Sound Devices Astral

The full Astral range comprises three receivers, three transmitters, and a host of accessories.

Wireless audio takes a leap forward with this incredibly flexible system from Sound Devices.

Wireless radio‑mic systems have come a very long way in their sophistication and reliability in the 40‑plus years since I started using them professionally. Of all the enhancements over that time, the most influential change has undoubtedly been the adoption of digital transmissions, bringing a step improvement in sound quality, reception robustness, and remote‑control features.

Today, most of the well‑known microphone manufacturers have their own radio‑mic systems, and there are several specialist manufacturers too. However, it’s a mature market, and at each price level these systems are quite similar in their performance specs and capabilities. That is until a couple of years ago, when Sound Devices — best known for very high‑quality location sound recording equipment — introduced a truly remarkable ‘next generation’ radio‑mic system called Astral, which I think has genuinely revolutionised the multi‑channel radio‑mic market.

Traditionally, wireless mic systems have required a separate receiver for each radio mic transmitter (although some systems offer dual‑ or even quad‑channel rackmount receivers). And most standard receivers occupy a half‑rack format, meaning a 16‑channel rig (which is not unusual for a theatre or a large band) would typically require 8U of rack space just for the receivers, plus more for the aerial distribution amplifiers and power supplies. In other words, a substantial 19‑inch rack full of receiving equipment is required.

By contrast, Sound Devices’ Astral A20‑Nexus provides up to 16 receiver channels within a single half‑width unit. A pair of A20‑Nexus receivers would provide up to 32 receiver channels in just 1U of rack space! That feature alone is astonishing — but it’s only the tip of this technological iceberg.

Although it’s clearly intended mainly for portable film‑shoot style applications, where it benefits from impressive integration with Sound Devices’ 8‑Series digital recorders, the Nexus system can also be used in any other situation requiring very high‑quality multi‑channel digital radio‑mic systems: theatre, broadcast, conference/events, music live sound, or houses of worship.

For portable film/TV shooting applications, the A20‑Nexus has been designed to piggyback onto Sound Devices’ 833, 888 or Scorpio recorders via a beautifully engineered Quick‑Dock mounting plate. But, for independent applications, it can also be used as a standalone unit, or mounted on a custom 1U rack shelf (along with a second unit or mains power supply, if required).

The A20‑Nexus can also be remotely controlled from a smartphone, tablet or computer via a network connection, thanks to a built‑in web server, and even powered over the network, too. These features make it perfectly practical to locate the receiver close to the transmitters on stage or set, while retaining full operational remote control.

Sound Devices offer three Astral transmitters for use with the A20‑Nexus receiver system: the A20‑HH handheld microphone transmitter, and the A20‑Mini and A20‑TX bodypack transmitters (see ‘Transmitters’ box). All three have a wealth of unusual or even unique features enhancing their versatility.

The A20‑Nexus offers an unprecedented number of channels for its size: eight as standard, and optionally up to 16, in a half‑width rack!

A20‑Nexus

As standard, the A20‑Nexus provides eight channels of true‑diversity receivers in the half‑rack unit. However, the number of receiver channels can be expanded through a software licensing scheme to 12 or 16, if required. These optional four‑channel licences can be purchased permanently or as time‑limited expansions for seven or 30 days, so when the owner of a standard eight‑channel system is asked to work on an unusually large project, the option is there to add four or eight additional receiver channels and hire a few extra transmitters, just for the duration. That kind of flexibility and cost‑effectiveness is very attractive indeed for freelance sound recordists or equipment rental services seeking maximum versatility and cost‑efficiency from their investments. Licences are loaded via a thumb drive through the USB port on the rear of the A20‑Nexus, as are firmware updates.

One of the most challenging aspects of running multiple radio‑mic channels is that of frequency allocation — accessing legal and clear channel frequencies within the horrendously congested RF spectrum. This is made more complicated by the different frequency restrictions in different regions and nations. For example, channels that are clear and available in the UK might not be usable in Europe or America.

Most radio‑mic transmitter and receiver systems operate across quite narrow frequency ranges, mostly due to the nature of conventional RF technology. As a result, to be usable in different parts of the world, radio‑mic products are typically offered in a number of variants, each covering different parts of the total RF spectrum.

