Hi @Simon-in-Suffolk et all,
Quite a hot subject this one. Lots of various questions. Will try and nail as many as possible:
Q: "Why does the transport stream influence audio quality - data is data"
A: If there are no playback synchronisation needs then within reason (aka - its a data rate that can reliably go over a typical home network) the above is true. As soon as any AV or multiroom synchronisation is added into the design then it starts to get a lot more complex. Primarily one device (typically the iPhone/iPad/control source) is clocking the audio out at one rate (for example, the audio clock might by 44.1006kHz with a slow drift on it and the playing device is playing it out at say 44.0999kHz. The system needs ability to ensure the playback rate is at the real rate and clocks samples out at the right time. On cheaper solutions sample rate conversion is used which has quite undesirable sound implications. In Naim devices we use hardware programmable fractional clock generators to track the Apple audio clock derived from PTP. This is still not as good as for example local UPnP playback or Roon where we lock the clocks into centre position and bypass all the fractional clock generation. On decent kit all these factors can knock a bit off the sound quality.
Q: Why not use Apple MusicKit
A: This is intended so apps & websites inside the Apple eco-system can have basic high level access to Apple Music tracks. They however, have no way to get at the physical music streams - a dedicated playback engine does that and hence keeping the licenced music streams secure. It is not a solution for standalone players.
Q: What about Amazon HD
A: Currently that generates a 16bit/44.1kHz ALAC stream in ‘legacy’ mode when using Airplay.
Q: So could Airplay2 be extended to support higher rates?
In theory yes, although Apple would have to roll new code out to Airplay2 devices to support it. There are a lot of complexities. For example:
- A If a Naim device supports upto 24/192 ALAC on Airplay2, but an Apple Homepod gen1 only supports 24/48kHz. Would the system be forced down to the lowest rate when doing a multiroom session, or have to try and stream two different streams which in turn could start to generate a huge amount of traffic on the users home network and internet connection.
- Battery life on the source device could take quite a kicking
- Buffers and RAM needed in AIrplay2 devices would need to be increased by around a factor of 4-5 to give reliable buffered streaming. Currently Airplay2 uses 10MBytes which when streaming HD is only about 10-12secs of audio.
From Naim’s side we are close Apple partners and all our kit has latest AIrplay2 tech built in and implemented in its higher quality form. If Apple require updates to expand higher sample rate playback beyond 44.1kHz then we’ll be one of the first to get it into the products.
Best wishes
Steve Harris
Software Director
Naim Audio Ltd.