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EE2F2 - Music Technology

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2. Stereo and Multi-track Recording. EE2F2 - Music Technology. Stereo Recording. The human hearing system is incredibly good at localising sound. Question: How do we tell what direction sounds are coming from? Amplitude differences between the ears Time/phase differences between the ears

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stereo recording
Stereo Recording
  • The human hearing system is incredibly good at localising sound.
  • Question: How do we tell what direction sounds are coming from?
    • Amplitude differences between the ears
    • Time/phase differences between the ears
    • Spectral differences in the sound content between the ears
    • Correlation with head movement
    • Visual cues
rayleigh s duplex theory
Rayleigh’s Duplex Theory
  • At low frequencies (< 1 kHz)
    • Level difference is small (sound diffracts around head)
    • Phase difference is significant
    • Brain uses phase difference to localise sound
  • At high frequencies (> 4 kHz)
    • Phase difference is ambiguous (±n.360°)
    • Level difference is significant (up to 20 dB or more)
    • Brain uses level difference
  • At mid-range frequencies (1-4 kHz)
    • Some combination of the two
blumlein stereo

At low frequencies

  • Each ear hears the sum of both speakers (one of which is delayed)
  • If L & R are different levels, this creates a phase difference.
  • The brain uses this as a spatial cue
Blumlein Stereo
  • Invented in 1931 and still the most popular stereophonic technique
    • Two speakers are used
    • Virtual sources (known as phantom images) are created by playing the same sound through each speaker, but at different amplitudes

L

R

blumlein stereo1
Blumlein Stereo
  • Invented in 1931 and still the most popular stereophonic technique
    • Two speakers are used
    • Virtual sources (known as phantom images) are created by playing the same sound through each speaker, but at different amplitudes
  • At high frequencies
  • Sound cannot diffract around the head
  • The head ‘shadows’ sound from one speaker
  • The brain uses amplitude differences as spatial cues

L

R

recording blumlein stereo

Angle between mics, 90°-180°

  • Directional mics (usually cardioid, sometimes figure-8)
  • NB. Sound is recorded over broad angle (up to 360°) but played back within 60° arc between speakers
  • The sound-stage is squeezed into a narrower space
Recording Blumlein Stereo
  • Requirements
    • Left and Right channels must be in-phase (no time delays)
    • Amplitude difference between channels determines angle
  • Implementation: coincident microphone pair
directional response

Level difference [dB]

90

120

60

150

30

Source angle

180

0

Phantom image angle

210

330

300

240

270

Source angle

Directional Response

Left microphone

Right microphone

time difference stereo

Williams curves

Time Difference Stereo
  • Stereo imagery can be created by:
    • Amplitude differences (especially at high frequencies)
    • Time delay (especially at low frequencies)
    • Combination of both
microphone array options

Near-coincident directional pairs: Based on Williams curves. Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

Near-coincident omni-directional pairs: Delay differences only are recorded. Good sense of ‘space’ but phase cancellation likely.

Spaced arrays (e.g. Decca Tree): Delay differences and some amplitude differences are recorded. Phase cancellation problems but it works well in practice.

Microphone Array Options

Coincident pairs: Only amplitude differences are recorded.

surround sound recording
Surround Sound Recording
  • Similar principles are applied to surround sound recordings
  • Popular configurations:
    • Near-coincident array of cardioids (pictured)
    • Spaced arrays
    • ‘Soundfield’ mics
  • Surround recording techniques are still in their infancy...
multi track recording
Multi-track Recording
  • Most modern recordings are made using much more than just two different tracks
  • These tracks are down-mixed to a stereo pair (or to 5.1 surround channels) before mastering
  • Important advantage of multi-track machines:
    • You don’t have to record all of the tracks at once
    • Examples:
analogue multi track tape recorders
Analogue Multi-track Tape Recorders
  • Features
    • Any track can record or playback at any time.
    • Auto-repeat facility for multiple takes.
    • Noiseless and gapless ‘punch-in’
  • Limitations/Disadvantages
    • High running costs
    • Head alignment problems
    • Analogue process so each stage adds noise
  • Studer Multi-track
    • 24 tracks
digital multi track tape
Digital Multi-track Tape
  • Features
    • Digital recording implies low-noise, perfect reproducibility etc.
    • Video cassette technology requires much less maintenance
    • Has all the features of a conventional analogue machine
  • Limitations
    • No new features compared with analogue
    • Tape-based so access time is slow – random access is virtually impossible
    • Expensive
  • Tascam DA-98HR
    • 8 tracks @ 44.1 kHz
hard disk recording
Hard Disk Recording
  • Any modern PC can record and playback digital audio
  • Multiple tracks can be recorded in sequence and played back simultaneously
  • Limitations:
    • Disk speed (limits number of tracks)
    • Standard stereo sound cards only give two inputs and two outputs
    • Professional sound card desirable featuring multiple ins/outs
features
Features
  • Tracks recorded on a hard disk can be randomly accessed. This allows:
    • Cutting, copying and pasting of segments or whole tracks
    • Looping or repeating sections
    • Time stretching, pitch shifting etc.
  • Also, using the processing capacity of an average PC you can also:
    • Digitally mix and equalise/pan/fade etc. multiple tracks
    • Apply real-time DSP effects on playback
    • Generate and mix additional sounds on playback using virtual software instruments
    • Examples of all this in another session…
digital audio interfaces
Digital Audio Interfaces
  • Standard noisy analogue connections are undesirable in the digital studio
  • We’d like to transmit digital audio in between pieces of equipment
  • Solution: either the professional AES/EBU or the domestic S/PDIF digital audio interfaces
    • They’re both asynchronous serial interfaces (like RS232 or MIDI)
    • The only differences between them are the signal levels and the physical connections (balanced/unbalanced, electrical or optical)
aes ebu and s pdif
AES/EBU and S/PDIF
  • AES/EBU & S/PDIF Data Format
    • For each audio sample, a ‘frame’ is transmitted consisting of two 32-bit sub-frames (for left and right channels)
    • Data rate depends, therefore, on the sample rate.
      • E.g. At 48 kHz, 2x32 bits must take 1/48000 seconds. Data rate is, therefore, 3.072 Mbit/sec
    • Each sub-frame contains:
      • 4 bit Sync marker
      • 24 bits of data
      • Validity, User-defined , Channel-status and Parity bits

Subframe - A

Subframe - B

Sync

Sample data – 24 bits

Sync

Sample data – 24 bits

V U C P

V U C P

summary
Summary
  • Multi-track techniques are an essential part of a modern recording studio.
  • Analogue multi-track tapes have been replaced by digital equivalents.
  • These days, both are being replaced by hard disk recording digital audio workstations.
  • A respectable digital audio workstation can be built from a standard PC.
  • This can replace the instruments, the mixers, the tape machines, the effects units and the mastering process.
  • All that they don’t replace is the performers… yet.