Digital Routing and Mixing

July 1, 2011


Here's what to consider in making the TDM vs. AoIP decision.

So with the economy turning around, and the immediate future looking (at least somewhat) brighter, perhaps the time is right to plan that studio rehab project, or perhaps that completely new facility that has been in the offing. When it comes to specifying and buying new consoles, what are the choices?

Going with analog consoles is still an option (AudioArts, Arrakis, Radio Systems, Mackie and others come to mind) but this time we'll consider the digital types.

Twenty years ago the state-of-the-art radio station not only used analog consoles, but analog routers as well. They were located in a separate room - typically called master control or some variant - and these routers took the place of rack upon rack of distribution amps, multiple-input-by-stereo output mechanical switches, and perhaps even patchbays.

Having a router system in place gave you the obvious advantage of quick and easy audio routing around the facility, and usually simplified the initial construction wiring as well.

After digital audio sources made their appearance around radio stations, digital routers soon followed. Even the analog inputs were digitized, though, and the router designs had changed drastically. It was no longer necessary for the console to actually contain the audio; all the switching/routing/mixing could be done inside the router itself. The console became no more than a user-interface, with a form-factor familiar to air talents. A PC attached to the system could control all the same functions as a console. Systems such as this appeared in the late 1990s.

As we all know, during the 1990s, computer networking around the offices and studios of a radio station became commonplace; Ethernet became practically ubiquitous. It should be of no surprise then that Ethernet later became an alternative to TDM (Time Division Multiplex) communications between nodes of a digital routing system (such as previously described). The audio-over-IP (AoIP) type system was born. This occurred in the mid 2000s.

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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.

TDM backbone

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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Here's what to consider in making the TDM vs. AoIP decision.

The way AoIP systems distribute signals is via the wire. A source is available to any destination on the network because the source is multicast onto the network. Let me explain a little bit about what that means.

TDM spoke

Recall that with a Layer 2 switch (like you see in use for the office network) any device can have a unicast connection to any other device that connects to the same switch. (Those two devices are going to need to be in the same subnet and the same VLAN as well.) A broadcast is a message that the Layer 2 switch floods throughout that entire VLAN. A multicast is a message that is meant to be heard by many hosts on the subnet (as opposed to a unicast, which is a one-to-one communication - and broadcast - which is heard by every host on the subnet). If every stream that represents an audio source is multicast on the local subnet, then any member of the multicast group will be able to not only hear that stream, but to further process it for use. This is fundamentally how the AoIP system types distribute audio. Whereas a TDM system has time-slots that are assigned to sources, and wired connections that carry specific signals to and from the router core, an AoIP system distributes audio over the shared network fabric.

Differ in practice

The sharing of the medium is the basic advantage of Ethernet (and IP communications in general). Fundamentally, networking multiple sources together is simplified by the use of Ethernet, mainly because of that fact that Ethernet is a standard. (This is to say the carrier - Ethernet - is a standard. The AoIP message within the Ethernet carrier is proprietary to the manufacturer.) The fact that it is used so much means that many sources are available for its components, and many sources means lower prices. That's just economics.

There is a caveat with the shared medium though. Whereas a TDM system has a throughput delay that is insignificant, AoIP systems do have a very small amount of delay because of the shared nature of the medium. It takes a finite amount of time to build up the packets, for the Layer 2 switch to process them, and then for the destination device to reassemble all the packets in the correct sequence (because it is possible for them to get out of sequence as they transit the network). The amount of delay that is objectionable to the human ear has been studied thoroughly in VoIP engineering and what has been learned in that field applies to AoIP as well. While those delays are not insignificant they are manageable, and minimized in the system engineering.

AoIP spoke

Both TDM and AoIP systems are scalable in that they can be increased in size after the original design and installation is done. In a TDM system, typically another input card or output card will be installed in a frame when and if more inputs or outputs are needed. Those new cards essentially live in the hub of the system. AoIP differs here in that the inputs/outputs are never part of the hub of the system; they are always at ends of the spokes. A new input/output module would be added to an AoIP system by putting it in a rack somewhere in the facility, and then making it part of the network by adding a single Ethernet cable from it to the local switch.

In terms of wiring one type of system versus the other, it really comes down to your wire management style. Typically TDM systems are based on card-cages, with connectors that carry dozens of circuits at a time. For this reason they are typically landed on external punch blocks because the only real practical way to deal with those large connectors is to terminate all of their associated circuits at the same time during the installation phase. AoIP systems are based around smaller nodes that have fewer inputs and outputs (usually eight pairs of inputs and/or outputs). The installer could chose point-to-point wiring on all those nodes; or, like with a TDM system, all those circuit connections could be landed externally on punch blocks. It really depends upon how you want to organize the wiring. In either case, the number of ingress/egress points is obviously going to be the same for systems that have the same capability (i.e., 128 by 128 TDM vs 128 by 128 AoIP). In other words, wiring is wiring. Both systems can be as simple or complex to install as the installer desires.

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Here's what to consider in making the TDM vs. AoIP decision.

One definite advantage that AoIP systems seem to have over their TDM cousins is that it is possible to add a computer workstation to the network, and to then have access to its associated audio output and input stream(s) and control logic, for the entire network, by way of the installation of software drivers and network cable. For a TDM system, the alternative way (which is what we've been doing for years) is by installing sounds cards in computers, which can be an expensive proposition with high-end sound cards. A logic control interface from this same computer would likely be done via a serial connection, which of course represents more work and expense in the installation phase. More recently, some TDM manufacturers have developed IP driver interfaces, which eliminates the sound card requirement and serial control connection.

A natural extension of the AoIP network is into the STL portion of your facility. Having a WAN and/or LAN connection at the transmitter site means that devices on that end can become nodes in the system. In practice though, sharing a LAN for STL purposes means either using QoS (and all interim devices must support that along the route) or maintaining a totally separate LAN just for STL purposes.

The final decision

As I mentioned earlier, a TDM-based digital router is really an evolution from the analog router type that many of us grew accustomed to over the years, and as such, it is familiar, tried-and-true technology. If you intend to go with an AoIP system, you'll not only need to learn the details of the new system itself, but you'll also have to become much more familiar with networking in general. In this day and age that is certainly a good thing to do. If you don't already have a good handle on networking, ask yourself if you want to learn that at the same time as an entirely new console type and system type. If networking is just one of your IT skills though, then likely an AoIP system will be something to seriously consider. Finally choosing the type of system might boil down to your design philosophy: Whether you want to centralize the intelligence of the system in your MCR (as with TDM) or whether you want to distribute the intelligence of the system (as with AoIP).


Resource Guide

AEQ
954-581-7999
www.aeqbroadcast.com

Audioarts Engineering
252-638-7000
www.wheatstone.com

Axia Audio
216-241-7225
www.axiaaudio.com

Harris
800-622-0022
www.broadcast.harris.com

Lawo
+49 7222 1002 0
www.lawo.de

Logitek
800-231-5870
www.logitekaudio.com
   .    .    Qphonics
+49 89-45672-300
www.klotzdigital.com

Radio Systems
856-467-8000
www.radiosystems.com

Sierra Automated Systems
818-840-6749
www.sasaudio.com

Studer
818-920-3212
www.studer.ch

Wheatstone
252-638-7000
www.wheatstone.com


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.



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