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AAF Streams: What You Need to Know


AAF stands for Audio Application Format, and it is one of the media formats that can be transported over AVB streams. AVB stands for Audio Video Bridging, and it is a set of IEEE standards that enable time-sensitive audio and video applications on local area networks (LANs).

AVB provides features such as time synchronization, bounded transmission latency, resource management and application interoperability.

What are AAF streams?

AAF streams are AVB streams that use the Audio Video Transport Protocol (AVTP) to encapsulate audio samples in Ethernet frames. AVTP is a protocol specified in IEEE 1722-2016 standard that defines the format and behavior of AVB streams. AAF streams use 32-bit audio samples for quadlet-aligned media and default samples per fram.

AAF streams are designed to be flexible and efficient for transporting audio over AVB networks. They support different sample rates, channel counts, encoding schemes and presentation time stamps. They also allow dynamic audio mapping and channel selection for listeners.

How do AAF streams work?

AAF streams work by sending audio samples in AVTP packets over an AVB network. An AVB network consists of AVB-capable devices that are connected by AVB-capable switches.

An AVB network uses the generalized Precision Time Protocol (gPTP) to synchronize the clocks of all devices and switches. It also uses the Stream Reservation Protocol (SRP) to reserve bandwidth and resources for each stream.

An AAF stream source is a device that generates and transmits an AAF stream over an AVB network. An AAF stream sink is a device that receives and consumes an AAF stream from an AVB network.

An AAF stream source can send multiple AAF streams with different media formats and channel counts. An AAF stream sink can receive multiple AAF streams and select the desired audio channels from each stream.

To send an AAF stream, an AAF stream source first reserves bandwidth and resources for the stream using SRP. Then, it generates audio samples according to the media format and channel count of the stream. It encapsulates the audio samples in AVTP packets with appropriate headers and trailers. It sends the AVTP packets over the reserved path on the AVB network with a specified transmission interval and presentation time.

To receive an AAF stream, an AAF stream sink first listens for SRP announcements from the AAF stream source. It registers its interest in receiving the stream using SRP. Then, it receives the AVTP packets from the reserved path on the AVB network. It extracts the audio samples from the AVTP packets according to the media format and channel count of the stream. It presents the audio samples at the specified presentation time

How do AAF streams compare to other AVB stream formats?

AAF streams are one of the media formats that can be transported over AVB streams. Other media formats include AM824, MIDI, SMPTE, MPEG video with audio, MJPEG video, SDI video with meta data, or control data1.

Each media format has its own advantages and disadvantages depending on the use case and requirements. Some factors to consider when choosing a media format are:

Transport efficiency: how much bandwidth and resources are consumed by each stream

Channel count limit: how many channels can be transported per link or per device

Stream size: how many channels are contained in each stream

Device compatibility: how many devices support each media format

Application interoperability: how easy it is to integrate different applications using each media format

For example, AM824 is a media format that uses 24-bit audio samples with 8-bit labels for each sample. It supports different encoding schemes such as IEC 60958 or AES. It has higher transport efficiency than AAF but lower channel count limit per link or per device.

MIDI is a media format that uses 8-bit bytes to represent musical instrument digital interface messages. It supports different types of messages such as note on/off, pitch bend, control change, etc. It has lower transport efficiency than AAF but higher application interoperability for musical applications1.

FAQs: Ultimate Guide To your questions

Q: What is Libavtp?

A: Libavtp is an open source implementation of AVTP specified in IEEE 1722-2016 standard. It provides functions and data structures for creating and parsing AVTP packets for different media formats including AAF2.

Q: How can I use AAF streams on Linux?

A: You can use ALSA or GStreamer plugins that leverage Libavtp to enable AAF streams on Linux systems. ALSA is a low-level framework that provides audio functionality on Linux. GStreamer is a higher-level framework that provides multimedia functionality on Linux2.

Q: What are some examples of applications that use AAF streams?

A: Some examples of applications that use AAF streams are:

Professional audio systems that require high-quality audio transport with low latency and jitter Automotive infotainment systems that require synchronized audio playback across multiple speakers
Industrial control systems that require deterministic audio feedback for operator

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