By way of example, frequencies available for professional radio mics in the UK are scattered between 173MHz and 2.5GHz, fitting delicately in between countless other RF services. However, two particularly popular UHF bands are the PMSE shared‑spectrum (606.5‑613.5 MHz) and the licence‑exempt band (863‑865 MHz). Now, taking Sennheiser’s EW‑D digital radio‑mic products as a typical case, the shared‑spectrum band is covered by the S1‑7 models, but the licence‑exempt band needs the U1‑5 models! Sennheiser offer 10 other model variants, covering alternative portions of the UHF spectrum between 470MHz and 1.8GHz. As you can imagine, ensuring the correct transmitter and receiver models are available to use legal frequencies in different regions or territories can be an expensive and logistical nightmare for a touring band or equipment hire company.

Sound Devices’ A20‑Nexus neatly circumvents that entire problem by offering an industry‑leading tuning range that spans from 169MHz up to 1.5GHz, in 25kHz steps, all within the standard unit. That vast spectrum coverage includes pretty much all the available VHF and UHF licensed and licence‑free channels available in most countries, which is both very impressive and fantastically helpful for sound engineers working on projects all over the world! And the system is clever enough that when you choose a country location in the setup menu, it automatically pre‑selects available legal channel frequencies and blocks certain illegal or sensitive frequencies. (If a licence has been granted to use frequencies that would normally be restricted, an unlock code can be obtained from Sound Devices to regain access.)

When it comes to managing frequency selection in the A20‑Nexus, the unit includes a fully integrated multi‑channel real‑time RF spectrum analyser. Its output can be shown across the unit’s four front‑panel colour OLED touchscreen displays, and it offers the ability to scan selected frequency bands to find clean channels and allocate them automatically or manually to linked transmitters.

RF Technology & Architecture

This cutting‑edge RF technology has been christened SpectraBand by Sound Devices and, in the A20‑Nexus, it involves over 40 SAW‑based front‑end band‑tuning filters, along with a very wide‑ranging, dual‑stage local oscillator system, and high dynamic range (low‑noise, high‑headroom) RF amplifiers. SAW stands for Surface Acoustic Wave (an advanced type of RF filtering technology), and SAW filters are used extensively here because they provide remarkably steep, almost brickwall cutoffs at the upper and lower frequency limits of a given band. Such steep and precise filtering maximises immunity to out‑of‑band interference, thereby increasing significantly the useful reception range and signal reliability for the wanted channels.

Highlighting the efficiency of these SAW filters, Sound Devices’ literature includes measurement plots illustrating 45dB attenuation of close out‑of‑band signals and compares that performance to some competitors’ products which achieve only 12dB attenuation or less. This represents a significant technological advantage.

Not surprisingly, the RF digital modulation system employs a proprietary format, with Standard (low latency), T&M and more robust Long‑Range (high latency) protocol options. Sound Devices claim the Long‑Range mode offers the greatest transmission distance of any digital radio‑mic system currently on the market, while the T&M (Test & Measurement) mode guarantees flat frequency and phase responses to allow radio mics to be used for PA loudspeaker analysis and optimisation. Impressively, these different modulation schemes can be selected on a per‑channel basis.

In terms of the A20‑Nexus’ internal architecture, no expense has been spared! This is a true spatial‑diversity receiver, which means it has two complete RF signal paths, one for each receiving antenna. Not all diversity systems are born equal, and while they all make use of two physically separated receiver antennas, the way they extract and process the captured RF signals differs greatly, as does the resulting performance (see ‘Diversity Systems’ box).

The A20‑Nexus uses a true‑diversity architecture, but the receiver chains are more complex than in typical systems. Each antenna signal passes through a SAW filter array that divides the spectrum into multiple tightly filtered frequency bandwidths (typically 24MHz). Then a low‑noise amplifier boosts the signal for maximum sensitivity with a high overload capability. A second SAW filter array follows to further attenuate out‑of‑band signals, before the signal is combined with the local oscillator output to down‑convert the received RF signal to an intermediate frequency (a process called superheterodyning). This is then routed through a third SAW filter array for further rejection of interference and to prevent aliasing as the signal is converted to a digital format for demodulation processing. And that’s why there are over 40 different SAW filters involved in this RF processing section!

