DOCSLIB.ORG

Simple Synchronisation for Open Sound Control

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Simple Synchronisation for Open Sound Control Simple Synchronisation for Open Sound Control Sebastian Madgwick Thomas Mitchell x-io Technologies University of the West of England Bristol, UK Bristol, UK [email protected] [email protected] Carlos Barreto Adrian Freed x-io Technologies CNMAT, UC Berkeley Bristol UK CA, USA [email protected] [email protected] ABSTRACT of timing and rhythm in music [4, 5], and the continually emerging applications of networked computational devices Clock synchronisation is a mature and important aspect of for musical purposes [6, 7, 8, 9, 10], there is a growing need distributed computing systems. Despite the importance of for a widely applicable synchronisation solution for computer accurate timing in music, there are relatively few widely music applications. applicable synchronisation solutions available to computer This paper presents a simple Open Sound Control (OSC) music practitioners. In this paper we present a simple synchronisation procedure for distributed systems, which has OSC-based synchronisation method for wired and wireless been designed to satisfy the following objectives: applications, which is designed to be easy to apply and is shown to offer accuracy appropriate for fine-grained music • simple to implement on a wide range of platforms applications. The proposed solution relies on a single master (heterogeneous network nodes) sending a synchronisation message to all slaves. Empirical • ‘transport independent’ operating entirely through the studies with a heterogeneous network of 17 Wi-Fi slaves and exchange of OSC messages 5 Ethernet slaves demonstrate that each homogeneous group • meets the fine-grained synchronisation requirements of is able to achieve a relative synchronisation accuracy of 166 computer music applications (i.e. sub-ms error [11]) us and 100 us respectively, offset from the master time by their respective network latencies. An acoustic localisation system The paper begins with an introduction to network is implemented to demonstrate an application that requires synchronisation with particular focus on computer music both accurate synchronisation and benefits from wireless systems. The proposed synchronisation method is then connectivity. The system is shown to precisely locate a sound presented, followed by a detailed empirical analysis and source with a standard deviation of 1.8 mm. discussion of its performance. A practical application example of the synchronisation precision is then provided in the form of a wireless indoor acoustic localisation system. 1. INTRODUCTION The paper closes with concluding remarks and proposals for future developments. In networks of distributed computational devices, individual network nodes are equipped with a local clock from which events may be accurately scheduled or timestamped. In 2. BACKGROUND practice this clock is derived from an on-board crystal or oscillator circuit, which, due to imperfections in the timing Clock synchronisation is a long and established component of hardware, will tend drift with respect to the clocks on other distributed computing systems, underpinning the technology network nodes [1]. Scenarios demanding precise temporal that enables GPS, mobile telecommunications, wireless data coordination have stimulated the development of many communications, networked file system integrity, sensor synchronisation procedures designed to establish an accurate, fusion, localisation, object and motion tracking [1, 12]. network-wide notion of time [2, 3]. Given the importance The Network Time Protocol (NTP) [13] has for decades kept internet clocks running to within a few milliseconds of Universal Coordinated Time and now forms one of a plethora Copyright: c 2015 Sebastian Madgwick et al. This is of available synchronisation methods that have since been an open-access article distributed under the terms of the proposed. Usually, these methods operate by propagating Creative Commons Attribution License 3.0 Unported, which permits a global or master time through the network from which unrestricted use, distribution, and reproduction in any medium, provided the slaves modify their local clocks to maintain synchronisation. original author and source are credited. The mechanisms of each synchronisation method differ based upon the the relevant network topology and the application 3. PROPOSED SOLUTION requirements in terms of accuracy, precision and energy. Comprehensive overviews can be found in the literature, see 3.1 Algorithm derivation for example [14] and [15] for a more recent comparison of The proposed synchronisation solution relies on a single synchronisation methods for wireless sensor networks. synchronisation master periodically broadcasting a measurement of global time to all slaves on a communication network; a method resembling the ‘simple’ synchronisation Synchronisation has also played an important role in solution proposed by Dannenberg [29]. distributed music systems to ensure audio and visual The synchronisation messages sent by the master provide media alignment. As technology has pervaded the sonic each slave with a precise observation of global time. and visual arts, a number of established synchronisation However, this observation will incorporate a varying error protocols and interfaces have been proposed and adopted due to the system latencies; the most significant being that enable distributed devices to maintain a common communication latency, and task scheduling within the slave. time frame. For example, MIDI clock, MIDI timecode Figure 1 shows the error in the observed global clock for a and SMPTE have provided reliable (although relatively single slave. This plot was obtained for a Wi-Fi node using imprecise) device synchronisation for professional media the experimental setup described in Section 4.1 and a fixed applications [16, 17]. Research into novel musical interfaces synchronisation message rate of 5 messages per second. and laptop/mobile phone performances are making increasing use of interconnected networks of wired and wireless 0 devices to sense and capture expressive control input and to schedule precisely timed output. For example, The −5 Princeton Laptop Orchestra [18], the Carnegie Mellon Laptop Orchestra [19] and the LOLC laptop ensemble [20] −10 implement their own synchronisation algorithms to ensure −15 timely coordination amongst players. The former alludes −20 to an OSC synchronisation procedure with synchronisation Error (ms) precision estimated in the order of 30-40 ms, while the latter −25 two incorporate messaging processes based upon Christian’s algorithm [21]. A third live coding synchronisation process −30 developed by Ogborn [22] for the Cybernetic Orchestra −35 encapsulates the complexities of the synchronisation process 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (minutes) within a dedicated application called EspGrid, which exposes global beat resolution event messages to clients running Figure 1 OSC compatible music software. While this abstraction is . Error in the observed global clock for a single slave. elegant for synchronising to salient musical events, it is not designed to provide random access to the real-time clock, The visualisation of the error in the observed global clock precluding high-precision synchronisation applications such shown in Figure 1 clearly indicates a fixed minimum error. as localisation and sensor fusion. In their 2014 live coding Although errors may be as high as 30 ms, a significant report, Blackwell et al [23] draw attention to a growing number of observations have a precise error of 2.2 ms. This abundance of bespoke synchronisation methods noting that represents the minimum system latency, and corroborates cooperation is currently hampered by a lack of agreement on Wi-Fi round-trip latency measurements made with similar a single protocol. devices in [30, 31]. The distribution of the error will vary between hardware and software platforms, and communication interfaces. However, This work is concerned with cases in which many for a network of homogeneous slaves, the minimum system established synchronisation protocols may not be applicable latency will be equal for all slaves. A slave may therefore because they depend upon specific physical interfaces or achieve relative synchronisation with other homogeneous other specific OSI model technologies such as TCP/IP. The types if it is able to identify synchronisation messages that proposed synchronisation method is designed to operate in are likely to represent the minimum system latency. The situations where nodes are able to communicate by OSC and proposed solution achieves this through the following three have real-time access to an onboard physical clock. The heuristics: OSC protocol has been chosen as it has now become a key technology within the computer music community [24]. The 1. The slave incorporates a fixed slave clock drift to time tag [25] argument type was introduced to the OSC ensure that the slave clock is slower than the global protocol as an enabling feature for device synchronisation. clock. Despite several subsequent calls for an OSC synchronisation 2. If the observed global clock is ahead of the current method [26, 27, 28], a standardised approach is still to be slave clock then update the slave clock to equal the established. observed global clock. 3. If the observed global clock differs from slave clock by 4.1 Experimental setup more than a specified threshold then update the slave The experimental setup included 17 Wi-Fi slaves, 5 Ethernet clock to equal the observed master clock.
Load more

Recommended publications
  • Lecture 12: March 18 12.1 Overview 12.2 Clock Synchronization
    CMPSCI 677 Operating Systems Spring 2019 Lecture 12: March 18 Lecturer: Prashant Shenoy Scribe: Jaskaran Singh, Abhiram Eswaran(2018) Announcements: Midterm Exam on Friday Mar 22, Lab 2 will be released today, it is due after the exam. 12.1 Overview The topic of the lecture is \Time ordering and clock synchronization". This lecture covered the following topics. Clock Synchronization : Motivation, Cristians algorithm, Berkeley algorithm, NTP, GPS Logical Clocks : Event Ordering 12.2 Clock Synchronization 12.2.1 The motivation of clock synchronization In centralized systems and applications, it is not necessary to synchronize clocks since all entities use the system clock of one machine for time-keeping and one can determine the order of events take place according to their local timestamps. However, in a distributed system, lack of clock synchronization may cause issues. It is because each machine has its own system clock, and one clock may run faster than the other. Thus, one cannot determine whether event A in one machine occurs before event B in another machine only according to their local timestamps. For example, you modify files and save them on machine A, and use another machine B to compile the files modified. If one wishes to compile files in order and B has a faster clock than A, you may not correctly compile the files because the time of compiling files on B may be later than the time of editing files on A and we have nothing but local timestamps on different machines to go by, thus leading to errors. 12.2.2 How physical clocks and time work 1) Use astronomical metrics (solar day) to tell time: Solar noon is the time that sun is directly overhead.
