EE2F2 - Music Technology 2. Stereo and Multi-track 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 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 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 LR 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 LR At high frequencies Sound cannot diffract around the head The head ‘shadows’ sound from one speaker The brain uses amplitude differences as spatial cues

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 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

Directional Response Left microphone Right microphone Source angle Level difference [dB] Source angle Phantom image angle

Time Difference Stereo Stereo imagery can be created by: Amplitude differences (especially at high frequencies) Time delay (especially at low frequencies) Combination of both Williams curves

Microphone Array Options Coincident pairs: Only amplitude differences are recorded. 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.

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 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 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 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 kHz

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 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 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 & 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, Mbit/sec Each sub-frame contains: 4 bit Sync marker 24 bits of data Validity, User-defined, Channel-status and Parity bits SyncSample data – 24 bitsSyncSample data – 24 bits V U C P Subframe - ASubframe - B

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.