How to Measure Your Wireless Unit for Linear Time Invariance

How to Measure Your Wireless Unit for Linear Time Invariance

If you're considering purchasing a wireless unit for your measurement setup and don't know where to start, we recommend checking out our "Wireless for Measure: Things to Consider" article, available here. It compares different wireless units and serves as a comprehensive breakdown of which features are required for measurement.

However, what if you already own a wireless unit but you're unsure whether it's suitable for measurement? What if you're considering a unit that's not listed in the "Things to Consider" article? This companion article will serve as a guide to testing the usefulness of a wireless unit already in-hand.

As mentioned in the "Things to Consider" article, it is absolutely critical that a wireless unit is Linear Time Invariant. To qualify as such, it should have an output level that closely matches the signal being fed into, and it must have a mostly flat frequency response. Additionally, it must be time-invariant: the wireless signal's propagation time should remain consistent when measuring conditions aren't changing.

While innovations in wireless transmission have come a long way with thanks to Lectrosonics and other manufacturers, the true golden standard for linear time invariance will always be a physical cable. Unless using an internal/virtual software-based loopback, your transfer function reference signal is almost always a physical loopback cable fed directly out of one of your I/O unit's outputs and back into one of its inputs.

When using a loopback cable (or digital loopback) for both your reference channel and your measurement channel, you will see both a completely flat magnitude trace, phase trace, and impulse response. This makes sense, as the two signals are 100% correlated.

It then follows that the best way to gauge the linear time invariance of a wireless unit is to compare its response to a loopback cable. 

To do so, follow these steps:

1. Use a loopback cable to connect output 2 of your I/O device to input 2. This will serve as the reference channel.

2. Connect the wireless transmitter to the I/O device's output 1, with the receiver plugged into 

input 1. Your measurement system should resemble this:

3. In Transfer Function view, click "+ New TF Engine" or use the [Shift] + [T] hotkey to bring up the New TF Measurement dialog. Set input 1 as your Measurement channel and input 2 as your Reference channel.

Red Flags

Utilizing this measurement setup, you can identify three potential issues that disqualify a wireless system from measurement:

1. After the measurement has been timed using the delay finder, the phase trace continues to fluctuate. Constantly changing phase shows fluctuating in timing, making it impossible to accurately quantify time-based phenomena within your SUT. Since time-based phenomena also affect a system's frequency response, having time variance in your measurement system is a no-go.

2. The Live IR is spread out after the impulse, showing multiple arrivals. This behavior means some frequencies are arriving later than others, showing an inconsistent frequency response. If your measurement system is voicing your measurement signal too much, it will distort the measured frequency response of your SUT, which can lead to making either incorrect or unneeded frequency-domain decisions.

3. There is excessive band limiting shown in the frequency response. Most commonly, many wireless units exhibit a high pass filter on the low end of the spectrum. If this filter affects too much of the frequency spectrum or attenuates the low end too drastically, it can produce measurements that can affect subwoofer measurement or give you a false impression of two speakers' crossover frequency. Filtering in the high end can be detrimental to measurement-based decisions, as well.

Wireless Response Examples

This is the response of a Lectrosonics TM400 that was tested in-office. Note that while not as perfect as the cable-to-cable measurement, its magnitude response is nearly completely flat. It does have a gentle high pass filter in the lowest frequencies (which is common among wireless units) but it isn't so drastic a change as to cause any major issues when measuring. Its consistency in the majority of the frequency spectrum leaves it vastly useful for measurement in general. It also exhibited consistent timing data (although this is difficult to illustrate in a screenshot).

This is the response of a Sennheiser EW-DP unit, also measured in-office. It is nearly completely flat as well, save for a tiny bit of voicing in the extreme high and low frequencies. The Sennheiser also showed consistent time-domain data.