While this RF processing is undoubtedly very impressive and state‑of‑the art technology, the really clever stuff happens next, in a field‑programmable gate array (FPGA) — a form of bespoke, highly customised, parallel microprocessor. It is this FPGA which performs further filtering, frequency conversion and demodulation of up to 16 radio‑mic channels, as well as diversity‑combining the outputs of the two RF receiver chains at the digital bit level. The processed outputs of the FPGA are routed to a set of D‑A converters for the analogue outputs, and to AES3 and Dante digital formatting stages.

NexLink

The A20‑Nexus is not only a multi‑channel receiver: it also incorporates its own 2.4GHz transmitter. This system, called NexLink, communicates with assigned radio‑mic transmitters in the Wi‑Fi band (using a bespoke protocol) to share transmission frequencies, transmission modes and timecode synchronisation, amongst other things.

The NexLink transmission range is designed to be slightly greater than the A20‑Nexus’ reception range, such that if a radio mic can be heard it can also be controlled! The supplied 2.4GHz aerials can be attached conveniently to the sides of the A20‑Nexus when installed in a recordist’s bag, or mounted externally in a variety of ways.

Connections

Unsurprisingly, given the A20‑Nexus’ capabilities and versatility, it is festooned with connections. On the RF side there are two standard 50Ω BNC sockets at the front for connecting the included Monarch diversity receiver aerials. A 12V ‘bias’ voltage (200mA from each BNC socket) is available for active antennae or head amps. These Monarch aerials — so‑called because of their similarity to butterfly wings — are omnidirectional wide‑bandwidth designs covering the entire 470‑1525 MHz range. Photography‑standard quarter‑inch mounting threads allow attachment to supplied articulating arms with multifunction clamps to mount these antennae as required.

Another pair of 50Ω BNCs on the rear of the unit normally provides buffered (‑3dB) RF outputs for cascading antenna connections to other units. However, they can be configured as alternative aerial inputs, if preferred. A pair of SMA screw connectors provide antennae outputs for the supplied NexLink transmission aerials.

The audio output options on the A20‑Nexus are nothing short of mind‑blowing!

The audio output options on the A20‑Nexus are nothing short of mind‑blowing! Two AES59‑compatible D‑Sub sockets can be configured individually to provide eight channels of either mic‑level (‑20dBu for 0dBFS) or line‑level (+20dBu for 0dBFS) balanced audio outputs, and selected to output channels 1‑8 or 9‑16, if present. Alternatively, they can be switched (individually) to carry eight channels in the AES3 digital format (at 44.1, 48 or 96 kHz). If the A20‑Nexus is upgraded for 12 or 16 receiver channels, both AES59 sockets must be used together to access all the output signals, of course.

Dante Audio‑over‑IP (at 44.1 or 48 kHz) is also available as a standard feature, via either a pair of RJ45 sockets or an optional SFP optical‑fibre transceiver module, which can be slotted into the left‑hand side of the unit. Any or all of these three ports can also be used for remote control of the unit over an Ethernet network. Such connection versatility therefore allows the eight‑channel A20‑Nexus model to provide (balanced mic or line) analogue, AES3 and Dante outputs simultaneously, which is extraordinary!

The sample rate of the AES3 and Dante digital outputs can be selected from internal or external clock sources, and a 75Ω BNC socket is provided to receive either an external word clock signal or a linear timecode input (which can still be used to synchronise the internal clock). The sample rate is selectable from internal crystals at 44.1, 48 or 96 kHz, or the BNC word clock (or timecode) input, the Dante network connection or (if the Nexus is docked to an 8‑Series recorder) the recorder’s current sample rate.

If connected to a Dante network, the receiver’s source name defaults to A20‑Nexus XXXXXX, where the Xs are the last six characters of the unit’s MAC address. However, a bespoke name can be configured instead, and the individual receiver outputs are identified as 20Nexus_Tx1, 20Nexus_Tx2 and so on — all much as expected. However, what I definitely didn’t expect is an option to route incoming Dante channels 1‑8 and/or 9‑16 across to one or both of the AES59 sockets as analogue or AES3 outputs! This is a really useful feature which can be used in many different ways. For example, if the A20‑Nexus is physically located near the stage or film set, picking up signals from body‑pack transmitters and sending them back to the location mixer over Dante, playback, foldback and comms signals can all be sent back to the receiver over the same Dante connections for use by the crew and actors/performers.