    [Show full text]
  • Distributed Synchronization
    Distributed Synchronization CIS 505: Software Systems Communication between processes in a distributed system can have Lecture Note on Physical Clocks unpredictable delays, processes can fail, messages may be lost Synchronization in distributed systems is harder than in centralized systems because the need for distributed algorithms. Properties of distributed algorithms: 1 The relevant information is scattered among multiple machines. 2 Processes make decisions based only on locally available information. 3 A single point of failure in the system should be avoided. Insup Lee 4 No common clock or other precise global time source exists. Department of Computer and Information Science Challenge: How to design schemes so that multiple systems can University of Pennsylvania coordinate/synchronize to solve problems efficiently? CIS 505, Spring 2007 1 CIS 505, Spring 2007 Physical Clocks 2 The Myth of Simultaneity Event Timelines (Example of previous Slide) time “Event 1 and event 2 at same time” Event 1 Node 1 Event 2 Event 1 Event 2 Node 2 Observer A: Node 3 Event 2 is earlier than Event 1 Observer B: Node 4 Event 2 is simultaneous to Event 1 Observer C: Node 5 Event 1 is earlier than Event 2 e1 || e2 = e1 e2 | e2 e1 Note: The arrows start from an event and end at an observation. The slope of the arrows depend of relative speed of propagation CIS 505, Spring 2007 Physical Clocks 3 CIS 505, Spring 2007 Physical Clocks 4 1 Causality Event Timelines (Example of previous Slide) time Event 1 causes Event 1 Event 2 Node 1 Event 2 Node 2 Observer A: Event 1 before Event 2 Node 3 Observer B: Event 1 before Event 2 Node 4 Observer C: Event 1 before Event 2 Node 5 Note: In the timeline view, event 2 must be caused by some passage of Requirement: We have to establish causality, i.e., each observer must see information from event 1 if it is caused by event 1 event 1 before event 2 CIS 505, Spring 2007 Physical Clocks 5 CIS 505, Spring 2007 Physical Clocks 6 Why need to synchronize clocks? Physical Time foo.o created Some systems really need quite accurate absolute times.
    [Show full text]
  • Clock Synchronization Clock Synchronization Part 2, Chapter 5
    Clock Synchronization Clock Synchronization Part 2, Chapter 5 Roger Wattenhofer ETH Zurich – Distributed Computing – www.disco.ethz.ch 5/1 5/2 Overview TexPoint fonts used in EMF. Motivation Read the TexPoint manual before you delete this box.: AAAA A • Logical Time (“happened-before”) • Motivation • Determine the order of events in a distributed system • Real World Clock Sources, Hardware and Applications • Synchronize resources • Clock Synchronization in Distributed Systems • Theory of Clock Synchronization • Physical Time • Protocol: PulseSync • Timestamp events (email, sensor data, file access times etc.) • Synchronize audio and video streams • Measure signal propagation delays (Localization) • Wireless (TDMA, duty cycling) • Digital control systems (ESP, airplane autopilot etc.) 5/3 5/4 Properties of Clock Synchronization Algorithms World Time (UTC) • External vs. internal synchronization • Atomic Clock – External sync: Nodes synchronize with an external clock source (UTC) – UTC: Coordinated Universal Time – Internal sync: Nodes synchronize to a common time – SI definition 1s := 9192631770 oscillation cycles of the caesium-133 atom – to a leader, to an averaged time, ... – Clocks excite these atoms to oscillate and count the cycles – Almost no drift (about 1s in 10 Million years) • One-shot vs. continuous synchronization – Getting smaller and more energy efficient! – Periodic synchronization required to compensate clock drift • Online vs. offline time information – Offline: Can reconstruct time of an event when needed • Global vs.
