libcw/flow/flow_doc.md

Flow Documentation:

flow is an experimental audio processing framework which supports the execution of real-time and non-real-time music and audio processing networks and the application of network state data.

The framework is designed to easily build a certain flavor of audio processing network which is defined by the ability to arbitrarily and seemlessly update the state of the network in real-time.

A flow program is a data flow network formed from a collection of interconnected processing units. A processing unit is referered to as a proc. Each processing unit has a function which operates on a set of variables. The network is formed by connecting processors together via their variables.

Fig. 1 shows a simple network non-real time network where the output of a sine oscillator is written to an audio file.

Building Neworks:

A Simple Network.

Use $ prefix to a filename. Use proc_expand_filename() to prepend proj_dir to a file prefixed with $.

Polyphonic Network.

Network with sub-nets.

Proc Instance Syntax:

<label> : { 'class':<class>, "in":{<in_stmt>*}, "preset":<class_preset_label>, "log":<log_dict> "argLabel":<arg_preset_label>, "args":<args_dict> }

Proc instance and variabel labels consist of two parts a leading identifier and a numeric suffix. The numeric suffix is referred to as the label_sfx_id. A proc or variable label without a numeric suffix is automatically assigned the label suffix 0.

args : This is a dictionary of named variable value records. preset : This string references a class preset to use for initializing this proc instance. argLabel : This string references an args dictionary parameter set to be applied after the preset class preset. If this argument is not given then it is automatically assigned the value "default". (What if there is not arg record named default? What if the are no arg records at all?) log : This is a dictionary of <var_label>:<sfx_id> pairs whose value should be printed to the console when they change at runtime.

Notes:

1. All fields except 'class' are optional.
2. The class preset named by preset is always applied before the arg values referenced by argLabel.

• When a preset is given as a list of values each entry in the list is taken as the value for the associated channel. When a list value is given to a var that is of type 'cfg' the list is passed as a single value.

Processing Unit Class

type	Description
string
bool
int	int32_t
uint	uint32_t
float	f32
double	f64
srate	(float) Sample rate type
sample	(float) Audio sample type
coeff	(float) Value that will be directly applied to a sample value (e.g added or multiplied)
ftime	(double) Fractional seconds
runtime	The type is left up to the processors custom 'create' function. These vars are not automatically created.

See list in cwFlowTypes.cpp : typeLabelFlagsA[]

Processing Units (proc)

A 'process' or proc is a set of functions and variables.

A network is a set of proc's with interconnected variable. By default most proc variables can be connected to, or act as sources for, other proc variables. A variable consists of a label, a type (e.g. int,real,audio,...), some attribtes (more about those below), a default value, and a documentation string.

One of the goals of 'flow' is to naturally handle multi-channel audio signals. To this end many audio processors create multiple internal sub-processes to handle each audio channel based on the number of input audio channels.

For example an audio gain processor consists of three variables: an input audio variable, an output audio variable and a gain coefficient. When the gain unit is created it will create an independent sub-process to handle each channel. If the input audio signal has multiple channels than the gain processor will internally duplicate each of the three variables. This allows independent control of the gain of each of the audio channels. In 'flow' parlance each of the sub-processes is referred to as a channel. A variable that can duplicated in this way is referred to as a multi-channel variable.

'flow' has a second form of variable multiplicity. This form occurs when a variable is duplicated based on how it is connected in the network. For example a simple audio mixer might have an undefined number of audio inputs and a single audio output. The number of audio inputs is only defined once it is connected in the network. This notion of variable multiplicity is different from a channel because each of the incoming audio signals may themselves contain multiple channels - each of which should be individually addressable. However, it should also be possible to address each of the incoming signals as a single entity. To accomplish this we use the concept of the mult-variable. A mult-variable amounts to creating an array of variables based on a single variable description, where the array length is determined at network compile time. mult-variables are distinguished by labels with integer suffixes.

The functions are:

Name	Description
create	Implements the custom elements of the proc instantiation.
destroy	Destroy resources that were acquired in create().
value	Variable values will pass through this function as they are assigned.
exec	Implements the custom execution functionality of this process.
report	Print the state of the process.

Var Syntax

label : { type: type, { value: value }, {proxy: proxy}, {flags:[{flag}*]}, doc:"q-string" }

Part	Description
label	Variable name
type	Variable type. See Data types below.
value	The default value of the variable.
proxy
doc	Documentation string for this variable.
flags

Notes:

• Whenever possible default values should be provided for the variable - even if the value is meaningless - like 0.0. This is important because during proc instantiation, prior to the custom create() call, variables listed in the proc instance's 'in' statement are connected to their respective sources. If the source does not have a valid value then the instantiation will fail. This consitutes a failure because it is guaranteed that when the custom create() function is called all variables in the 'in' statement will be resolved to a source variable with a valid value. This allows the proc instance to have the information it needs to configure itself.

There is one subtle case where the default value becomes important. When the variables in an 'in' statement are initially connected to their source they are connected to the 'any-channel' source variable because they do not have a specific channel yet. Specific channel can only be known during or after the custom create() function is called. Since the way a given proc. distributes channels will vary from one proc. to the next.

If during initial variable connection the source happens to be a variable with channels then the values that were assigned to those channels, in the source proc. create() function, will not show up on the 'any-channel'. If no default value was assigned to the source variable then the 'any-channel' will have no value, and the connection will fail with an error message like this:

"The source value is null on the connection input:foo:0 source:blah:0.bar:0".

Note that although variables are initially connected to the 'any-channel' source variable, prior to the their proc's create() call, after the create() call, when the variables do have a valid channel number, they are reconnected to a specific channel on the source variable.

Var Semantics

Var Types:

• Variables final types are determined during their owner proc instantiation. Once the type is set it never changes for the life of the proc.

• When reading a variable value the value will be coerced to the type of the output variable. For example: int v; var_get(var,v) will coerce the value of var to an int.

• When writing a variable the value will be coerced to the type of the variable. For example: If the type of var in var_set(var,float_val) is double then the value of float_val will be coerced to a double.

• The type a variables value is set in variable_t.type and always consists of a single bit field. (i.e. assert(isPowerOfTwo(variable_t.type)))

• The type of the value assigned to a variable (variable_t.value->tflag) must always exactly match variable_t.type.

Preset Syntax:

Data Types:

Types	Description
bool	bool
int	int32_t
uint	uint32_t
real	double
audio	multi-channel audio
spectrum	multi-channel spectrum
cfg
srate	platform defined sample rate type
sample	platform defined audio sample type
coeff	platform defined signal processing coefficient type

Variable Flags:

Flag	Description
init	This variale is set at proc instantation and never changes.
src	This variable must be connect to a source variable in the instance 'in' statement or be set to a default value. See 1.
no_src	This variable may not be connected to a source variable. See 1.
no_dflt_create	This variable is not created automatically as part of the proc instantiation. See 2.
mult	This variable may be duplicated in the instance 'in' statement. See 3.

Notes:

1. Unless the no_src attribute is set any variable may be connected to a source variable in the proc instantation 'in' statement. no_src variables are output variables whose value is calculated by the proc and therefore don't make sense to be fed from some other entity.

2. By default all variables are created prior to the proc create() function being called. Variable with the no_dflt_create attribute will not be created. This is useful in cases where information needs to be accessed in the create() function in order to determine some basic parameters of the variable For example a proc that needs to create multiple output variables based on the number of input audio channels cannot know how many output variables it may need until it accesses the number of audio channels it has been instantiated with.