Having said all that, if the A20‑Nexus is mounted directly onto a Sound Devices 8‑Series recorder (via the bespoke mounting plate, which uses a special multi‑pin connector on the bottom panel), both AES59 sockets and the Dante connections are automatically disabled — presumably as a power‑saving feature.

Channel output is on two DB25 connectors, and can be either analogue (mic or line level) or digital (AES3 or AES59). There is provision for multiple antannae, Ethernet control and PoE, dual‑redundant power supplies, and Dante networking. The USB‑A port is for connecting a keyboard for naming channels, pairing with transmitters, battery charging and firmware/licence updates.

Powering

Talking of power… the A20‑Nexus requires 10‑18 V DC (supplying at least 2A) which can be connected via a pair of dual‑redundant TA4 sockets (the one receiving the highest voltage is given priority). That DC power can come from any standard battery system, or from an optional Sound Devices XL‑WPTA4 60W mains power supply. The A20‑Nexus consumes 11W at idle and up to 35W when it’s really busy!

Alternatively, the unit can be powered via its Ethernet sockets from a 30W (803.at) PoE supply (not the older 803.af standard which can only provide 15W). If mounted to an 8‑Series recorder the A20‑Nexus is automatically powered via its Quick‑Dock baseplate connections, from the recorder.

A pair of standard four‑pin Hirose DC output sockets are also included, and they can be switched individually from the menu screens to link power through to external low‑power devices (providing up to 3W or 250mA each).

Both USB‑A and USB‑C sockets are present on the rear panel, although the latter is currently reserved for factory use only. The USB‑A socket is used for connecting a keyboard to name channels and other parameters, or to mount a thumb drive for firmware updates and save or load settings. It is also used to connect handheld and body‑pack transmitters for pairing and sync’ing their internal timecode generators. A removable panel to the right of the RJ45 sockets accesses factory configuration ports that don’t hint at future feature expansions.

Moving around to the front panel, there are the two BNC receiver antenna inputs as already mentioned, plus a 3.5mm headphone socket and a volume control/data encoder knob.

Configuration & Use

Configuration is handled through four crisp, clear, touch‑sensitive, colour OLED screens, which extend across 6.2 inches of panel width. There are far too many setup and user options to describe in detail here although, despite the A20‑Nexus’ enormous complexity, the user manual has only 70 pages (in large part because there is a lot of assumed knowledge, as befits an experienced, professional user base).

The receiver unit is powered up by pressing the orange triangle button on the right‑hand side. Once running, the same button cycles through various operating modes (receiver view, main menu, spectrum analyser view), and is also used to back out of lower menu pages. Most functions are selected by touching the screens, and parameters are adjusted by touching drop‑down options or by using the rotary encoder, which also doubles as the headphone volume control when not navigating the menus. Sensibly, the screen touch‑sensitivity function can be disabled to prevent accidental operation, and this is achieved simply by pressing the headphone knob for more than two seconds (same again to re‑enable). A thin orange border appears around each screen to indicate the locked state. A ring of LEDs around the encoder knob indicates the unit’s current status: blue while powering up; red if the headphone amp is clipping; flashing orange if there is a digital audio clocking error; and green when the front panel is locked out.

When first powered up, the default view shows all eight receiver channels, two per OLED screen. Other display options show one, four, or all (12 or 16) channels. Selecting the Main Menu page accesses 12 further sub‑menus for functions like the real‑time spectrum analyser, RF configuration (country, antenna connections, tuning bands...), channel auto‑assign, audio output allocations and formats, power options, timecode settings, NexLink functions and transmitter pairings, network control, and core system settings (amongst others).

When fully configured and operating, those eight receiver screens provide comprehensive information for each receiver channel, although the layout varies if fewer channels are displayed. In the eight‑channel mode a user‑entered channel name occupies the top of the screen, with displays below for battery status, NexLink communication status, recording mode (in the transmitter itself), and the channel frequency.

Moving on down, there is an RF history meter, and below that four vertical bargraphs showing the signal strength from each antenna for this specific channel, the overall received signal quality, and the audio output level.

I have to say configuring and controlling the A20‑Nexus, although inherently complex, is remarkably logical and straightforward, with excellent graphics and intuitive menu operations — much like the operating paradigms employed on other Sound Devices products, such as the MixPre recorders. Full familiarisation and confidence would take some time, naturally, but the learning curve is not steep, and I felt quite comfortable using the receiver within a day of exploring it.

