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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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2 stereo and multi track recording

2. Stereo and Multi-track Recording

EE2F2 - Music Technology


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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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 Amplitude and delay differences are recorded. Good sense of ‘space’ but phase cancellation possible.

  • 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.


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