EE2F2 - Music Technology

1. 2. Stereo and Multi-track Recording EE2F2 - Music Technology

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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...

11. 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:

12. 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

13. 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

14. 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

15. 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…

16. 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)

17. 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

18. 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.