Connections
Digital Audio Transmission
The real difference is where the digital work is done
Digital interfaces are often compared as though they are just different connections.
They are not. Choosing one shifts certain roles between the DAS and the DAC.
That is why Ethernet, async USB, S/PDIF, AES3 and I²S are not simple winners and losers. They divide the job differently between the source and the DAS (Digital Audio Source), and so what works best depends on the DAS and DAC concerned.
First, separate the issues
Three different things are often mixed together:
- PCM and DSD are ways of encoding audio
- Ethernet, USB, S/PDIF, AES3 and I²S are transport methods
- DoP and Native DSD are ways of framing and signalling DSD over a given link
Unless those are kept separate, the discussion quickly becomes confused.
Ethernet: convenient isolation, but much more work in the DAC
Some DAC manufacturers argue that Ethernet is best. That can make sense if they assume the source is a noisy general-purpose computer and want to isolate the DAC from it.
But Ethernet does not simply deliver a prepared audio stream to a passive DAC input.
An Ethernet DAC has to do far more work, to do it well. It is not just a DAC. It becomes a network endpoint. That means network stack activity, packet handling, buffering, control protocols, and more software complexity - generating interference inside the DAC enclosure.
Saying Ethernet is best is a trade-off decision, not a general rule.
Ethernet can provide useful electrical and functional separation from the server, but it moves much digital processing from the DAS to the DAC.
That may work very well in some designs. But it is not a free pass, and it is not automatically superior. It is a different way of dividing the job.
Async USB: the DAC can take more control over timing
The main advantage of asynchronous USB is that the DAC can buffer incoming data and time conversion against the DAC’s own clock.
That matters.
It means the DAC does not have to follow the timing of the arriving stream in the way it normally does with S/PDIF or AES3. The DAC can receive the data, hold it in a buffer, and decide when to convert it.
That is a real architectural strength.
But USB has a cost. It is a packet-based computer interface. That brings packet handling, protocol activity, and more digital noise into or near the DAC.
So the async USB trade-off is clear: more clock control at the DAC, but usually more digital activity there too.
If that is handled brilliantly, async USB can be excellent. If it is not, the clock advantage can be undermined by the extra processing burden.
Synchronous links: S/PDIF and AES3
S/PDIF and AES3 solve the problem differently. Their bandwidth is limited, which affects not only support for higher data rates, but also the margin available for preserving signal integrity and accurate clock recovery.
S/PDIF and AES3 send a continuous stream with timing embedded in it. The DAC normally has to recover timing from that arriving stream and work from it, unless one device is explicitly slaved to the other.
That gives the DAC less freedom than async USB, because it must work from the arriving timing.
Their advantage is that these links avoid pushing high levels of packet-based or network-style activity into the DAC. The DAC can remain more focused on receiving and converting a prepared stream.
So their trade-off is almost the opposite of USB: less computer-like activity at the DAC input, but more dependence on upstream timing and clock recovery.
That is why source quality matters so much with these connections. The source is not just sending data. It is shaping the conditions from which the DAC must derive timing.
AES3 and S/PDIF are similar in principle, not identical in practice
AES3 and S/PDIF share the same broad architecture. Both embed timing with the audio stream. Both usually require clock recovery at the DAC.
But they are not identical in practice.
AES3 can offer stronger signal integrity in practice.
S/PDIF is the domestic version, usually coaxial or optical.
Optical is touted to provide galvanic isolation, but it comes with timing penalties. Another trade-off - not superiority.
Coaxial often preserves edge integrity better than Optical.
Again, there is no free pass. Only different trade-offs.
I²S with clock slaving: potentially the cleanest split
I²S becomes especially interesting when combined with clock slaving. Like S/PDIF and AES3, it is used for synchronous audio transfer. But unlike them, it can keep clock and data on separate lines, keep noisy computer activity out of the DAC, and still allow high bandwidth. Adding clock slaving allows the DAC’s clock to become the timing reference.
In principle, this gives a very attractive division of labour:
- the DAC clock controls timing
- the source follows that timing
- some of the extra packaging and protocol burden of USB or Ethernet can be avoided
That has the potential to be the cleanest architecture of all.
But only in principle. Like every good architectural idea in digital audio, it depends utterly on excellent execution.
It is not something that can be assumed to work well simply because of the architecture. For this reason, where we offer I²S with clock slaving, it is specified for particular DACs, so that we can deliver superior implementation.
Bespoke Connections
Where appropriate, we also work with DAC manufacturers on their bespoke connections. These bespoke connections are often strong in concept, but non-standard. By working on the end-to-end implementation across DAS and DAC, we can make them robust and predictable.
Conclusion
Digital transmission is not just about moving bits. It is also about where the data is buffered, where clock authority sits, where the stream is reconstructed, and where the noisy digital work is done.
Ethernet can isolate the DAC from the server, but usually turns the DAC into more of a network endpoint.
Async USB lets the DAC buffer data and time conversion against its own clock, but usually brings more digital activity into the DAC.
S/PDIF and AES3 usually ask the DAC to recover timing from the arriving stream, but can avoid some of that computer-style burden at the DAC input.
I²S with clock slaving has the potential to be the cleanest split of roles, but will only be superior with DAC-specific execution.
Bespoke connections can be excellent in concept, but require end-to-end implementation to deliver their potential.
So there is no universally correct interface.
There are only different ways of distributing the burden, alongside different bandwidth limits and implementation demands.
And in digital audio, where that burden sits matters a great deal.