In contrast, an analog signal moving through a wire is traveling at nearly the speed of light, and that’s fast! Modern converters are pretty fast and humans cannot usually detect any delay.
But networking the signal introduces opportunities for latency to accumulate, and critically accumulated latency destroys the intimacy of the original analog signal in time-sensitive applications such as live performance.
The bottom line: With basic fidelity issues addressed, digital connectivity makes a lot of sense in the audio world because it is limited only by conversion with its upside being longer, thinner wires and easier splits,
Digital connectivity is a necessary ingredient in the evolution of audio networking, but networking puts a greater strain on digital’s primary limiting factor, conversion.
DIGITAL AUDIO NETWORK CHALLENGES
It’s not surprising that there’s a lot of interest in networking digital audio.
The basic advantages of digital – long cable runs with a lot of channels and loss-less copies – encourage going not just from point to point but from point to point to point.
But what we’ve discovered is that moving from a point-to-point model to one where lots and lots of nodes are tied together with audio swimming all around magnifies many of those basic challenges of digital audio, such as dock integrity (jitter and wander) and system latency.
The real goal is to address those challenges without limiting the flexibility a network model promises.
Clock Quality: In a network, signals hop from one node to another. Remember that each node must receive the stream of 1s and 0s (the digitized audio), as well as the dock which governs the time alignment of those data bits.
There are two techniques for transporting an audio dock in a digital system: either running a separate wire for the dock alongside the data or embedding the dock with the data stream (like in an AES3 stream).
True digital audio networks require the embedded clock technique. The problem, though, is that as data hops about the network, the embedded dock tends to accumulate distortion in the form of jitter and wander. Remember that audio samples depend on a clean, distortion-free clock to place the data stream properly in time.
Thus, jitter can directly degrade the performance of an audio network (see Jitter and Wander sidebar). The key to transparent digital audio is to keep variations in clock frequency so small that the audio is not distorted.
This challenge can be mitigated to some extent by very narrowly defining the sample rate capture range of the network, but this cuts into the real virtue of a network. Real-world devices in real-world systems need to sync to each other and to different dock sources.
If analog infrastructure is to be replaced by a DAN, the network needs to be as accommodating as a simple wire to even slight shifts in the clock and able to sync to external devices and clock sources.
Ideally DANs would be able to lock to and distribute the house clock. Virtually all current DANs can sync to external clocks only within a very narrow capture range.
The downside of this is that systems often require sample rate converters to connect to other digital audio equipment. The problem with sample rate converters, besides adding latency, is that they forever change the audio data, adding noise and distortion to the digital audio signal that can never be removed.
A related challenge is wander, low-frequency jitter caused by the accumulation of jitter as clock information is re-transmitted from device to device within the network. Wander occurs when digitally clocked devices are daisy chained. The jitter from each device adds up as the dock is passed on until the slight jitter errors become a large error.
While wander is not audible when listening to a single channel, it becomes a problem when sample alignment between multiple digital audio streams is required, as could be the case when DANs are connected to mixing consoles or digital recorders using multiple AES streams.
The good news is that if the jitter is managed successfully, wander can be managed along with it.