Radio mics or wireless microphones are two terms commonly used for a number of different types of devices. In the broad sense the terms refer to any wireless system used to transmit audio signals over a wireless connection. In a narrow sense the terms refer to a wireless enabled microphone such as the Shure SM58.

Wireless systems are common in concert, drama and film applications both for PA and monitoring. Singers performing on stage use dynamic or condenser hand held microphones with a wireless transmitter built in. In drama/theatre or film it is mostly lavalier microphones connected to a wireless transmitter body pack that are being used. All of these systems usually use the UHF 470 MHz to 870 MHz bands for transmission. Which frequencies you will be able to use is dependent on country specific regulations. Often you will need a license to be allowed use of the equipment.

Wireless microphone systems operate in the same frequency ranges as TV channels and wireless internet (wireless DSL). As of 2012 the new LTE (4G) mobile phone networks will impact the usage of wireless mics and wireless personal monitors quite heavily. Strict usage regulations to be expected.

Manufacturers like AKG, Sennheiser and Shure have wireless systems in their product offering but these are geared toward stage performance. For field recording one needs to look at wireless systems for ENG/ EFP applications. To me only two manufacturers offer systems potentially of interest to the field recordist: Lectrosonics and Micron. Their hardware fits in an audio bag, the build quality is rock solid and the audio quality is excellent. I am ruling out Zaxcom because their systems low pass everything at 16kHz.

The obvious advantage of wireless is mobility. No cable clutter and less weight to carry around. But what about audio quality?

I tested the Micron TX700B transmitter SDR550 receiver combination with a DPA 4060 miniature microphone.

Frequency response of this system is 80Hz to 20kHz. S/N ratio is better then 100dB. The receiver uses a diversity design meaning two antennas are used to prevent audio from dropping out. This works really well. I have experienced no audio drop-outs even in the RF hostile city where I live.

Much of the discussion on audio quality in wireless systems evolves around the companding that is used on the radio connection. The dynamic range of the audio is compressed on the transmitter side, usually in a 2:1 ratio. An expander restores the audio to the original values at the receiver side. Depending on the technology used side effects referred to as “breathing” can sometimes be heard as a result of this signal processing. The “breathing” can occur by a sudden rise in the noise floor on quiet sections of the audio.

Here again the Micron system performed extremely well without any side effects.

In all I am quite happy using this system for field recording use. I can position the DPA microphones in locations I would be unable to reach using cables. The audio quality is more then acceptable. I have had no trouble with signal strength with distances as far as 80 meters.

I will continue the testing with phantom powered condenser microphones, probably the Sennheiser MKH 8020.

Framework Radio; www.frameworkradio.net; run by Patrick McGinley and broadcast on-line on Resonance 104.4 FM. Dedicated to phonography, field recording and it’s use in composition,

Fluid Radio; www.fluid-radio.co.uk; true internet radio for experimental acoustic and vocal work,

Klara; radio.klara.be; the Brussels, Belgium based radio station’s Mixtuur show airs and streams live Monday though Thursday. Arnold Schönberg to Max Richter and everything in between and on the side,

Mixcloud; www.mixcloud.com; while very diverse in broadcast categories has selections brought together by Taylor Deupree, Seaworthy and Matt Rösner, Celer, … and check out Mary Ann Hobbs,

Take Away Shows; www.takeawayshows.com; music video documentaries from all over produced and directed by Vincent Moon (and others). A minimal approach brings out the love for the artists and the artists’ passion for their music,

Touch Radio; www.touchradio.org.uk; published by Touch at regular intervals always comes up with creative work in areas such as field recording, sound installations and performances. New and unexpected with every issue.

Not all of the work listed was published in 2010. And not all of the work are albums or CD releases. Field recording artists use Bandcamp and Soundcloud as much as any other artist. Perhaps even these media are more important to sound artists compared to others to get their work the exposure it deserves.

My selection is not exclusively ‘field recording’. Some of the releases are musical compositions that take field recordings as their building blocks. All of the releases I feel are explorations in sound.

Jana Winderen, Energy Field, Touch
Seattle Phonographers Union, Seattle Phonographers Union,  and/OAR
Chris Watson, Jana Winderen and others, Sleppet, +3dB Records
Francisco Lopèz and others, Dérives, Universinternational
Seaworthy & Matt Rösner, Two Lakes, 12k
Richard Ranft, Rainforest Requiem, British Library Sound Archive
Solo Andata, Ritual, Desire Path Recordings
Mark Brennan, wildearthvoices, soundcloud.com/wildearthvoices
Louis Antero, Sound Narratives vol. 3, mimi
Surround2011, Zushi Sound Map, surround2011.bandcamp.com

While the 96 kHz, 24 bit suggests high quality audio recordings it is the integrated microphones that spoil it all.  Unfortunately not many manufacturers document the specs of the microphones on their devices. Even elementary information like whether the mics are cardioid or omni is often lacking. Instead we are left with descriptions like “Internal Stereo Microphone”. No noise performance indications or frequency response information. Clearly a sign most of these devices are not positioned as devices for the professional sound recordist.

Luckily a few websites come to our rescue. Of note here are www.wingfieldaudio.com who provide excellent noise floor samples and www.avisoft.com who provide both noise floor samples for a few models and EIN data for a fairly complete range of portable recorders available in the market today.

