Measuring Latency for Video Surveillance Systems

Rhys Hill, Christopher Madden, Anton van den Hengel, Henry Detmold, Anthony DickAustralian Centre for Visual Technologies

School of Computer ScienceThe University of Adelaide

http://www.acvt.com.au/research/surveillance/

Abstract—The increased flexibility and other benefits offeredby IP network cameras makes them a common choice forinstallation in new and expanded surveillance networks. Onecommonly quoted limitation of IP cameras is their high latencywhen compared to their analogue counterparts. This causessome reluctance to install or upgrade to digital cameras, andis slowing the adoption of live, intelligent analysis techniquesin video surveillance systems. This paper presents methodsfor measurement of the latency in systems based upon digitalIP or analogue cameras. These methods are camera–agnosticand require no specialised hardware. We use these methods tocompare a variety of camera models. The results demonstratethat whilst analogue cameras do have a lower latency, most IPcameras are within acceptable tolerances. The source of thelatency within an IP camera is also analysed, with prospectsfor improvement identified.

Keywords-Video Surveillance; Camera Comparison; La-tency; Digital; Analogue

I. INTRODUCTION

There are many aspects that contribute to a useful andeffective video surveillance system; however one which hasreceived little attention in the literature is the latency ofthe system. The two important classes of latency within asurveillance system are image latency and command latency,both of which depend on other factors like the video frame-rate. For the purposes of this paper, image latency willbe defined as the difference in time between the scene infront of a surveillance camera changing, and that changeappearing on an appropriate monitor. Command latency,applicable only to PTZ cameras, is the time between acommand being issued to change a cameras view, and whenthe view on an appropriate monitor changes in response. Theframe-rate is the frequency at which video frames arrive atthe observation point.

Image latency contributes directly to command latency,and is the more important of the two classes. Where imagelatency is low, in the order of a few milliseconds, a surveil-lance operator is said to be observing events in real time,as there is no perceptible delay. When the latency is high,in the order of few seconds, the operator may experience

This work has been partially funded by the Defence Science TechnologyOrganisation and the South Australian Premier’s Science and ResearchFund.

confusion or be unable to effectively respond when eventsoccur in the scene. Proponents of analogue systems claimthat high command latency in digital Internet Protocol (IP)camera systems is reason enough to dismiss them, since itmakes direct PTZ steering difficult.

An analysis of surveillance camera latency is importantfor a number of reasons. First, automated surveillancecamera analysis is becoming more common as the qualityof available algorithms improves. As surveillance networksbecome progressively larger, it is likely that their operatorswill desire software to command more aspects of the system,and to steer PTZ cameras in particular. A number of papershave begun to explore this area [1], [2], but controllingmechanical devices is difficult if their latency characteristicsare unknown.

Latency analysis is also important for those choosingwhat type of surveillance system to install. The majorityof deployed surveillance systems are networks of analoguecameras, but an increasing number of digital cameras arebeing installed [3]. The key difference between the analogueand digital systems is the link between the camera andthe display components. Analogue systems use a seriesof dedicated direct links, usually constructed from coaxialcable, between each camera and a screen for display. Digitalsystems generally employ standard IP networks, where eachcameras utilises in-built video compression hardware to cre-ate a digital video stream that can be transferred across thenetwork. This digital stream is then normally decompressedon a PC and displayed on a computer screen. If the analoguestream passes through a digitiser on its way to the screen,then the system is effectively digital, and loses any latencybenefits a pure analogue system provides. Digital systemsprovide a large number of additional, well known, benefits,such as improved image quality, resolution, frame–rate andscalability. However, these will not be discussed in detail.

This paper investigates the latency within analogue anddigital systems over a variety of different digital cameramodels. First, the core of both analogue and digital surveil-lance systems are detailed. Next, the methods used to testboth the image and command latency across a variety ofcamera models are detailed. Finally, the results of theselatency experiments are then discussed along with theirimplications in video surveillance systems.

II. VIDEO SURVEILLANCE SYSTEM

Video surveillance systems can utilise either analogueor digital video cameras to observe their surveillance area.Both systems utilise similar imaging technology, typicallyconsisting of a CCD or CMOS sensor that records lightfocussed upon the sensor by a lens. The sensor convertsthe light into either an analogue signal in the case of a CCDsensor, or a digital signal in the case of a CMOS sensor.In an analogue camera, a CCD is generally employed, andits signal is fed directly into the output connection on thecamera. In a digital camera with a CCD sensor, the analoguesignal is first digitised then compressed, before being sentout over the network. A CMOS–based digital camera willsimply skip the digitisation step. In all cases, the final signalis sent through an appropriate communication medium, andis then received and displayed by an output device.

