Why Digital Wireless?
Wireless systems are designed to replace signal cables. While cables are simple and reliable, many times going ‘hardline’ is impractical or impossible. A wireless system, such as the A10 Digital Wireless System, that sounds and performs as much like a cable as possible is the next best thing.
Wireless systems can be digital or analog. In a digital wireless system, audio performance remains perfect until range is exceeded and the system drops out. In analog systems, the noise floor gradually increases as the edge of range is approached. Although digital and analog systems behave differently, the result is the same. When the system is out of range, the audio is unusable.
So why choose a digital system? With regulators around the world selling off frequencies in the UHF band to mobile phone companies, it is becoming more difficult to operate wireless systems. Analog systems require very careful frequency coordination to operate without intermodulation effects. Typically, only 8 systems can be used in one 8 MHz TV channel.
Digital wireless systems can make more efficient use of the limited spectrum by being able to operate up to 20 wireless systems in one 8 MHz TV channel. With the A10 Digital Wireless System, 20 transmitters can be spaced evenly with only 400 kHz in between. Additionally, digital systems benefit from up to 10 dB greater immunity from interference compared to analog wireless systems. This means that your digital wireless system will provide perfect audio in the presence of interference, albeit with reduced range. In an analog system, the interference would render the audio unusable.
Getting the best results from a wireless system requires proper antenna selection and use, and a reliable, consistent RF performance.
Let’s explore how to get the best from digital wireless.
Picking the Right Antenna for Digital Wireless Audio
Antennas are critical components of a wireless system. In many ways, they perform like microphones with receivers, and loudspeakers with transmitters. Just like with microphones and loudspeakers, there is no single antenna suitable for all applications. A strong, predictable RF performance requires the use of the right antenna.
¼ Wave Whip
¼ wave whip antennas are included with most wireless systems, including the A10 Digital Wireless System, for both transmitters and receivers. Whips are omnidirectional antennas suitable for many applications, and are used on bodypack transmitters for their simplicity and portability. ¼ whip antennas must be mounted directly to the transmitter or receiver. They cannot be remotely mounted because they rely on the chassis of the transmitter or receiver for their ground plane.
When using bodypack transmitters, keep some distance around the antenna. Allowing the antenna to touch skin or damp clothing directly will dramatically reduce the transmitter’s RF output. Just a few mm of distance from the skin can result in 10 dB more output, which has a direct impact on the range of the system. It is also important to use the proper length ¼ wave antenna for the operating frequency. Folding or bending the antenna by inserting antenna-first into a pocket, etc, dramatically reduces the effectiveness of the antenna.
When to use: ¼ wave whip antennas are normally used in portable systems, such as one or two RXs being deployed in a bag, or when the RX is mounted on and linked to a camera.
¼ wave whip antenna included with the A10 System
Half Wave Dipole
Compared to ¼ wave whips, half wave dipole antennas have an improved system range. Like ¼ wave whips, dipoles are omnidirectional. An important benefit of the half wave dipole is its ability to be remotely mounted due to its built-in ground plane.
Dipole antennas have an operating bandwidth of approximately 50-60 MHz, so check that the antenna is designed for the intended frequency range. This is especially important when using receivers which tune across a broad range. Some dipoles are frequency adjustable, which allows the center frequency to be tuned.
When to use: For bag use, portable dipole antennas can be clipped to a bag or strap and connected via cable to an RF distribution amplifier or a slot mount rack, such as the Sound Devices SL-6. Dipole antennas can also be used on carts, especially in small studio environments.
Photo of ½ wave dipole antenna pic, courtesy of Badland Ltd
LPDA antennas, or log periodic dipole array, are a common directional antenna used for wireless microphone systems. On a sound cart, LPDA are used for both receiver and IFB transmitter applications. LPDA, sometimes referred to as “shark fin” antennas, are remotely mounted. These directional antennas have a theoretical gain advantage of up to 8 dB over a ¼ wave whip. Some LPDA antennas include active circuitry. Active antennas have built-in RF amplifiers which can be engaged to make up for signal loss with long cable runs.
When transmitters are in close proximity to an LPDA antenna, there is a potential to overload the receiver. Reduce TX power or deploy RF attenuation after the antenna, before the receiver.
