Wireless for Measurement: Things to Consider

Wireless for Measurement: Things to Consider

In the world of measurement, there are many options for wireless transmission. These can be incredibly useful, especially in larger venues where you'd need excessively long cables. While wireless units can be a more lightweight and convenient option, they are not without their downsides (even in extremely high-end units). 

Some of these downsides, in fact, disqualify certain wireless units from measurement entirely. When looking into wireless units for measurement, it is critical that they are Linear Time Invariant.

There are 2 key requirements for a measurement system to be considered Linear Time Invariant:

1. Linearity in Level - Frequency-shifting or gain effects are not desirable, as they will hurt coherence and alter the resulting measurement's response. When sending a signal through an undamaged cable, it's perfectly reasonable to expect that its output is at a similar level, with a largely untouched frequency response. The same should be true of any useful wireless unit, but this is not always the case. Many wireless units make use of companding, which can negatively affect linearity beyond the point of usefulness. Companding and its effects are described in greater detail below.

2. Time Invariance - Any time-variant phenomena must be avoided. Linear delay values are integral for evaluating measurements (especially phase), so any changes in timing between your measurement and reference signals will make your data unusable. For instance, if you capture a measurement through your wireless unit and find a delay value of 3.29 ms, then measure again without making any changes and see a delay of 3.52 ms, this variation in time can throw off your phase measurements.

What is Companding?

The dynamic range of a microphone can exceed 100 dB from the noise floor to the maximum SPL before distortion occurs. An analog wireless transmission link, however, can only have a dynamic range of less than 60 dB from the link's Radio Frequency (RF) noise floor to the maximum transmitted signal level. In order to transmit a 100 dB range over a link with only a 60dB dynamic range, the signal needs to be compressed. Then, at the receiving end, a process called expansion is used to reverse the compression and restore the full dynamic range of the performer. A compressor followed by an expander is then known as a compander.

Not all companders are created equal, however. A majority of analog wireless mics use a single-band companding scheme, which applies the same levels of compression and expansion across the entire frequency range. This type of companding can produce undesirable audible artifacts. To combat this, many companders feature signal level-dependent ratios of compression and expansion. Unfortunately, this could render the system's EQ ineffective at extreme high or low signal levels. Additionally, most wireless system companders are optimized for voice, so they may not be as effective when trying to get an unbiased measurement of an audio system due to the changes they can induce on frequency, and their inherently non-linear definition.

Digital Wireless: A Solution?

With the advent of digital signal processors, other approaches have been tried to improve the signal-to-noise ratio of wireless systems. Sending digital audio across a wireless link is difficult because it requires too much RF bandwidth to be used in the common frequency bands used for wireless mics. Pure digital wireless microphones typically do not transmit the entire digital audio signal that comes out of the analog-to-digital converter for this reason. This leads to the question: what if the required RF bandwidth could be reduced? 

There are two main schemes used to reduce the required RF bandwidth: 

1. The first is to use digital bit rate reduction. These schemes remove portions of the incoming audio, which the ear is unlikely to miss (but measurement software will almost certainly catch). 

2. The other main scheme is to make use of complex encoding that fits more data into less RF bandwidth. The tradeoff of these complex encoding systems is that they tend to have a more reduced transmission range. 

Most digital wireless systems on the market use a carefully selected blend of both schemes. Some even allow the user to select the blend used to allow a trade-off between audio performance and the number of wireless mics that can be used in a given RF bandwidth.

Digital Hybrid

Lectrosonics has devised a scheme that tries to blend the best of analog and digital wireless systems, which they've dubbed digital hybrid. It is for this reason that Lectrosonics is widely considered to be the gold standard for wireless measurement systems.  More info on their digital hybrid scheme can be found on their website.

Lectrosonics wireless systems fall in two categories - Digital Hybrid and Fully Digital. The Rational Acoustics online store offers two versions of the Digital Hybrid systems in inventory, both based around the HMa transmitter. Check out our Lectrosonics Wireless Measurement Buyers Guide for more information.

Wireless Units We Recommend

• The Lectrosonics TM400 is a digital hybrid wireless wireless unit. When purchased from the Rational Acoustics store, the TM400 includes an HMa transmitter, R400a receiver, XLR pin-swap adapter (required for use with Earthworks microphones), and a foam-fitted water resistant hard case.

• The Lectrosonics ZS-LRHMa is a digital hybrid wireless unit that functions similarly to the TM400, but is a smaller and more portable unit. 

• For people looking to try wireless measurement with less of a financial commitment, the Rational Acoustics store also features a less expensive, entry-level alternative: The Xvive U3C Wireless System. It is important to note that at roughly 1/10th of the price, the performance of the Xvive is not comparable to that of the Lectrosonics units.
  
  
• In between these two extremes lie MiPro's wireless units:
  
  • The ACT-818 Kit is a single-channel full-digital wireless solution that comes with a TA-80 plug-on transmitter and charger. It is a portable, lightweight wireless option.
    
    
  • The ACT-5814 is also full-digital, but features 4 channels of simultaneous wireless measurements. It is available from the Rational Acoustics store as a kit with 4 transmitters.
    
    
  • The MiPro ACT-848 Kit is another 4-channel unit , which comes with 4 transmitters, external chargers, and is powered by a rechargeable lithium ion battery.

Xvive U3C Wireless System

The Xvive U3C is a compact, plug-and-play digital wireless unit with a fairly flat frequency response. It is a single-microphone unit, ideal for small-to-mid-sized venues due to its 90ft of range.

The Xvive units are a great solution for someone's first wireless unit in a non-critical situation. However, since they use WiFi as transmission they are susceptible to radio interference in RF-heavy environments. This interference can sometimes cause additional latency, but this latency should not be an issue so long as it stays stable. A common workaround for wireless unit-originating latency is to take an electronic measurement through the wireless unit (via a physical loopback) to investigate exactly how the wireless affects the system, then simply account for the found latency when discussing measurement delays. This being said, the unit is incredibly effective for use in smaller venues in more rural areas where interference is less common.

When setting your measurement delay in Smaart, you should compensate for any wireless-created latency. I typically recommend that customers do an electronic measurement through their wireless units (just with cables, no speakers/microphones) to investigate exactly how the wireless affects the system. 

Some U3C users have reported some drift in latency, (especially in busy RF environments)  but choose to sell them for the pro-sumer level of wireless needs. We have had success using them for measurement, especially in rural areas. Most gigs Xvive can be used for can be done with a single microphone and a 100' cable. If used in larger spaces, or where frequency range is more crowded, then the Lectrosonics is a better solution.