    [Show full text]
  • Clock Synchronization
    Middleware Laboratory Sapienza Università di Roma MIDLAB Dipartimento di Informatica e Sistemistica Clock Synchronization Marco Platania www.dis.uniroma1.it/~platania [email protected] Sapienza University of Rome Time notion Each computer is equipped with a physical (hardware) clock It can be viewed as a counter incremented by ticks of an oscillator i At time t, the Operating System (OS) of a process y r o t reads the hardware clock H(t) of the processor a r o b a L e r a C = a H(t) + b w Then, it generates the software clock e l d d i M C approximately measures the time t of process i B A L D I M notion among processes among notion isas known evolve executions distributed (DS) Distributed in Systems Time Problems: how to analyze a in DS factor Time a key is The technique used to coordinate a common time time common a techniqueThe used coordinate to common notion time common of a process state during distributed a computation However, it©s important processes for a share to However, Lacking of a global reference time: it©s hard to know Lackingthe time:it©sknow to hard of a global reference ClockSynchronization MIDLAB Middleware Laboratory Clock Synchronization [10] The hardware clock of a set of computers (system nodes) may differ because they count time with different frequencies Clock Synchronization faces this problem by means of synchronization algorithms y r Standard communication infrastructure o t a r o No additional hardware b a L e r a w e l Clock synchronization algorithms d d i External M Internal B
    [Show full text]
  • PTP Background and Overview
    PTP Background and Overview by Jeff Laird, July 2012 Introduction Precision Time Protocol (PTP) allows computers in a local area network to synchronize their clocks to within a microsecond of each other. The standard was originally defined in 2002. The second and current version of the standard was published in 2008. It is known as “IEEE 1588-2008” (IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems). PTP compared to other synchronization methods PTP is more accurate than NTP but less accurate than GPS. NTP NTP (Network Time Protocol) was standardized in 1985 and is the most common clock synchronization method, being used by tens of millions of client computers to synchronize to NTP servers over the Internet. NTP is primarily used in wide-area networks, where it can achieve accuracy to 10 ms. On a local-area network NTP can achieve accuracy to 200 μs. NTP includes SNTP (Simple NTP), which is stateless (no averaging) and therefore suitable for embedded devices, but less accurate. In NTP terminology, stratum-0 clocks are atomic clocks, GPS satellites, and radio clocks not directly on a network. They provide the time to stratum-1 clocks (network time servers) using a PPS (pulse-per- second) signal over an RS-232 serial connection. Stratum-2 clocks typically synchronize with multiple stratum-1 clocks. Stratum-3 clocks synchronize with one or more stratum-2 clocks, and so on. An NTP timestamp is 64 bits long—32 bits for seconds (to 136 years) and 32 bits for fraction of a second (to <0.25 ns).
    [Show full text]
  • STP Recommendations for the FINRA Clock Synchronization Requirements
    STP recommendations for the FINRA clock synchronization requirements This document can be found on the web, www.ibm.com/support/techdocs Search for document number WP102690 under the category of “White Papers". Version Date: January 20, 2017 George Kozakos [email protected] © IBM Copyright, 2017 Version 1, January 20, 2017 Web location of document ( www.ibm.com/support/techdocs ) ~ 1 ~ STP recommendations for the FINRA clock synchronization requirements Contents Configuring an external time source (ETS) via a network time protocol (NTP) server with pulse per second (PPS) ......................................................................................................... 3 Activating the z/OS time of day (TOD) clock accuracy monitor .............................................. 4 Maintaining time accuracy across power-on-reset (POR) ...................................................... 5 Specifying leap seconds ....................................................................................................... 6 Appendix ............................................................................................................................. 7 References ........................................................................................................................... 8 Trademarks ......................................................................................................................... 8 Feedback ............................................................................................................................
    [Show full text]
  • Distributed Systems: Synchronization
    Chapter 3: Distributed Systems: Synchronization Fall 2013 Jussi Kangasharju Chapter Outline n Clocks and time n Global state n Mutual exclusion n Election algorithms Kangasharju: Distributed Systems 2 Time and Clocks What we need? How to solve? Real time Universal time (Network time) Interval length Computer clock Order of events Network time (Universal time) NOTE: Time is monotonous Kangasharju: Distributed Systems 3 Measuring Time n Traditionally time measured astronomically n Transit of the sun (highest point in the sky) n Solar day and solar second n Problem: Earth’s rotation is slowing down n Days get longer and longer n 300 million years ago there were 400 days in the year ;-) n Modern way to measure time is atomic clock n Based on transitions in Cesium-133 atom n Still need to correct for Earth’s rotation n Result: Universal Coordinated Time (UTC) n UTC available via radio signal, telephone line, satellite (GPS) Kangasharju: Distributed Systems 4 Hardware/Software Clocks n Physical clocks in computers are realized as crystal oscillation counters at the hardware level n Correspond to counter register H(t) n Used to generate interrupts n Usually scaled to approximate physical time t, yielding software clock C(t), C(t) = αH(t) + β n C(t) measures time relative to some reference event, e.g., 64 bit counter for # of nanoseconds since last boot n Simplification: C(t) carries an approximation of real time n Ideally, C(t) = t (never 100% achieved) n Note: Values given by two consecutive clock queries will differ only if clock resolution is sufficiently smaller than processor cycle time Kangasharju: Distributed Systems 5 Problems with Hardware/Software Clocks n Skew: Disagreement in the reading of two clocks n Drift: Difference in the rate at which two clocks count the time n Due to physical differences in crystals, plus heat, humidity, voltage, etc.
