Synchronization and Alignment: ST 2110 / AES67

Presenters : Andreas Hildebrand - ALC NetworX

Year : 2019

dlp_document_download : # 1 - Andreas Hildebrand – RAVENNA Technology Evangelist ALC NetworX, Munich ST2110 Technical: Synchronization & Alignment (Audio) # 2 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Andreas Hildebrand , RAVENNA Technology Evangelist • more than 25 years in the professional audio / broadcasting industry • graduate diploma in computer science • R&D, project & product management experience • member of AES67 TG and ST2110 DG ALC NetworX GmbH , Munich / Germany • established 2008 • R&D center • developing & promoting RAVENNA • Partnerships with > 40 manufacturers RAVENNA • IP media networking technology • designed to meet requirements of professional audio / broadcasting applications • open technology approach , license -free • fully AES67/ST2110 -compliant ( built -in ) # 3 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 • Media bit -transparency → no sample rate conversion → streams need to run on same media clock • Concurrent operation of different sample rates on same network • Determinable (low) end -to -end latency • Time alignment between media streams • Replacement for "house clock" distribution (word clock, black burst etc.)  Clock reassembly from stream data not appropriate  Distribution of master clock beats not sufficient  Common understanding of absolute time required ("wall clock") Timing & Synchronization – General Requirements # 4 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 • Audio applications have highest time accuracy & precision demands:  Sample accurate alignment of streams ( ± ½ sample) − @ 48 kHz: ± 10 µs − @ 96 kHz: ± 5 µs − @ 192 kHz: ± 2.5 µs  "Distribution" of word clock reference (AES11 calls for ± 5% max jitter / wander): − @ 48 kHz: ± 1 µs − @ 96 kHz: ± 500 ns − @ 192 kHz: ± 250 ns Timing & Synchronization – Accuracy Requirements # 5 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & Media Clocks • All nodes are running local clocks • Local clocks are precisely synchronized to a common wall clock via PTP PTPv1 standardized by IEEE in 2002 (IEEE 1588 -2002) PTPv2 followed in 2008 (IEEE1588 -2008) PTPv1 and PTPv2 are not compatible! Skip PTP # 12 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & Media Clocks • All nodes are running local clocks • Local clocks are precisely synchronized to a common wall clock via PTP • Media clocks are generated locally from synchronized local clock # 13 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & Media Clocks Master Clock Slave Clocks (nodes ) Media Clocks PTP # 14 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & Media Clocks • All nodes are running local clocks • Local clocks are precisely synchronized to a common wall clock via PTP • Media clocks are generated locally from synchronized local clock • Generation of any desired media clock (sample rate) possible • Concurrent operation of different media clocks possible • Phase accuracy of AES 11 ( ± 5% of sample period) achievable by deployment of PTP -aware switches (BC or TC) • Synchronization across facilities possible by reference to absolute time (TAI / GPS) • Essence data (audio samples or video frames) is related to the media clock upon intake -essentially receiving a generation "time stamp" with respect to the media clock # 15 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & media clocks • 3 type of clocks in the system: • Wall clock -provided by Grandmaster − local copy of the wall clock in each node • Media clock – derived from the local clock (i.e. 48 kHz for audio, 90 kHz for video) • RTP clock (stream clock) – derived from the media clock # 16 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 • Offset R is established on stream start -up • R may be random to defeat crypto - text attacks • This offset will be constant throughout the stream’s lifetime Synchronization & media clocks • The offset ( R) will be conveyed via SDP (a= mediaclk:direct =<offset>) Sender Receiver Reference clock (PTP Grandmaster) Local clock Local clock Media clock RTP stream clock Stream data (copy ) (copy ) R SDP Media clock PTP PTP – must be "0" in ST2110 R=0 # 17 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 RTP Packets (Layer 5) • Consist of RTP header, optional payload headers and the payload itself • RTP header (overhead) = 12 bytes , RTP payload (linear audio data) = up to 1440 bytes • RTP Timestamp = media clock counter (for linear PCM audio) = 32 bits (4 bytes) # 18 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 • Generation of any desired media clock (sample rate) possible • Concurrent operation of different media clocks possible • Phase accuracy of AES 11 ( ± 5% of sample period) achievable by deployment of PTP -aware switches (BC or TC) • Synchronization across facilities possible by reference to absolute time (TAI / GPS) • Essence data (audio samples or video frames) is related to the media clock upon intake -essentially receiving a generation "time stamp" with respect to the media clock • Fixed / determinable latency by configuring a suitable link offset ("playout delay") Synchronization & Media Clocks • All nodes are running local clocks • Local clocks are precisely synchronized to a common wall clock via PTP • Media clocks are generated locally from synchronized local clock # 19 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Synchronization & Media Clocks -Link offset # 20 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 • Generation of any desired media clock (sample rate) possible • Concurrent operation of different media clocks possible • Phase accuracy of AES 11 ( ± 5% of sample period) achievable by deployment of PTP -aware switches (BC or TC) • Synchronization across facilities possible by reference to absolute time (TAI / GPS) • Essence data (audio samples or video frames) is related to the media clock upon intake -essentially receiving a generation "time stamp" with respect to the media clock • Fixed / determinable latency by configuring a suitable link offset ("playout delay") • Inter -stream alignment by comparing and relating the time stamps of individual essence data Synchronization & Media Clocks • All nodes are running local clocks • Local clocks are precisely synchronized to a common wall clock via PTP • Media clocks are generated locally from synchronized local clock # 22 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Production Workflow Timing Image courtesy of Andy Rayner ( Nevion ) Buffer # 23 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 How to synchronize streams across various processing stages • Problem: − Any stream leaving a (processing) device is a new stream − New alignment of (processed) essence to wall clock time − Alignment of original essence is lost • Possible solutions: − Use of original time alignment for new stream (RTP timestamps adjusted to those of original essence) o Offset increases, might be too large for downstream Rx buffer o Which timestamps serve as reference when mixing essence? o How does the (processing) host know the exact relationship between ingress and egress essence? − Carry origin timestamps as in -band meta data o Requires new payload format (audio essence data + audio meta data), or o Needs to make use of (experimental) RTP header extensions mechanism (which in turn may result in variable / decreased audio payload segments) − Carry origin timestamps as out -of -band meta data o Requires new standard (in the works → AES X242, SMPTE ST2110 -41/42, NMOS IS -?? ) # 24 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 How to synchronize streams across various processing stages • Problem: − Any stream leaving a (processing) device is a new stream − New alignment of (processed) essence to wall clock time − Alignment of original essence is lost • Intermediate (?) / current solution: − Leave alignment task to management layer (i.e. Broadcast Controller) o Devices report processing delays to BC (or have fixed / configurable delays) o BC configures required Rx delay for subsequent stages (playout delay) # 25 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Production Workflow Timing Image courtesy of Andy Rayner ( Nevion ) Buffer Broadcast Controller # 30 ST2110 Technical: Synchronization & Alignment (Audio) Andreas Hildebrand, AIMS / AES AVoIP Theatre @ AES 147 th, Oct . 2019 Andreas Hildebrand Technology Evangelist Contact information : ALC NetworX GmbH Am Loferfeld 58 81249 Munich Germany ravenna@alcnetworx.de www.ravenna -network.com