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Porting Guide

Status of Ports

The status of ports is kept in the main module README.

Overview

ZikiChombo sio code has a porting process that is unconventional due to the nature of sound support on host systems. Usually, to port a system, one takes some desired cross-host functionality, such as a filesystem, and implements it with the OS or OS/hardware combination of interest and that's that (to make a long story short).

For sound, the problem is that hosts usually have multiple different software components as entry points to playing and capturing sound, and moreover these entry points have some mutually exclusive properties, such as lower bounds on latency, availability of duplex synchronised by hardware audio clock, ability to be shared accross many users or applications, etc.

Although the main CPU hardware architectures can certainly be a concern, it has not been so far in our experience. So we focus on organizing around host, which is taken to be the value of runtime.GOOS in a Go program.

Hosts

A host often has a hardware abstraction layer, hardware drivers, and some levels of interacting with these layers to coordinate and make secure demands related to playing and capturing sound on a device.

To simplify the end use case, we want to make all this transparent to the consumer of sio, and let them simply work with sound.{Source,Sink,Duplex} via sio.{Capture,Play,Player,Duplex}.

For each Host, ZikiChombo defines a default entry point to accomplish this.

All entry points are available both to the consumer of ZikiChombo and either with ZikiChombo or as 3rd party implementations. So if you're working with VoIP and have precise duplex echo cancellation needs, or a music app that listens and plays in real time with feedback, or writing a PulseAudio replacement, you're more likely to be interested in using a specialized entry point than the default.

There is a directory ZikiChombo/sio/ports/{runtime.GOOS} for each host.

Entry Points

An entry point is an interface defining a subset of the functionality listed in the main README. ZikiChombo defines for each host (runtime.GOOS) a list of entry points which refer to the software layer with which the go program will communicate. These are named after the respective entry points in the main README.

Each entry point defines a base set of functionality related to what the entry point supports: device scanning, device change notifications, playback, capture, duplex. Note the README excludes some functionality from some entry points and entry points may be incomplete.

The functionality is slightly more rich than implementing sio.{Capture,Play,Player,Duplex} to allow callers to achieve latency or other requirements. Package sio automatically uses the Entry point to apply defaults so that the caller may simply call sio.{Play,Player,Capture,Duplex}. Package sio also enforces that only one entry point may be in use at a time in one Go program.

Entry points can then be registered by a package implementing them in their init() function. Consumers of ZikiChombo may optionaly control which package implements a given entry point. The default is chosen by package initialisation order.

See host for details.

The list of entry names for each host is defined in host/entry_{host}.go under the function host.Names().

Supporting concepts Devices, Inputs, Outputs, Duplex, Packets, Cbs

To implement an Entry Point, ZikiChombo provides some support code in libsio. The only required part of this code to reference is libsio/Dev to implement an Entry.

Other parts of this code, libsio.{Input,Output,Duplex,Packet,DuplexPacket} provide interfaces for synchronising with the host via Go channels and implementations to adapt these structures to sound.{Source,Sink,Duplex}.

There is also a libsio.Cb, which is a mechanism for interfacing the blocking calls in sound.{Source,Sink,Duplex} to lower level callback interfaces in C. Cb is tuned for the case the lower level C API runs the callback in a different thread, possibly with a different scheduling priority. see gddo for details.

Import directions

Package zikichombo.org/sio imports the package implementing entry points for registration side effects

import _ "zikichombo.org/sio/ports/{runtime.GOOS}"

Packages implementing the entries should import "zikichombo.org/sio/host" and call

host.RegisterEntry(mySuperReliableEntry)

These packages may also import zikichombo.org/libsio.

Duplex

Duplex support is intended for synchronized input/output. Systems which simply buffer underlying independent I+O and loosely synchronize with the slack that results from the buffering should consider not implementing Duplex and just letting the caller use sound.{Source,Sink} synchronously. Ideally, duplex should be audio hardware clock synchronized as in Apple's Aggregate devices. Duplexing I+O ringbuffers which are independently clocked to the same sample rate are acceptable but less reliable especially for long sessions. PortAudio actually supports duplex connections which have different sample rates. We recommend that this not be done to preserve the reliability of latency in Duplex implementations.

Build tags

Submitted ports and tests which produce or capture sound should use the build tag "listen" so that they do not run by default but are easily invokable with

go test zikichombo.org/sio/... -tags listen

3rd Party Ports

To have an independently distributed port listed here, please file an issue. We only list the most recent zc version/port version pairs here.

Port zc version Port version
- v0.0.1-alpha.2 vX.Y.Z

References

The following may be useful references for those considering sio ports.

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