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Back to Stereo: Stereo Imaging and Mic Techniques

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Back to Stereo: Stereo Imaging and Mic Techniques Huber, Ch. 4 Eargle, Ch. 11, 12 Localization/Phantom Images Two loudspeakers recreate a stereo sound stage, or ... – PowerPoint PPT presentation

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Title: Back to Stereo: Stereo Imaging and Mic Techniques


1
Back to StereoStereo Imaging and Mic Techniques
  • Huber, Ch. 4
  • Eargle, Ch. 11, 12

2
Localization/Phantom Images
  • Two loudspeakers recreate a stereo sound stage,
    or stereo field.
  • Sonic images appearing between the speakers are
    referred to as Phantom Images.
  • Localization cues
  • Interaural Phase (Time) differences
  • Interaural Intensity (Loudness) differences
  • Pinnae, head, and shoulders

3
More Localization
  • Phase typically more important for low
    frequencies.
  • Intensity more important for high frequencies.
  • Discrete time delays at high frequencies also a
    factor in localization.
  • Brain processes all cues, creates a perceptual
    image.

4
Stereo Miking Techniques
  • The use of two (or more) microphones to obtain a
    coherent stereo image.
  • Coincident Near Coincident Spaced
  • Coincident yields excellent stereo image
    (individual placement of sound sources).
  • Spaced yields a more spacious-sounding result.
  • Near Coincident techniques attempt a compromise.

5
Coincident X/Y Arrays
  • Two directional microphones (same make and model)
    placed as close together as possible without
    touching. (coincident technique)
  • Generally at angle of 90 to 135.
  • Midpoint faces towards middle of source.
  • Directionally comes entirely from amplitude
    differences.
  • No phase problems.
  • Cardiod or bi-directional

6
Blumlein Array
  • Eargle refers to this as the most notable
    example of an X/Y array.
  • Crossed bi-directional, 90 angle.
  • Pickup advantages result from relationship of
    sine and cosine angles.
  • Same math as with a panpot (panoramic
    potentiometer)

7
Blumlein(2)
  • Excellent front quadrant pickup
  • Equally good back quadrant pickup, but out of
    phase with front.
  • Shuffling introduction of in-phase crosstalk
    between two channels, above 700 Hz. (to match
    high-frequency image localization due to phase)

8
Blumlein Summary
  • Produces excellent stereo stage lateral imaging
    (due to panning aspect of pickup)
  • Creates an excellent sense of acoustic space (due
    to pickup of reverberant signals)
  • Can create difficulty in mic placement. Wide
    array of performers will require a placement
    distance that can compromise presence.

9
More X/Y stuff
  • 90 angle produces too much apparent center
    stage, with wide sources appearing much narrower
    in recorded image.
  • Widen up to 135 to compensate, but image can
    still appear narrow.
  • Super- or hyper-cardiod pickups at 135 angle
    produce better stereo image results, plus add a
    bit of room reflection. (but less than Blumlein)

10
X/Y summary
  • Crossed cardiod produces center-oriented stereo
    image.
  • Wider image when mic angle is wider (up to 135 -
    splayed)
  • Excellent mono capability
  • Splayed angles with super- and hyper-cardiod
    pickups offer widest stereo image possibilities.

11
Mid-Side Summary
  • Excellent stereo-to-mono compatibility
  • Flexibility in postproduction remixing
  • Easy to implement in field

12
Spaced Microphone Arrays
  • Recreates phase differences for localization.
  • Blurred, soft-edge image (preferred by many, like
    a soft-focus photo)
  • Omnis and subcardiods preferred.
  • Distances larger than 1m should incorporate a
    center microphone to avoid a center hole (Decca
    Tree).

13
Spaced Summary, 2 omni mics
  • Microphones usually placed no further than 1 m
    apart.
  • Array can be place relatively close to performers
    (intimacy)
  • Used when precise imaging not preferred.

14
Spaced Summary, 3 omni mics
  • Wide spacing creates ambience through inclusion
    of early reflections
  • Center mic fills in hole, but not precisely.
  • Lots of room pickup (reverberation and
    reflections) without losing significance of
    direct sound.

15
Near-Coincident
  • Compromise
  • ORTF (French Broadcasting)
  • 17 cm separation of cardiod pickups, with 55
    outward angle
  • NOS (Netherlands Broadcasting)
  • 30 cm separation of cardiod pickups, with 45
    outward angle.
  • ORTF is more accurate for Lc and Rc sources NOS
    creates a wider Lc and Rc image.

16
Near-Coincident Summary
  • Combines image specificity of coincident arrays,
    with spatiality of spaced arrays.
  • Allows for considerable choice in splay angle and
    mic spacing.

17
Head (Binaural) Microphones
  • Two pressure (omni) microphones at ear position
    in an artificial head (dummy).
  • Intended solely for headphone reproduction.
  • Shuffling can modify recording for speaker
    playback.

18
Stereo Listening Setups
  • Loudspeaker-listener angle should be kept
    moderate
  • 45 or 50 in Eargle
  • Often use the rule of equilateral triangle (60)
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