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 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
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.
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.
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
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)
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)
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.
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)
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.
Mid-Side Summary • Excellent stereo-to-mono compatibility • Flexibility in postproduction remixing • Easy to implement in field
Spaced Microphone Arrays • Recreates phase differences for localization. • Blurred, soft-edge image (preferred by many, like a soft-focus photo) • Omni’s and subcardiods preferred. • Distances larger than 1m should incorporate a center microphone to avoid a center hole (Decca Tree).
Spaced Summary, 2 omni mic’s • Microphones usually placed no further than 1 m apart. • Array can be place relatively close to performers (intimacy) • Used when precise imaging not preferred.
Spaced Summary, 3 omni mic’s • 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.
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.
Near-Coincident Summary • Combines image specificity of coincident arrays, with spatiality of spaced arrays. • Allows for considerable choice in splay angle and mic spacing.
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.
Stereo Listening Setups • Loudspeaker-listener angle should be kept moderate • 45° or 50° in Eargle • Often use the rule of equilateral triangle (60°)