Archive for the 'Uncategorized' Category

Radio Microphones and Field Recording

April 28, 2011

I have been wanting to test the use of radio microphones for my field recording work for a while now. I finally got round to it.

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.

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Field Recording 2010 Top 10

December 11, 2010

2010 has been a year with amazing field recording work hitting my radar screen. So much so I decided to compile a top 10 of the year with the work that has made the most impression on me or influenced me to considerable extent.

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 Mapsurround2011.bandcamp.com


Esta casa está sonada sound art festival

March 5, 2010

March 5 through March 6 the Esta casa está sonada (This house is a sound experience) sound art event takes place in a house in el barrio de Santa Lucia, Maracaibo, Venezuela (UTC -4:30 hours). The event is part of the annual Veleda Santa Lucia art festival. During the festival residents of the Santa Lucia neighborhood make their house available for the exhibition.

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.

Seesaw installation by Alvin Lucier in Max/MSP

December 6, 2009

From December 21, 1983 though January 18, 1984 Alvin Lucier exhibited his ‘Seesaw’ sound installation in the Film and Video Room of the Whitney Museum of American Art in New York. The installation aimed at having the visitor experience sound waves as a physical presence.

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

Mid-Side recording and Microphone Sensitivity

November 1, 2009

Small technical backgrounder this time.

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.