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...