    [Show full text]
  • BLAS: Broadcast Relative Localization and Clock Synchronization for Dynamic Dense Multi-Agent Systems Qin Shi, Xiaowei Cui, Sihao Zhao, Shuang Xu, and Mingquan Lu
    1 BLAS: Broadcast Relative Localization and Clock Synchronization for Dynamic Dense Multi-Agent Systems Qin Shi, Xiaowei Cui, Sihao Zhao, Shuang Xu, and Mingquan Lu Abstract—The spatiotemporal information plays crucial roles better capabilities beyond only a single agent, in terms of in a multi-agent system (MAS). However, for a highly dynamic robustness, scalability, and effectiveness. and dense MAS in unknown environments, estimating its spa- Realizing this vision will require MASs to overcome some tiotemporal states is a difficult problem. In this paper, we present BLAS: a wireless broadcast relative localization and clock new unique challenges. Among these, real-time precise relative synchronization system to address these challenges. Our BLAS localization and clock synchronization are two key challenges. system exploits a broadcast architecture, under which a MAS Relative localization is the process of determining the multi- is categorized into parent agents that broadcast wireless packets agent dynamic topology, and clock synchronization provides a and child agents that are passive receivers, to reduce the number common time reference for distributed agents. The spatiotem- of required packets among agents for relative localization and clock synchronization. We first propose an asynchronous broad- poral determination is crucial for MASs to perform basic oper- casting and passively receiving (ABPR) protocol. The protocol ations: 1) Coordination: Spatiotemporal coordination between schedules the broadcast of parent agents using a distributed time agents is necessary for a MAS to collaboratively carry out division multiple access (D-TDMA) scheme and delivers inter- tasks effectively. The difficulty of the coordination operations agent information used for joint relative localization and clock strongly lies in the knowledge of the relative position and clock synchronization.
    [Show full text]
  • Lecture 12: March 4 12.1 Review 12.2 Clock Synchronization
    CMPSCI 677 Operating Systems Spring 2013 Lecture 12: March 4 Lecturer: Prashant Shenoy Scribe: Timothy Wang 12.1 Review Objects and resources in distributed systems have names, which are mainly used to resolve to objects that your application is trying to access. DNS is an example of a naming system. DNS resolves a global sets of clients, and thus itself must be distributed. LDAP is another example of distributed naming system, which provides a more general lookup mechanism. 12.2 Clock Synchronization Assume that a machine has a system clock that tells you the current time. Every application can request the current time from the system clock. Time is unambiguous in centralized systems, so there is no syn- chronization problem in centralized systems. In a distributed system,however,each machine has its own clock. Note that clocks in machines are usually crystal oscillators. They are not very accurate. Thus, the asynchronization among the clocks will cause problems. Let’s use ”make” as an example. Make is an utility that allows you to incrementally compile every source file that was modified since the previous time it was run. The way it works is to compare the time stamps on the source files and the object files. If these files are located in different machines, the clocks on these machines may not be synchronized. Then if you save a source file on a machine with slower clock, its time stamp may be earlier than the object file, and make won’t recompile that source file. Many problems like this occur because of lack of synchronization.
