The MP 4 Structured Audio Language By Van

The MP 4 – Structured Audio Language By Van Bucsko May 7, 2008

Intro – What is SAOL? �SAOL is a standard programming language that specifies sound as a computer program that generates audio when it is executed, as opposed to just audio data. �It is pretty similar to C++ as far as logic, expressions, and math go, but there are several differences in variables, the kinds of files each can play and so on.

Numbers and Names � Numbers � Names� reserved words � Declarations � Instr Parameters & Variables � Scalars � Arrays � Global variables – must be imported or exported � global blocks � Standard Names � Global Parameters: � 5 kinds � difference between global and instr

Expressions and Statements � Arithmetic Expressions Unary (-) takes precedence over both the binary classes (+, - and *, /) � All binary and unary operations in SAOL follow a rate rule – the rate of an expression is the rate of its fastest subexpression. � i-rate is the slowest, k-rate is faster and a-rate is the fastest rate � Arrays and Other Expressions � declared exactly like in C++; use the same syntax and numbering � indexed arrays in SAOL are scalars – the array takes on the fastest rate possible � Ex. ivar i[2]; ksig k; asig a[3]; � SAOL basically uses the same operators for less than, greater than, less than/equal to, greater than/equal to, and not equal � exceptions – if both operands are scalar width (1), with &&, the right subexpression is only evaluated if the left one is true - opposite for || - right subexpression is evaluated only if the left one is false � Switch – switch operator takes the logical value of the first non-zero operand (there are 3 operands with switch) � if at least one subexpression has width greater than 1, all three subexpressions are evaluated

More Statements and a Loop � Assigment Statements � the syntax for these statements is lval (a instr parameter or signal variable that receives the new value) = expr (an expression evaluated to generate the value to assign). � these statements also have rules about width because an lval can be a scalar or array variable – both the lval and expression have to be scalar width � If & If – Else � these statements work basically the same as in C+ + � � � � if (note > 5) // expr i-rate { decay = 0. 8; // k-rate } else { decay = 0. 9; // k-rate } � While Statement � only loop used in SAOL; executed basically exactly like in C++ � statement in parentheses must be scalar width, and (along with all the statements in the set), must run at the same rate

Core opcodes � What are they? Core Opcodes basically serve the same function as library functions do in C++. � Opcode calls basically act like parameters or variables in expressions. � Transcendentals : � Trigonometric Opcodes - forward trig opcodes take angle parameters in units of radians, and inverse trig opcodes return angle values in radians � The trig opcodes are asin, acos and atan. � there are no tan or hyperbolic functions in the opcode library, so there are replacements in the Slib library. � There also log, log 10 and sqrt opcodes.

Loudness, Tempo and Pitch opcodes � Loudness -Waveform amplitude is often converted to the decibel (d. B) scale to express loudness. � Ex. opcode dbamp(xsig x) computes 90 + 20 log 10(x) for x > 0 � SAOL has two rate-polymorphic core opcodes that handle conversion between amplitude and d. B units: the dbamp and ampdb. � Pitch – a set of 12 rate-polymorphic core opcodes handles conversions between three note name encodings and cycles per second. � the opcodes are named by the abbreviations for the four kinds of pitches, cps, midi, pch and oct. � the oct and pch types are octave-fraction and pitch class notation respectively, which use float numbers; the integer part of the number for both types is the octave number from a full keyboard. Also oct opcodes may return values between the 12 note steps. The pch opcodes give values rounded to the nearest semitone (in. 01 increments). � Tempo – the global tempo can be changed in SAOL from the default 60 beats per minute through the k-rate core opcode settempo. � Ex. kopcode settempo(ksig x) opcode gettempo([xsig dummy])

SAOL Wavetables �Declarations: �First you have to declare a table (and follow the naming rules). � To import tables, an instr parameter accesses a global wavetable by declaring an instr wavetable with the same name as the global wavetable. �SAOL has a special data type called the tablemap which is an array of wavetables to support some wavetable applications that create a set of sample buffers.

