Reproduced Sound 25 Dr Bruce Wiggins Tom Spenceley Signal Processing Applications Research Group University of Derby DISTA...
Ambisonics - What is it? <ul><li>What is Ambisonics? </li></ul><ul><ul><li>A system where the spatial encoding of the audi...
Ambisonic Encoding – B Format <ul><li>The encoding, or recorded format is known as B-Format, which consists of 4 channels,...
Proximity/Near Field Effect <ul><li>Magnitude/Phase changes at low frequencies due to spherical wave front. </li></ul><ul>...
Proximity/Near Field Effect <ul><li>To obtain the first-order component response to this pressure, the time-integral of th...
Proximity/Near Field Effect <ul><li>As the two terms are in quadrature, the 3dB point is when the two terms are equal. </l...
Speaker Compensation <ul><li>If the speakers reproducing these signals emit plane waves, then just reproduce these signals...
Near-Field Compensation <ul><li>Proximity effect will result in increasingly high LF gain with increasing orders… </li></u...
NFC Filters for 1m (solid) and 3m source with a speaker distance of 1.5m
Aims of this Work <ul><li>Proximity effect, and therefore the curvature of the wave front, is only available at low freque...
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 Degre...
Procedure <ul><li>Impulse response of the W (Omni) and X (F8) channels taken at 25cm intervals from 25cm to 3m. </li></ul>...
Results – ST350
Results - ST350
ST350 Calibration Amplitude
ST350 Calibration Phase
Results - MKV
Results – MKV
Results – MKV Amplitude
Results – MKV Phase
Results - Summary <ul><li>ST350 </li></ul><ul><li>MKV </li></ul>
Conclusions <ul><li>The ST350 and MKV microphones are using different calibration filter schemes. </li></ul><ul><li>The ST...
Ambsionic Decoding in Software <ul><li>These settings can be realised in the WigWare VST decoders: </li></ul>
Further Work <ul><li>Does this calibration actually effect the decoded audio. </li></ul><ul><li>If these cues are only ava...
Questions? <ul><li>? </li></ul>
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Distance Coding And Performance Of The Mark 5 And St350 Soundfield Microphones And Their Suitability For Ambisonic Reproduction

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A paper presented at the Institute of Acoustics Reproduced Sound 25 conference in 2009 looking at the response of two SoundField Ambisonic microphones to sound sources at different distances from the microphone.