When to use: LPDA antennas are best used outdoors or to increase the range of the system. LPDA are directional, so ensure that the transmitters are in the field of view of the antennas when in use. These antennas operate over a wider range of frequencies than the Yagi type (below).
Betso LPDA antenna. Photo courtesy of Betso.
Yagi antennas are another type of directional antenna. The benefit of a Yagi is its high gain and directionality. The downside of using a Yagi is its narrow operating frequency range and the narrowing angle of pick-up as the number of elements on the antenna increases. For users requiring frequency agility, a Yagi may not be practical.
When to use: Like LPDA antennas, Yagi antennas are best used outdoors, or to increase the range of the system. Yagi antennas have higher gain than LPDA antennas.
7-Element Yagi Antenna
Circularly Polarized Helical
Another high-gain directional antenna is the helical antenna. On-talent transmitters with fixed ¼ whip antennas can often end up in unpredictable orientations, or polarity, relative to receiving antennas. Helical, circularly polarized antennas operate with the same efficiency regardless of antenna polarization, resulting in fewer dropouts.
When to use: Helical antennas are often used for in-ear monitoring transmitter applications as well as receiver applications, and to circumvent issues with polarity on-set.
Professional Wireless helical antenna
Choosing a Cable
Quality 50 Ω RF cables are an important component of a wireless system. As cable lengths increase, so too does signal loss. RG58 cable is commonly used for antenna-to-receiver connections because it is reasonable in size, flexibility, and cost. It has an RF loss of 4 dB every 10 m at 400 MHz (see chart below). When cable losses approach more than 6 dB, consider applying gain to make up for the loss. The goal is to achieve unity gain, no more.
WBC400 cable is much lower loss at the equivalent frequency, but it is also unwieldy and more suitable for fixed installations than location use.
Typical cable loss chart at various UHF frequencies.
Note: Coaxial cables used to interconnect video signals are typically 75 Ω with 75 Ω BNC connectors. While they may look identical to 50 Ω cables and connectors, there is an added signal loss when using 75 Ω cables.
Minimizing RF Problems when using Digital Wireless Audio
Minimizing RF interference is essential to achieve the best wireless audio possible. Several common issues which result in dropouts and short range include:
- Interference from other RF sources (RFI). This includes DTV broadcast television, gigantic video walls, and RF video links. RF video systems can be the source of interference, even though most of these systems operate outside the UHF band at 5.8 GHz. From our experience, wireless video links can interfere with any UHF-band audio system by any manufacturer, whether analog or digital. Solution: when possible, work away from, or turn off, other RF sources. Look for other possible sources of interference.
- Transmitters are too close to the receiver. With digital systems in particular, it is recommended to keep at least ten feet/three meters between all transmit and receive antennas. Transmitters placed too close to receiving antennas can overload the receiver’s front end, rendering the system unstable and prone to dropouts. Solution: Reducing transmitter output power can improve performance in close-proximity situations.
- Camera, IFB transmitters, and walkie-talkies are in close proximity to receivers. This is a variation of the issue above. A transmitter in close proximity to a receiver can desensitize it, reducing range. Solution: Maximize the distance between receiver antennas and transmitter antenna in a bag with both wireless receivers and transmitters (for camera hops or IFB).
- Too much RF gain in the system. Directional antennas inherently add “passive gain.” A typical shark fin reduces the pickup pattern to 120 degrees and adds 7 dB of gain. Active antenna amplifiers are great tools, but are only intended to compensate for cable loss: adding 12 dB of amplifier gain to overcome 6 dB of cable loss does not give 6 dB of range in addition. A directional antenna plus an antenna amplifier can be particularly problematic, and overload the system if the transmitters get too close to the antenna. RF gain applied when the system already has sufficient gain can overload the receiver front end and cause dropouts. Solution: reduce RF gain and avoid antenna amplifiers when unnecessary.
- Antenna frequency mismatch. Antennas operate most efficiently at a specific frequency. Mismatching frequencies negatively impacts range. Solution: Make certain to use both transmit and receive antennas that are the correct length, or tuning.