More Wavetables � Periodic Waveforms – 3 wavetable generators produce a single cycle of a periodic waveform � harm, the simplest generator, specifies a waveform as a harmonic series of N zero-phase sine waves; can be used to generate a pure tone � harm_phase generator specifies an amplitude parameter and a phase parameter to each sinusoid (or curve of the sine function), and the periodic generator also adds a frequency parameter � buzz generator specifies an algorithm for generating a family of pulselike waveforms using only a few parameters. � Envelope Waveforms – 5 generators (step, lineseg, expseg, spline, cubicseg) produce wavetables useful for signal envelopes � For each one, the envelope is specified as a piece-wise waveform, and the generators interpolate between the break points to create a waveform � the cubic equations should not be too complex, they are way too complex to calculate manually, so it’s best to use a computer program

Sound Files � 2 nd parameter of generator sample specifies the name of the audio file to be read into the table. � 1 st parameter specifies the size of the wavetable. �Only certain types of AIFF (the kind of music file on a CD) or WAV files are supported by sfront, a translator that converts MP 4 -SA files into efficient C programs. �The opcode speedt shrinks the size of the wavetable without changing the pitch in the sample.

Table Playback � 4 core opcodes play back wavetables. �Each core opcode has an internal pointer to the wavetable that always starts at the first sample. � 2 of them, koscil and doscil are specialized for creating signals by repeating a cycle of a waveform stored in a table.

Table Playback (cont’d. ) �The doscil opcode plays back the wavetable t once and then returns 0. It also considers the wavetable to be a recording of a pitched sound. �The loscil opcode enhances doscil by playing back the wavetable at a set pitch. �Ex. aopcode oscil(table t, asig freq [, ivar loops]) kopcode koscil(table t, ksig freq [, ivar loops])

Low-Level Opcodes � 9 opcodes produce settings for the wavetable t: �fsetsr and fsetbase set the sampling rate and base frequency, respectively. �ftsetloop and ftsetend change the loop start and ending points respectively. �ftlen, ftsr, ftbasecps, ftloop and ftloopend return the values of the length, sampling rate, base frequency, and loop start and end points. �Tableread and tablewrite read and write values into the wavetables.

Low-level Generators �empty fills a wavetable with zero values. �data fills a wavetable with parameter values. �polynomial fills a wavetable with nth order polynomial values. �concat takes a list of wavetables as parameters, joins them together and then initializes its own wavetabl �Random fills a wavetable with pseudorandom values, and a distribution number of 1 -5 tells the distribution type. �Window produces a wavetable to one of a set of windowing functions.

Output Statement �SAOL instruments have one audio output port each, and each one holds the instrument’s audio output. �Every SAOL program includes an output_bus, the width of which is set by the global parameter outchannels. �After all instances of execute, the output_bus is summed onto the system’s final audio output, which usually falls into the range of -1 and 1, and is sent to a file to be stored or to a converter, so one can listen to it.

Effects Instruments �An instrument that processes audio output is called an effects instrument. �An effects instrument usually processes the value held by the standard input name, and writes a modified version to the audio output port. �i = 0; while (i < inchan) { w[i] = sfactor; i = i + 1; }

Send Statements �Create instances of effects instruments; these statements can only appear in global blocks. �Have three sections: � 1 st specifies the name of instrument to be created. � 2 nd is an expression list, whose values initialize the instrument parameters of the instance. � 3 rd specifies the audio input port for the instrument as a list of at least one bus.

Route Statements �By default, an instrument sends its audio input port to output_bus, but route statements can change it to go to a user-defined bus. �A statement consists of a bus name and a list of instruments. �Several route statements can target the same bus.

Signal Flow Graphs �A set of send and route statements act together to form a diagram. �Send statements create the boxes in the diagram, connected by lines to and from the boxes that represent the route statements.