In my experience an Equivalent Input Noise number of -120dBu, unweighted and at maximum gain setting is the minimum for the field recordist to be able to make decent recordings. Also, a frequency response of 40Hz through 20kHz is a minimum specification to be met.

On the noise spec that rules out popular recorders such as the M-Audio Micro Track II and the Edirol R-09HR. These might prove effective in high dBFS music recording environments but are pretty useless to record in quiet environments.

Two recorders in my view perform particularly well: the Sony PCM-M10 and the Sony PCM-D1.

The PCM-M10 has very quiet omnidirectional built in microphones. Omnidirectional microphones are less sensitive to handling noise (nice to have on a handheld device) and have extended low frequency response.

The PCM-D1 has Analog Devices pre-amps that are virtually noiseless. The microphones are cardioids mounted in an XY coincident pattern. While this would suggest a narrow stereo field Sony managed somehow to overcome this and give recordings made this device a wide stereo image. Might be a device that is hard to find these days as it seems to have been discontinued in some countries in favor of the PCM-D50.

The microphones on both recorders are high sensitivity so dealing with quiet environments is no problem. The D1 has better high-end response compared to the M10.

Samples of recordings I made:

PCM-M10: JanuarySleet.wav, ThunderandRain.wav

PCM-D1: QuietForestSpringEvening.aif, LakeSide2.wav

One of the houses is transformed into a a space to experience sound: “The sound house will be a space for the transmission and experimentation of sound art during la Velada de Santa Lucia: a family home as a sound container.”

The event features my ‘ResonantAtrium’ sound piece on day two, 11PM local time.

In a small Max/MSP project I set out to recreate the experience.

The idea is to have two sine waves generated in a quiet room, one at a fixed frequency of 256Hz, the other sweeping between 255.8 and 256.2 Hz.

A spinning is noticed that accelerates to once every 5 seconds as the sweeping oscillator frequency drops from the reference 256 to 255.8 cycles per second. Movement is from the speaker with the highest frequency to the one with the lower. As the sweeping oscillator raises its frequency the sweeping reverses. For full experience the room must be free of reverb.

For now my patch requires manual operation of the sweeping oscillator. In the original installation the sweep down 0.2 cycles, up 0.4 cycles past unison and down to unison again would take 3 minutes. Automating this will be for version 2 of the patch.

The spectroscope window provides on screen visual feedback about the (slow) beats occurring.

Literature:

Alvin Lucier: Reflexions Reflexionen; Interviews, Scores, Writings 1965-1994; MusikTexte; Köln, 1995

When recording stereo it is essential to have equal gain settings for the microphones at the risk of the stereo image shifting to either left or right. More gain on the right mic channel will shift the image to the right and the other way around. Equal gain settings are especially important in XY, ORTF or any other spaced or coincident microphone positioning.

With mid-side recording the gain setting for the mid and side channels allow you control the stereo spread. More gain on the side channel compared to mid will widen the stereo image. More gain on the mid channel will narrow the image. Ultimately, no gain on the side channel will leave you with a centered mono signal from the mid mic.

Reading gain or trim pot settings is easy enough but you usually end up with differences in signal strength if you don’t take into account microphone sensitivity even with an equal gain setting on your recorder or mixer. Unless you have a matched pair, microphones have different sensitivity characteristics.

A typical MS setup is the Sennheiser MKH60/ MKH30 microphone pair.

The MKH60 has a sensitivity of 40mV/Pa, the MKH30 has 25mV/Pa. With equal gain you will end up with a much stronger mid signal and consequently a narrow stereo image.

The calculation to compensate for sensitivity is as follows:

±dB = 20 x log(MmV/SmV)

where

MmV = sensitivity of the mid mic in mV/Pa
SmV = sensitivity of the side mic in mV/Pa

In our example we need 4.1dB more gain on the side channel compared to the mid channel to get equal signal strength on both of the recorded channels.

This is easy to realize on mixers such as the Sound Devices 302. With MS stereo linking set trim pots allow you to set gain for the mid and side channels separately. The input 2 fader controls the overall gain for the pair.

Recorded with a Sennheiser MKH8020/ MKH30 MS stereo microphone configuration to a Sound Devices 702 recorder. MS converted to AB stereo at the recorder stage.

SoundCloud uploads are transcoded to 128 kbps mp3 format for streaming. That’s still twice as good as MySpace.

Without any exception these plug-ins were engineered with music in mind and that makes life for the field recordist slightly more difficult. While the Abbey Road TG12413 compressor/ limiter plug-in is a brilliant piece of software that does wonders on a drum kit, few field recordists will find a use for it. Vintage style EQ plug-ins can bring excessive coloration to a recording. Many plug-in developers will advertise the ‘warmth’ their processors will bring to your recorded material but that does not interest me, and I suspect most field recordists,  at all. What I am looking for is audio processors that keep the recorded material intact. No added warmth required, not looking for increased presence or transients adjustment to obtain better attack.

My search for the ideal field recording plug-in pack is an on-going thing but here is the result so-far:

EQ: Waves Q10 Paragraphic equalizer, DDMF LP10 Mastering Equalizer

Dynamics: Flux Pure Compressor II, PSP Xenon Limiter

Channel strip: Waves Audio Track

Audio restoration: iZotope RX

Analysis: RNDigital Labs Inspector XL

Go check these guys out:

http://www.myspace.com/adancingbeggar