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Air Chains: DAs vs. Transformers
I have written in the past regarding air chains, and how to eliminate single points of failure (SPOF). I'm a fan of using multiple, parallel air chains, but each should be simplified as much as possible.
Recently, a colleague and I were designing new air chains (based on AES) and we agreed on just about everything — except how to split the original (studio) source so multiple air chains are fed simultaneously. While I'm fine with using AES splitting transformers, he preferred using active distribution amps (DA). Since the intended air chains are meant to be parallel, I don't really have an issue with using an active device instead of a passive device, since there won't be any SPOFs. (Eliminating the studio as a SPOF is done simply by having another studio available to feed all air chains if necessary.) He planted a seed of doubt in me, so I did a little research and testing to see if there really was any substance to his discomfort.
What happens to the AES signal as I run it through the splitting transformer? Simple measurements (shown below) indicate the peak-to-peak level drops somewhat. That's not surprising since the very small amount of power put out by the AES "transmitter" is finite and divided by two by the transformer. By studying the AES3 standard, you learn that the AES data stream peak-to-peak level should be, at very minimum, 200mV peak to peak (referring to the "eye" pattern).
The AES3 system is balanced and thus has strong rejection of common-mode signals. Still, it is possible noise can be induced into the system that could interfere with the AES receiver's ability to decode the data. This super-imposed noise can show up in the decoded data stream as jitter. "Jitter on a digital signal can be observed as pulse transitions that occur slightly before or after the transitions of an ideal clock," according to Audio Precision. Jitter is measured in the unit interval (UI), which is proportional to the sample rate, and is defined as, "the shortest nominal time in the coding scheme." Again, looking at the AES3 standard, you'll note that the AES data stream should be decoded (for any jitter frequency) as long as the UI is below 0.25.
Now on to some measurements (see chart below):
> Figure 1 shows the baseline measurement, using a 1' piece of Belden 1800B between the output and input of the Audio Precision Portable 1/Dual Domain (AP).
> Figure 2 shows the AP driving the transformer input, and one output of the transformer feeding back to the AP through a 25' piece of Cat-5 cable. (The transformer is an ETS PA830.)
> Figure 3 is similar to Figure 2, but having an AES DA in place of the transformer.
> Figure 4 shows the AP driving the transformer, which in turn drives 50' of Cat-5 cable, with a Krone block interconnect at the 25' mark.
> Figure 5 is similar to Figure 4, but having an AES DA in place of the transformer.
> Figure 6 shows the AP driving the transformer input, and one output of the transformer feeding back to the AP through 75' of Cat-5 cable, with Krone block interconnects at the 25' and 50' marks.
> Figure 7 is similar to Figure 6, but having an AES DA in place of the transformer.
Let's look at the measurement results now:
I added the Krone block "interconnects" at intermediate points because I wanted to more closely simulate typical rack room construction. The trend indicates that I could have gone on with more and more cable length without any problem.
My conclusion is that the AES splitting transformers are not going to degrade the system performance.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at email@example.com.
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