Thursday, October 19, 2017

Audio over Ethernet cable

I recently learned that it is possible to send four channels of balanced audio over category 5 or 6 shielded ethernet cable. What makes this especially interesting to me is that it solves some audio distribution problems that I've been trying to see my way through. The tracking room currently has only one place where all microphones and instruments connect, a multi-channel snake sitting in the center of the floor. While this works it is a constant effort to maintain all the cables in an orderly fashion and out from under everyone's feet. Plus it just looks bad.

I would like to have XLR connectors mounted on the walls near where each musician sits so that the cables could be neatly tucked out of the way. This would also provide monitoring and really de-clutter the room.

There are also other rooms that would be great for extending capabilities. The living room has nine foot ceilings and a good vibe. Running mic cables here to isolate instruments or singers during recording would be a great option. But the path from the desk is long, meaning a bunch of long XLR cables or a multi-core snake that would have to be run for every session and then rolled up and put away after.

The actual distance is pretty short if I were able to go through the floor. That would mean putting a decent sized hole through 100 year old inlaid oak flooring. Not at all my first choice. I discovered a hidden spot where a previous owner had drilled a tv-cable sized hole near the edge of the baseboard. The diameter is too small for a multi-core snake or even a single XLR connector to fit through but a Cat5 cable passes through with just a slight push.

Commercially made products like the ETS Instasnake provide three or four balanced signals and terminate in XLR connectors. It feels kind of expensive at $185 per box (you need two to make a snake.)  Radial Engineering also has a nice product at similar prices. Redco has a product that runs $132 for the set of two boxes. In all cases you need to provide your own Cat5 Sheilded cable.

I really liked the idea of balanced audio over Cat5 cable so I looked at how I could build the termination boxes myself. It turns out that it is much easier than I expected. So here is how I approached it.

Cat5 Audio over Ethernet Snake

A basic audio over Ethernet snake consists of an Ethernet cable that plugs into a box on each end that has XLR (or TRS) connectors on it. A commercially made unit is a metal box with up to four audio connectors and one RJ45 connector. The RJ45 connector allows you to plug in a shielded Ethernet cable and provides four twisted pairs plus ground. The RJ45 connector is wired to the individual XLR or TRS connectors inside the box.

Finished snake input box and pigtail end
There are many different configuration options for each end. I decided to have a box with female XLR connectors on the input side for microphones or instruments and terminate the other end with a pigtail (four XLR  male cables ) to connect to my mixing board. This is a completely modular setup so it is easy to swap out the end components to suit the application.

The XLR Box

Parts Express sells an XLR panel box with four female connectors and 4 TRS jacks pre-mounted for under $18 shipped.

PartsExpress XLR box and MDFLY RJ45 Breakout board
To complete the box all I had to do was wire in a RJ45 connector. I chose an RJ45 breakout board from MDFLY Electronics because the connections are made via screw terminal and this would allow me to experiment easily. Soldering would probably be better but this was really easy to wire up as a prototype and allowed for mistakes. These were $2.48 each on ebay.

RJ45 connector mounted inside box

This made the total cost of the termination box $20.

The box has knockouts for passing cable through or mounting a connector . The location of the knockouts did not allow me to mount my connector without hitting the XLR and TRS connectors inside the box. I had to nibble s slot for the connector so that it was out of the way. Choosing a different type of connector might alleviate this issue. For my next boxes I will experiment with a connector type that has a pigtail already wired in. See below.


The Pigtail

There is no reason you cannot build a XLR box for each end of the snake but the box from Parts Express only comes with female connectors. I thought that would make for extra wiring and adapters to connect to the mixer so I made a simple pigtail instead.

I found inexpensive four channel XLR snakes on Amazon that I could cannibalize. Easier and cheaper than making my own as the XLR connectors are already soldered on and I just cut off the length I wanted. The RJ45 connector was mounted inside a cheap electrical junction box to protect the wiring and secure the cables.

Wiring

There is nothing complicated about the wiring. The signals just need to match at each end. I made a diagram for the connections and then used my multi-meter to test each connection as I made it. Then one final check after assembly.

There is probably an industry standard for wiring but I don't have it. I wired the connectors as shown below. All Color/White wires are Hot. All Solid Color wires are Cold and the Shield is Ground.


What I Might Do Differently

The placement of the knockouts in the XLR box make it challenging to mount a RJ45 connector. I don't have a punch for making the correct size and shape opening in the box so I would still use one of the knockout holes. The connector shown below looks like it would mount in the existing knockout hole and provide enough clearance. The plug end could be cut off and the individual pairs soldered to the XLR connectors directly. I could even mount one of the breakout connectors inside the box and use this type of connector like a short extension cable. This connector would also work well for the pigtail end where you would solder directly to the XLR or TRS cables and not need a box to mount the RJ45 connector in.

Caps to block phantom power on the TRS jacks would also be a good idea.


Cat5 Shielded Extension Cable

XLR box and Cat5 cable
Making the snake took me  a couple of hours of planning and soldering but in the end I saved somewhere between $100 and $300. I have four more to make so the savings is very real and gives me the option to customize during fabrication.