Maximizing Wireless Range When Using a Production Bag
When working out of a production bag, wireless receivers are generally carried in close proximity to each other and to mixers and recorders. In some cases, multiple receive antennas, generally whips, are directly connected to the receiver. To maximize range when working out of a bag, there are some basic guidelines to consider:
- Keep any transmit antennas (IFBs, walkie-talkies, cell phones) as far away from the receivers’ antennas as possible. The minimum acceptable distance is 8 inches. Even if the transmitters are on vastly different frequencies from the receivers, the receivers’ front ends can still become desensitised.
- Mount receivers in the bag so that the base of the antenna connector is above the level of the dividers in the bag, away from the body. The goal is to have the receiver antennas be in line-of-sight with any transmitting antennas. This is particularly important when using ¼ wave antennas mounted directly to the receiver.
- When using the A10-RX, we recommend the use of one straight and one right-angled antenna on each of the receiver’s antenna ports. Make sure the receiver’s antennas are perpendicular to the A10-TX.
- Mount ¼ wave whip antennas directly to the receiver, not remotely, since a ¼ wave antenna requires a ground plane. The shell of the antenna connector needs to make a solid electrical connection to the receiver’s chassis in order to function correctly.
- When practical, a slot receiver chassis, such as the A10-RACK with built-in RF distribution, or a portable antenna distribution, such as the SL-2, can improve performance by allowing multiple receivers to be connected to a single or pair of remote-mounted antennas.
SL-6 receiver chassis mounted to a Scorpio
Maximizing Wireless Range Indoors and Outdoors
When on stage or in the studio, transmitters are often in close proximity to receivers. In these situations, it is good practice to reduce the transmitter output power and to use omnidirectional ¼ wave whip or ½ wave dipole antennas. These can be mounted directly to the receiver, or to a slot receiver chassis like an A10-RACK that provides good coverage. The advanced digital diversity technology in the A10-RX receivers take advantage of multipath, or reflected RF signals.
A10-RACK with A10-RX mounted
When using wireless systems outdoors, multi-element Yagi or a LPDA receive antennas increase range. With active, wideband LPDA-type antennas, it is paramount that the gain on the amplifier is applied only to overcome the loss due to the cable run from the antennas to the system. The cable length on each side of a diversity receiver should be equal. Unequal lengths could favor one side of a diversity, making the system perform like a non-diversity system with more frequent dropouts.
A10 Digital Wireless Performance Optimization
Mixed Analog/Digital Environments
Frequency selection when using multiple A10 Digital Systems is simple: make certain selected frequencies are clear of interference and that systems are tuned at least 400 kHz apart. Since intermods are not an issue with the A10 Digital System, no frequency planning or software is necessary.
Frequency selection of multiple analog wireless systems requires an intermodulation-free frequency plan so that each system operates without interference. Similarly, in systems with both analog and A10 Digital Wireless Systems, the A10 Digital Wireless Systems frequencies need to be selected as if they were analog systems.
For example, the Audio Ltd RMS2040 system on CH 38 has the following intermodulation-free frequencies:
1 – 606.05
2 – 606.50
3 – 607.90
4 – 610.30
5 – 611.10
6 – 612.35
7 – 613.30
8 – 619.90
In this example, the first four italicized frequencies are used for 2040 analogue systems. Additional systems, whether analog 2040 systems or A10 Systems, need to be set one of the four bolded frequencies in order to continue with an intermod-free plan for the first four systems.
A similar process can be applied to other manufacturers’ intermodulation-free plans.
Because intermod distortion needs to be considered for a mixed environment, the scan operation on the A10-RX cannot be relied upon to generate usable frequencies. The A10 frequencies should be selected in a clean part of the spectrum, but also selected to ensure that the frequency is coordinated with an intermodulation-free plan specified by the manufacturer of the analog system.
A Note On A10 System Heat
Digital wireless systems like the A10 use more power compared to analog wireless systems. This is mainly due to the linearity requirements of the front-end receiver amplifiers and very linear transmitter power amplifiers. This linearity ensures very low distortion digital modulation.
Both the A10 transmitter and receiver are designed to operate over a very wide temperature envelope and will warm up, especially in ambient temperatures in excess of 25 degrees C. Many systems are being used in hot climates around the world. Customer reports of warm transmitters are often due to the use of NiMh batteries in the transmitter, which produce heat when in use. When lithium batteries are used, this heating effect is not much more than that of an analog transmitter. Thus, we recommend using lithium batteries in the A10 Digital Wireless System.