QNX Neutrino OS音频开发指南：QSA驱动与库应用详解

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本指南是针对那些希望利用QNX Sound Architecture (QSA) 驱动器和库编写音频应用的开发者编写的。它属于QNX Software Development Platform 6.6的一部分，专注于QNX Neutrino OS，这是BlackBerry Limited的子公司QNX Software Systems Limited提供的一个强大的实时操作系统。该指南于2000年至2014年期间发布，版权归QNX所有，并且强调了QNX在汽车、嵌入式系统和其他领域的广泛应用。 "QNX Software Development Platform 6.6" 提供了一个全面的开发环境，适合音频应用程序的开发，包括音频架构的深入了解。QNX Sound Architecture（QSA）是核心组件，它定义了音频处理的基础，包括卡片（如声卡）、设备管理以及数据格式的处理，如Pulse Code Modulation (PCM) 设备。 "控制设备" 和 "混音设备" 是QSA中的关键概念，前者负责音频输入和输出的控制，后者则用于整合多个音频流并混合它们。此外，指南详细解释了如何处理PCM设备，包括如何打开设备进行音频播放和捕捉。这涉及到软件层面的PCM混合技术，以及使用PCM插件转换器进行音频格式转换。 "PCM状态机" 是理解音频数据处理过程的重要部分，它展示了数据在传输过程中如何通过不同的状态进行编码和解码。软件PCM混合涉及到在内存中管理和混合不同来源的音频数据，确保音质和性能。对于开发者来说，本指南不仅提供了技术细节，还可能包含示例代码、调试技巧和最佳实践，以帮助他们优化音频应用的性能和用户体验。此外，附录和技术支持信息提供了开发者在遇到问题时寻求解答的渠道。 QNX Neutrino OS Audio Developer's Guide是一份实用的参考资料，深入探讨了如何在QNX平台上构建高效、稳定的音频应用，适用于音频驱动程序开发者和嵌入式系统工程师。无论是初学者还是经验丰富的开发者，都可以从中获取有价值的知识和洞见。

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QNX Sound Architecture

In order for an application to produce sound, the system must include several

components.

• hardware in the form of a sound card or sound chip

• a device driver for the hardware

• a well-defined way for the application to talk to the driver, in the form of an

Application Programming Interface (API).

This whole system is referred to as the QNX Sound Architecture (QSA). QSA has a rich

heritage and owes a large part of its design to version 0.5.2 of the Advanced Linux

Sound Architecture (ALSA), but as both systems continued to develop and expand,

direct compatibility between the two was lost.

This document concentrates on defining the API and providing examples of how to

use it. But before defining the API calls themselves, you need a little background on

the architecture itself. For those who want to jump in right away, full source for

examples of a “wav” player and “wav” recorder are included in the wave.c and

waverec.c appendixes.

Audio Architecture

Pulse Code Modulation (PCM) devices

PCM devices are responsible for converting digital sound sequences to analog

waveforms, or analog waveforms to digital sound sequences.

Each device operates only in one mode or the other. If it converts digital to analog,

it's a playback channel device; if it converts analog to digital, it's a capture channel

device.

The attributes of PCM devices include:

• the data formats that the device supports (16-bit signed little endian, 32-bit

unsigned big endian, etc.). For more information, see “Data formats (p. 20),”

below.

• the data rates that the device can run at (48KHz, 44.1kHz etc.)

• the number of streams that the device can support (e.g., 2-channel stereo, mono,

and 4-channel surround)

• the number of simultaneous clients that the device can support, referred to as the

number of subchannels the device has. Most sound cards support only 1

subchannel, but some cards can support more; for example, the Soundblaster Live!

supports 32 subchannels.

The maximum number of subchannels supported is a hardware limitation. On

single-subchannel cards, this limitation is artificially surpassed through a software

solution: the software subchannel mixer. This allows 8 software subchannels to

exist on top of the single hardware subchannel.

The number of subchannels that a device advertises as supporting is defined for

the best-case scenario; in the real world, the device might support fewer. For

example, a device might support 32 simultaneous clients if they all run at 48 kHz,

but might support only 8 clients if the rate is 44.1 kHz. In this case, the device

advertises 32 subchannels.

Data formats

The QNX Sound Architecture supports a variety of data formats.

The <asound.h> header file defines two sets of constants for the data formats. The

two sets are related (and easily converted between) but serve different purposes:

SND_PCM_SFMT_*

A single selection from the set of data formats. For a list of the supported

formats, see snd_pcm_get_format_name() (p. 210) in the Audio Library

chapter.

SND_PCM_FMT_*

Audio Architecture

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