Impeccable Sound Quality

As would be expected of any Sound Devices product, the sound quality is impeccable, and I couldn’t fault it in any way. The audio bandwidth extends between 10Hz and 20kHz ( ±1dB) and the transmitted dynamic range exceeds 130dB, with two of the transmitter options manging in excess of 140dB!

To achieve that, the microphone preamps in the transmitters are derived from those used in the flagship 8‑Series recorders, with a dynamic range of more than 140dB, and work the same patented multi‑stage floating‑point A‑D conversion system used in Sound Devices’ floating‑point audio recorders. The digital transmission system uses a floating‑point modulation scheme to maintain the dynamic range, too.

These technologies allow the full dynamic range of any microphone capsule to be conveyed over the digital radio link unimpaired. In practical terms, this means that there is no need for gain control or compression at the transmitter. The audio output signal level is controlled only at the receiver, simply by processing the received digital signal — a feature Sound Devices call GainForward. So, provided the mic capsule itself isn’t physically overloaded, the signal level (as well as a low‑cut filter and polarity inversion) can always be adjusted easily and instantly at the receiver while maintaining the full source dynamic range. To benefit fully from this technology, it is important to set a generous headroom margin to minimise any risk of clipping, while the floating‑point conversion guarantees an amazingly low noise floor.

All digital radio‑mic systems involve some signal latency, of course, because of the A‑D conversion, signal coding, error protection and transmission formatting involved. However, the A20‑Nexus’ latency is amongst the lowest currently available from any digital radio‑mic system, at just 2ms in the standard operating mode.

As mentioned earlier, the Nexus also offers a Long Range mode, which uses a more robust coding and modulation technique to protect against damage over longer transmission paths. Not surprisingly, this mode comes with a longer audio latency of 3.9ms, but for most film and TV shoot applications latency is largely irrelevant, and even 3.9ms is perfectly acceptable as it’s only about a 10th of a video frame. In this context, latency is only likely to cause problems if performers have foldback to earpieces, for example, and even then many will not notice and cope perfectly well!

The ability to control eight receivers across pretty much any frequency in the RF spectrum, from such a compact unit, and with so many and such versatile output options, just blows my mind.

Conclusion

I wasn’t sure what to expect when I unpacked the A20‑Nexus system and associated transmitters. I’ve used a lot of professional analogue radio‑mic systems over the years, and currently have a three‑channel digital Sennheiser EW‑D system operating in the PMSE shared‑spectrum band. The EW‑D system impresses me greatly in terms of its sound quality, signal robustness and reliability, performing very well in comparison to the A20‑Nexus (and beating it fractionally in terms of latency at 1.9ms). However, I was astounded at the transmission ranges I could achieve with the A20‑Nexus, even in standard mode, and I gave up walking away in the Long Range mode; it seemed unbreakable!

The ability to control eight receivers across pretty much any frequency in the RF spectrum, from such a compact unit, and with so many and such versatile output options, just blows my mind. If I were working as a sound recordist, this would be such a neat, versatile and immensely capable solution to multi‑channel radio‑mic usage.

I was also bowled over with all three transmitter options. The handheld is nicely balanced, feels very solid and robust, and sounds perfect. The built‑in recorder is a nice feature, as are the options for different third‑party capsules, switching functions, and even the counterweight. This transmitter has clearly had so much careful thought in its design, with input from experienced users.

The standard body pack is another mind‑melter! Not only unbalanced and balanced mic input options, but balanced line and digital inputs, and local recording too? It even boasts a very configurable guitar input mode — no‑one else offers such amazing and practical versatility in one unit. And for performers who need to conceal their body‑pack transmitter, the mini version is cleverly scaled down and should prove easy to hide, while sacrificing very few features.

The whole Astral family of products is exceptional in its user‑focused feature sets, its cutting‑edge technology, its robust engineering, and its sheer versatility and practicality. Yes, it is eye‑wateringly expensive, but there’s no doubting how much is provided for the money. Obviously, this is not pub stage gear. This is for serious professionals who genuinely need the flexibility and reliability for daily use in challenging situations, and I feel quite privileged to have been able to use the A20‑Nexus and enjoy a taste of this rarefied high‑end technology.