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Digital Routing and Mixing
Here's what to consider in making the TDM vs. AoIP decision.
TDM vs. AoIP
So as a potential buyer of a digital routing system, one thing you'll need to address is whether or not you want to make use of a TDM system, or go with an AoIP system. Perhaps it makes sense to first examine their similarities, and their differences thereafter.
Obviously the point of any router is to switch input sources and route them to destinations. Any digital router is going to be able to accommodate digital inputs (AES) as well as analog; likewise, any digital router will be able to provide output sources in either a digital or analog format. Both the TDM types and the AoIP types are built around a spoke-and-hub topology; what constitutes the hub is different between the types. (More on that later). Both types will have (as the terminal end of a spoke) an array of potential interfaces to the system - the most basic being a set of inputs and outputs, the most complex being a control surface (what we would typically call a console). The control surfaces will be similar to good old analog consoles in that each channel represents a source being added to a mix bus; however, digital systems provide flexibility so that the source for a channel can change based on router control. (Analog consoles would literally need their inputs rewired to do that - with the exception of pre-select inputs.) Both types will be configured via a computer workstation and as such will support off-site access. One common feature that I noted in the AoIP types, as opposed to TDM systems, is the ability to also route machine control in the same fashion as the audio is routed. The control is included in the data packet with the audio. TDM systems can also route machine control, and this information is carried in the same signal pair as the audio data.
Differ in design
So let's take a look at the differences now. As I mentioned earlier, both systems will be built around a spoke-and-hub topology. TDM systems, being derived from their analog ancestors, are based around a router core or engine inside of which all the switching and mixing and routing is really done. So, any time point A (a source) is connected to point B (the destination) it routes through that router engine. (All roads lead to Rome.) TDM systems use proprietary communications means to connect the spokes to the hubs; no standards have been developed, let alone implemented. (They may use CAT-5 cable and RJ-45 connectors too, by the way, but it isn't Ethernet. It's just a smart way to make use of cable and connectors that are readily available.) In using TDM though, those manufacturers have built a system with a designed-in, minimum data rate that effectively guarantees how many audio channels can be moved back and forth along the spoke. Time slots in a TDM system don't compete with one another; they each have a job to do (move an audio stream in one direction or the other) and they do it with next to no latency.
AoIP systems differ from their TDM cousins in that they don't use a router engine as the hub of the system; what they use is a very fast, very common type of Ethernet switch that makes the Layer 1 and Layer 2 (in IT parlance) connections between nodes of the entire system. And while the Ethernet carrier is a standard, the AoIP message inside is still proprietary to each manufacturer.
That is a fundamental difference between TDM and AoIP. The processing power needed to carry out the mixing/switching/routing functions in a TDM system has typically been located at the heart (or star-point) of the system in the technical center; whereas that same processing power is distributed in an AoIP system, because the processors that do that work are located typically in the same studio as their associated control surface, connected via Ethernet. The most recent versions of some TDM systems have implemented a distributed processor approach.
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