Determining Execution Order of Instruments � 1 st rule – instruments that produce a signal should run before instruments that use the signal. � 2 nd rule – if loops are created by the send and route statements, the order of the send statements creates the order of the instances. �By default, the instruments run in any order. �Any of the above rules can be overriden by the sequence statement, which has a list of instruments as parameters and these instruments are executed in the order listed.

System buses �The input_bus – a system bus that lets SAOl programs process external audio streams, like WAV files or live mikes. �The output_bus – holds the final output of the SAOL decoder. �Ex. send(rvb; inchan + outchan; input_bus); // output_bus in a send statement send(scale; 0. 5 ; output_bus);

SASL Scores �SASL score – a list of commands that trigger actions relative to the clock time the SAOL programs run by. �SASL commands are one line long and are a list of elements.

Instr command �Starts with an optional label � 1 st required element is the trigger time, which is a float value in units of beats that says when to create the instance. � 2 nd – the name of the SAOL instrument to be instantiated. �Then a float duration value that says how long it should execute.

Control command �Changes the value of a SAOL variable. �Begins with the trigger time, which tells when to change the value. �Then an optional label element is next, and then the keyword control is the name of the variable you want to change.

Tempo and End commands �When the tempo command executes, it changes the global tempo value. �Ex. 0. 283 tempo 130. 0 0. 12 tempo 4 �When end executes, the simulation ends, and no more audio is produced. �Ex. 129. 0934 end

Table command �The table command creates wavetables in a SAOL program. �Keyword table follows the table you want to create. �globals { table orig(data, 3, 1, 2, 3); }

Streaming and SASL �The MP 4 binary file format has provisions for certain SASL commands. �If these commands have timestamps, they follow the trigger semantics; if not, they are executed at the next possible opportunity. �A “*” can be added to each command to indicate that it’s higher priority.

MIDI Basics �What exactly is MIDI in reference to computers and music? �MIDI commands for start and end notes use integers for the notes on the keyboard (0 -127, with 60 as Middle C). �These commands specify a key number, the note value that was pressed or released, and a channel number (between 0 -15). �Many electronic keyboards have a pitch wheel on the side of the keyboard to bend a note. �MIDI Files can include tempo commands, which change the value of the global SAOL tempo value.

More MIDI �Note. On command creates an instance of the instrument that has the same number as the command channel. �An instance created on a specific channel executes until a Note. Off executes with that instance. �However, the instance can be delayed with either a Sustain Pedal on the keyboard, or through the SAOL extend statement to delay the end of the instance.

Standard Names in MIDI �There are five accepted names that contain info about MIDI: channel, preset, MIDIbend, MIDItouch, and MIDIctrl. �Channel holds the extended channel number of the command that created the instance, and preset holds the value of the preset extended channel value. �MIDIbend, MIDItouch, and MIDIctrl all have to do with the extended channels associated with the instance, along with certain commands they are also associated with.

SAOL Dynamic Instruments �The SAOL instr statement performs dynamic instrument instantiation, so that a child instrument can be copied several times from its parent. � 5 SAOL standard names (time, itime, released, dur and cpuload) let an instance monitor its execution status. �An instance can delay its termination time with the extend statement, which works the same as in MIDI. �Turnoff statement can force a program to terminate.

Execution of a SAOL program � 1 st – all the instruments and wavetables are initialized, and the instances are also produced with the send statements. � 2 nd – all SASL or dynamically generated instruments are initialized, all SASL table, tempo and control and MIDI events and tempo commands that are ready to execute are run, and instruments that are supposed to be terminated end. � 3 rd – like the first step, except all imports variables and table wavetables are set to an initial value. � 4 th – all local variables are set to the values of their corresponding global variables, and all imports exports wavetables are linked to their associated global wavetables.

Questions? Comments?

Bibliography �Lazzaro, John and Wawrznek, John. The MP 4 – Structured Audio Book. CS Division, UC Berkeley: 1999. <http: //www. cs. berkeley. edu/~lazzaro/sa/book/i n dex. html>

The End. Yay!