stream an AAC file, it will be compressed to ALAC, and then, decompressed to PCM ... but at the same quality of the original AAC file.
I am aware that the internal Texas DAC on the Airport Express is limited to handling 44kHz/16 bit streams but as I am using an external DAC, whose optical input supports up to 96k/24bit, should that matter?
At best, your external DAC will still receive input at 14.1kHz@16, so no, it shouldn't matter.
