The Design And Implementation Of Embedded Audio Network Synchronition Transmission System

In the era of digital network, people pursue dynamic acquisition and playback of audio stream at anytime and anywhere, which puts forward higher requirements for synchronization and delay of professional audio signal network transmission. Clock synchronization and low delay have always been difficulty problems of network transmission technology, so, the design of high-precision clock synchronization, real-time and low-cost audio network transmission is a hot research in the field of professional audio.Based on the research and analysis of the existing IP network transmission technology, we designed a set of audio network synchronous transmission system based on embedded platform for clock synchronization and low latency problems. It achieved sub-microsecond clock synchronization and low network latency, the specific work and innovation are as follows:1. A design scheme of embedded audio network synchronization transmission system is researched, which based on the ARM Cortex-A9 architecture and it adopts Exynos4412 as processor. We completed the system hardware platform structures according to the design scheme, specifically including audio signal collecting and processing, network transmission control, PTP(Precision Time Protocol)hardware support, synchronous media clock generated, etc. We completed software platform structures according to the design scheme, specifically including embedded linux operating system into the hardware platform, finished programs of network transmission control, PTP hardware support, and media clock generation module driver. We designed PTP program to achieve hign accuracy of clock synchronization, and designed RTP( Real-time Transport Protocol) program to achieve real-time transmission of professional audio signal.2. The design of clock synchronization. Audio network transmission equipment is easily affected by various external factors which can cause time error between equipments, and the error will accumulate over time, it greatly impact on the speed and quality of audio data transmission. In order to solve this problem, a kind of clock synchronization mechanism is designed which used to complete time synchronization and correction. In the process of synchronization, we introduced PTP precision clock synchronization technology to complete the interactions of time information between equipments, it can calculate time offset between master and slave equipment in real time. At the same time, we designed hardware circuit module to support PTP clock in the time correction mechanism, and used the above time offset to adjust local time, all of those made the equipment time synchronously to the source equipment.3. The design of audio media clock reconstruction. Traditional audio transmission equipments is used to distribute the audio clock signal through a special coaxial cable for each equipment to achieve consistent audio media clock, but the more transmission equipments in the system, the higher the cost. So we designed a method of reconfigurable media clock according to the absolute time, this method uses time offset which calculated in the process of synchronization to adjust local reference clock, and obtain audio media clock which aligned with source clock in clock frequency and phase, and we decomposed audio media clock by phase-locked loop technology to get other required clock in audio processing.