Distance Coding And Performance Of The Mark 5 And St350 Soundfield Microphones And Their Suitability For Ambisonic Reproduction

Reproduced Sound 25 Dr Bruce Wiggins Tom Spenceley Signal Processing Applications Research Group University of Derby DISTANCE CODING AND PERFORMANCE OF THE MARK 5 AND ST350 SOUNDFIELD MICROPHONES AND THEIR SUITABILITY FOR AMBISONIC REPRODUCTION [email_address] http://www.derby.ac.uk/staff-search/dr-bruce-wiggins

Ambisonics - What is it?
What is Ambisonics?
A system where the spatial encoding of the audio is separate from the decoding of that audio to a set of speakers.
A set of rules and equations that help to optimise and quantify the performance of a multi-speaker audio presentation.
A system based on coincident recording principles.
An extendable system based on Spherical Harmonics.
A future proof audio format.
Distance Cues
Simulation extensive studies by Daniel et al.
1 st order (pressure gradient) and higher order components exhibit proximity, or near field, effect.
The decoding of this reproduces the curvature of the wavefront from the finite number of speakers.

Ambisonic Encoding – B Format
The encoding, or recorded format is known as B-Format, which consists of 4 channels, W, X, Y & Z.
It is made up of 0 th and 1 st degree spherical harmonics (which equate to standard pressure and pressure gradient polar patterns).

Proximity/Near Field Effect
Magnitude/Phase changes at low frequencies due to spherical wave front.
Assuming the pressure amplitude is unity at 1m, the pressure p for an on-axis point x metres from a sine wave source can be found by:

To obtain the first-order component response to this pressure, the time-integral of the gradient of must be found:
Depends on Distance Depends on Distance & Frequency

As the two terms are in quadrature, the 3dB point is when the two terms are equal.
Source at 1m Source at 0.5m

Speaker Compensation
If the speakers reproducing these signals emit plane waves, then just reproduce these signals.
However, if the speakers are closer(!), then they will reproduce spherical waves.
For this reason, it’s necessary to ‘undo’ the proximity effect dependant on the speaker distance.
Essentially ‘focusing’ the audio so that low frequency amplitude/phase differences are zero for a source at the distance of the speakers.

Near-Field Compensation
Proximity effect will result in increasingly high LF gain with increasing orders…
…which will then be partially undone in the decoder due to finite speaker distance.
Daniel proposed that encoding should happen with a fixed speaker distance already taken into account.
This could then be undone and redone if the speaker distance is different.
This results in much more managable proximity filter frequency responses.

NFC Filters for 1m (solid) and 3m source with a speaker distance of 1.5m

Aims of this Work
Proximity effect, and therefore the curvature of the wave front, is only available at low frequencies in 1 st order Ambisonics.
However, recordings exists that exhibit good distance cues over a wider range of frequencies
Motorbike recording found at www.soundfield.com , for example
Carry out some simple test to ascertain whether:
Our two SoundField mics follow this theory?
SoundField ST350 and MKV microphones
If they deviate, what settings need to be entered into Ambisonic decoders to take this into account.

Why would the Mics be different? – A to B Format Pre-filter A Pre-filter B Pre-filter C Pre-filter D Degree 0 Filter Degree 1 Filter Degree 1 Filter Degree 1 Filter W = (A+B+C+D) X = (A+C)–(B+D) Y = (A+B)–(C+D) Z = (A+D)–(B+C)

Procedure
Impulse response of the W (Omni) and X (F8) channels taken at 25cm intervals from 25cm to 3m.
Swept sine wave technique using:
Aurora Plugin
Adobe Audition
Taken in large lecture theatre.
Room reflections windowed out.
27.1ms Kaiser with shape parameter of 5 π
Earthworks reference omni also used in test.
X Y x

Results – ST350

Results - ST350

ST350 Calibration Amplitude

ST350 Calibration Phase

Results - MKV

Results – MKV

Results – MKV Amplitude

Results – MKV Phase

Results - Summary
ST350
MKV

Conclusions
The ST350 and MKV microphones are using different calibration filter schemes.
The ST350
calibrated flat for both phase and amplitude differences between the pressure and pressure gradient responses for a distance of around 1 – 1.5m.
This is equivalent to the ST350 being calibrated for Near Field Compensation of 1 – 1.5m.
The MKV
For a flat frequency response at around 1.5m
but with the aim of not affecting the phase differences between the pressure and pressure-gradient response of the microphone.
According to Gerzon, the phase response is more important to compensate for.

Ambsionic Decoding in Software
These settings can be realised in the WigWare VST decoders:

Further Work
Does this calibration actually effect the decoded audio.
If these cues are only available at LF, what else is giving the illusion of distance?
Direct/reverb ratio
Separation of sources with distance
Apparent source width
Floor reflections (outdoors)
…

Questions?
?

Distance Coding And Performance Of The Mark 5 And St350 Soundfield Microphones And Their Suitability For Ambisonic Reproduction

by Bruce Wiggins on May 27, 2010

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Distance Coding And Performance Of The Mark 5 And St350 Soundfield Microphones And Their Suitability For Ambisonic Reproduction — Presentation Transcript