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

  1. 1. 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
  2. 2. Ambisonics - What is it? <ul><li>What is Ambisonics? </li></ul><ul><ul><li>A system where the spatial encoding of the audio is separate from the decoding of that audio to a set of speakers. </li></ul></ul><ul><ul><li>A set of rules and equations that help to optimise and quantify the performance of a multi-speaker audio presentation. </li></ul></ul><ul><ul><li>A system based on coincident recording principles. </li></ul></ul><ul><ul><li>An extendable system based on Spherical Harmonics. </li></ul></ul><ul><ul><li>A future proof audio format. </li></ul></ul><ul><li>Distance Cues </li></ul><ul><ul><li>Simulation extensive studies by Daniel et al. </li></ul></ul><ul><ul><li>1 st order (pressure gradient) and higher order components exhibit proximity, or near field, effect. </li></ul></ul><ul><ul><li>The decoding of this reproduces the curvature of the wavefront from the finite number of speakers. </li></ul></ul>
  3. 3. Ambisonic Encoding – B Format <ul><li>The encoding, or recorded format is known as B-Format, which consists of 4 channels, W, X, Y & Z. </li></ul><ul><li>It is made up of 0 th and 1 st degree spherical harmonics (which equate to standard pressure and pressure gradient polar patterns). </li></ul>
  4. 4. Proximity/Near Field Effect <ul><li>Magnitude/Phase changes at low frequencies due to spherical wave front. </li></ul><ul><ul><li>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: </li></ul></ul>
  5. 5. Proximity/Near Field Effect <ul><li>To obtain the first-order component response to this pressure, the time-integral of the gradient of must be found: </li></ul>Depends on Distance Depends on Distance & Frequency
  6. 6. Proximity/Near Field Effect <ul><li>As the two terms are in quadrature, the 3dB point is when the two terms are equal. </li></ul>Source at 1m Source at 0.5m
  7. 7. Speaker Compensation <ul><li>If the speakers reproducing these signals emit plane waves, then just reproduce these signals. </li></ul><ul><li>However, if the speakers are closer(!), then they will reproduce spherical waves. </li></ul><ul><li>For this reason, it’s necessary to ‘undo’ the proximity effect dependant on the speaker distance. </li></ul><ul><ul><li>Essentially ‘focusing’ the audio so that low frequency amplitude/phase differences are zero for a source at the distance of the speakers. </li></ul></ul>
  8. 8. Near-Field Compensation <ul><li>Proximity effect will result in increasingly high LF gain with increasing orders… </li></ul><ul><li>…which will then be partially undone in the decoder due to finite speaker distance. </li></ul><ul><li>Daniel proposed that encoding should happen with a fixed speaker distance already taken into account. </li></ul><ul><li>This could then be undone and redone if the speaker distance is different. </li></ul><ul><li>This results in much more managable proximity filter frequency responses. </li></ul>
  9. 9. NFC Filters for 1m (solid) and 3m source with a speaker distance of 1.5m
  10. 10. Aims of this Work <ul><li>Proximity effect, and therefore the curvature of the wave front, is only available at low frequencies in 1 st order Ambisonics. </li></ul><ul><li>However, recordings exists that exhibit good distance cues over a wider range of frequencies </li></ul><ul><ul><li>Motorbike recording found at www.soundfield.com , for example </li></ul></ul><ul><li>Carry out some simple test to ascertain whether: </li></ul><ul><ul><li>Our two SoundField mics follow this theory? </li></ul></ul><ul><ul><ul><li>SoundField ST350 and MKV microphones </li></ul></ul></ul><ul><ul><li>If they deviate, what settings need to be entered into Ambisonic decoders to take this into account. </li></ul></ul>
  11. 11. 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)
  12. 12. Procedure <ul><li>Impulse response of the W (Omni) and X (F8) channels taken at 25cm intervals from 25cm to 3m. </li></ul><ul><ul><li>Swept sine wave technique using: </li></ul></ul><ul><ul><ul><li>Aurora Plugin </li></ul></ul></ul><ul><ul><ul><li>Adobe Audition </li></ul></ul></ul><ul><li>Taken in large lecture theatre. </li></ul><ul><ul><li>Room reflections windowed out. </li></ul></ul><ul><ul><ul><li>27.1ms Kaiser with shape parameter of 5 π </li></ul></ul></ul><ul><li>Earthworks reference omni also used in test. </li></ul>X Y x
  13. 13. Results – ST350
  14. 14. Results - ST350
  15. 15. ST350 Calibration Amplitude
  16. 16. ST350 Calibration Phase
  17. 17. Results - MKV
  18. 18. Results – MKV
  19. 19. Results – MKV Amplitude
  20. 20. Results – MKV Phase
  21. 21. Results - Summary <ul><li>ST350 </li></ul><ul><li>MKV </li></ul>
  22. 22. Conclusions <ul><li>The ST350 and MKV microphones are using different calibration filter schemes. </li></ul><ul><li>The ST350 </li></ul><ul><ul><li>calibrated flat for both phase and amplitude differences between the pressure and pressure gradient responses for a distance of around 1 – 1.5m. </li></ul></ul><ul><ul><li>This is equivalent to the ST350 being calibrated for Near Field Compensation of 1 – 1.5m. </li></ul></ul><ul><li>The MKV </li></ul><ul><ul><li>For a flat frequency response at around 1.5m </li></ul></ul><ul><ul><li>but with the aim of not affecting the phase differences between the pressure and pressure-gradient response of the microphone. </li></ul></ul><ul><ul><li>According to Gerzon, the phase response is more important to compensate for. </li></ul></ul>
  23. 23. Ambsionic Decoding in Software <ul><li>These settings can be realised in the WigWare VST decoders: </li></ul>
  24. 24. Further Work <ul><li>Does this calibration actually effect the decoded audio. </li></ul><ul><li>If these cues are only available at LF, what else is giving the illusion of distance? </li></ul><ul><ul><li>Direct/reverb ratio </li></ul></ul><ul><ul><li>Separation of sources with distance </li></ul></ul><ul><ul><li>Apparent source width </li></ul></ul><ul><ul><li>Floor reflections (outdoors) </li></ul></ul><ul><ul><li>… </li></ul></ul>
  25. 25. Questions? <ul><li>? </li></ul>

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