    [Show full text]
  • Optimization of Time Synchronization Techniques on Computer Networks
    THÈSE Pour obtenir le grade de DOCTEUR DE L’UNIVERSITÉ GRENOBLE ALPES Spécialité : Informatique Arrêté ministériel : 25 mai 2016 Présentée par Faten MKACHER Thèse dirigée par Andrzej DUDA et coencadrée par Fabrice GUERY Préparée au sein du Laboratoire d’Informatique de Grenoble (LIG), dans l’École Doctorale Mathématiques, Sciences et Technologies de l’Information, Informatique (EDMSTII). Optimization of Time Synchronization Techniques on Computer Networks Thèse soutenue publiquement le 02 juin 2020, devant le jury composé de : Noel de Palma Professeur, Université Grenoble Alpes, Président Katia Jaffrés-Runser Maître de conférence, Université de Toulouse, Rapporteur Hervé Rivano Professeur, Université INSA de Lyon, Rapporteur Andrzej Duda Professeur, Grenoble INP, Directeur de thèse Fabrice Guery Responsable Innovation, Gorgy Timing, Invité 2 Abstract Nowadays, as society has become more interconnected, secure and accurate time-keeping be- comes more and more critical for many applications. Computing devices usually use crystal clocks with low precision for local synchronization. These low-quality clocks cause a large drift between machines. The solution to provide precise time synchronization between them is to use a refer- ence clock having an accurate source of time and then disseminate time over a communication network to other devices. One of the protocols that provide time synchronization over packet- switched networks is Network Time Protocol (NTP). Although NTP has operated well for a general- purpose use for many years, both its security and accuracy are ill-suited for future challenges. Many security mechanisms rely on time as part of their operation. For example, before using a digital certificate, it is necessary to confirm its time validity.
    [Show full text]
  • Adaptive Clock Synchronization in Sensor Networks
    Adaptive Clock Synchronization in Sensor Networks Santashil PalChaudhuri Amit Kumar Saha David B. Johnson [email protected] [email protected] [email protected] Department of Computer Science Rice University Houston, TX 77005 ABSTRACT 1. INTRODUCTION Recent advances in technology have made low cost, low Recent advances in technology have made low-cost, low- power wireless sensors a reality. Clock synchronization is an power wireless sensors a reality. Sensor networks formed important service in any distributed system, including sen- from such sensors can be deployed in an ad hoc fashion and sor network systems. Applications of clock synchronization cooperate to sense and process a physical phenomenon. As in sensor networks include data integration in sensors, sen- each sensor has a finite battery source, an important feature sor reading fusion, TDMA medium access scheduling, and of sensor network is energy efficiency to extend the network's power mode energy saving. However, for a number of rea- lifetime. sons, standard clock synchronization protocols are unsuit- As in any distributing computer system, clock synchro- able for direct application in sensor networks. nization is an important service in sensor networks. Sensor In this paper, we introduce the concept of adaptive clock network applications can use synchronization for data inte- synchronization based on the need of the application and gration and sensor reading fusion. A sensor network may the resource constraint in the sensor networks. We describe also use synchronization for TDMA medium access schedul- a probabilistic method for clock synchronization that uses ing, power mode energy savings, and scheduling for direc- the higher precision of receiver-to-receiver synchronization, tional antenna reception.
    [Show full text]
  • Time and Synchronization
    FYS3240- 4240 Data acquisition & control Time and synchronization Spring 2018– Lecture #11 Bekkeng, 02.04.2018 Why is time and synchronization so important? • Some examples: – In financial trading we must know the time accurately. – Important to reduce confusion in shared file systems. – Update databases (in parallel). – Tracking security breaches or network usage requires accurate timestamps in logs. – Used in electric power systems (fault recorders, billing meters, etc.). – Necessary in telecommunication networks. – Global Navigation Satellite Systems (GNSS), such as GPS, requires very accurate clock synchronization for position calculations. TU-artikkel : GPS mer enn bare navigasjon How good is a crystal oscillator (XO) ? • Interested in the long-term measurement stability and accuracy • Watch crystal oscillator: about 20 ppm, or worse – Error > 1.73 s in 24 hours (almost 1 minute drift in one month) • The accuracy can be improved using a: – Temperature compensated crystal oscillator (TCXO) – Oven controlled crystal oscillator (OCXO) • the oscillator is enclosed in a temperature controlled oven • Some DAQ card accuracy examples: – TCXO : 1 ppm – OCXO: 50 ppb Chip Scale Atomic Clock (CSAC) • Two orders of magnitude better accuracy than oven-controlled crystal oscillators (OCXOs). • Can keep track of the time if GPS-signals are lost (e.g. inside a building, or due to jamming). • Example: Microsemi CSAC – < 120 mW power consumption – < 17 cm3 volume – 35 g weight – two outputs; a 10 MHz square wave and 1 PPS (Pulse Per Second) The frequency of the TCXO is continuously – Maintains time-of-day (TOD) as a 32-bit compared and corrected to ground state hyperfine frequency of the cesium atoms, unsigned integer which is incremented contained in the “physics package”, which synchronously with the rising edge of the thereby improves the stability and 1 PPS output environmental sensitivity of the TCXO.
    [Show full text]