A. Analogue Systems

Analogue surveillance systems have been in use fordecades, and still dominate the surveillance industry. Thislong history has lead to the development of two key stan-dards, which maximise the interoperability between differentcomponent manufacturers, as well as defining resolutionfor storage and display. In an analogue system, there is adedicated link between each camera and the output device.An analogue video stream will conform to either the PAL(maximum of 720 × 576 pixels) or NTSC (maximum of720 × 480 pixels) video standard. Both PAL and NTSCare normally interlaced, meaning that each individual imageis only 288 or 240 pixels high respectively, where themaximum image size is used.

Scene Lens Sensor Cable Monitor

Figure 1. The typical layout of a pure analogue video surveillance system.The scene is imaged onto a sensor, whose output is fed directly through acable into a monitor. Everything other than the scene itself is consideredpart of the system.

Analogue links are generally constructed from coaxialcable, as shown in Figure 1, but radio links and optical fibrecan also be used. The video stream can be digitised andcompressed, for transfer using a standard data network [4],but this introduces the same degree of latency that digitalIP cameras exhibit, removing the only significant advan-tage that analogue cameras provide. Most Digital VideoRecorders (DVRs) provide a direct analogue pass–throughconnection, which may be connected to a standard television,providing the low latency associated with analogue cameraswith the digital storage of IP cameras. Another difficulty

associated with analogue cameras is the need to run powercables for each camera. This leads a decentralised power–supply architecture that increases the difficulty of employingprotective measures such as an Uninterruptible Power Sup-ply (UPS) to cope with power loss or fluctuations.

B. Digital Systems

Although they are the newer technology, digital surveil-lance systems have been in operation for over a decadeand their market share is growing significantly [3]. Digitalsystems are extremely flexible, allowing them to adapt tothe changing requirements of modern surveillance systems.Figure 2 shows an example of a minimal digital system.When comparing Figure 1 to Figure 2, the core reason for adigital systems increased latency become clear – a digitalsystem simply involves more components. Each of thesecomponents is important and contribute to a digital system’sflexibility, at the price of additional latency.

Scene Lens Sensor Monitor

Encoder Decoder

Network Stack Network Stack

Ethernet

Figure 2. The typical layout of a minimal digital video surveillance system.As with the analogue system, the scene is imaged, but the signal must thenpass through a range of components prior to arriving at the monitor.

C. Video digitisation and compression

A vital component of any digital system is the digi-tisation and compression of each camera’s video stream.Digital surveillance cameras have hardware within themto perform this task, whereas analogue cameras can bedigitised using an external device, such as a DVR or videoserver. The choice of compression method depends largelyon constraints with the network, such as available bandwidthand storage. A detailed overview of the possible codecscan be found in [5], [6]; however MJPEG and MPEG-4are the most common codecs used in existing IP cameras,with H.264 becoming available in more recent cameras. TheMJPEG standard encodes and compresses each video frameindividually, so every frame is a keyframe. MPEG-4 encodesthe video stream using a combination of key frames andB–frames, which contain pixel–based temporal differenceinformation. P–frames can also be included, which encodebi–directional temporal differences. However, P–frames adda large amount of additional latency and encoding com-plexity, so are normally not included in surveillance camera

video streams. H.264 is a newer version of MPEG-4 thatis more CPU intensive, employing object–based temporaldifferences, and achieves higher compression rates at thesame quality as a result. Both MPEG-4 and H.264 introducefurther latency into a digital system, due to additionalbuffering and compression complexity over MJPEG. For thisreason, MJPEG compression was used in all testing withinthis paper. Investigating the latency of systems based uponthe MPEG-4 and H.264 would be complex and may be thesubject of future work.

D. Network Latencies

The network itself, along with the transport used uponit, can be an important source of latency. Current digitalsurveillance cameras employ either HTTP over TCP/IP orRTP over UDP. Those cameras which use the MJPEG codecmost commonly use HTTP. A simple test was carried outto quantify this latency. A CGI program was created whichemulates an MJPEG video stream, but sends a time–stampand random data in place of each JPEG. Access to the CGIwas provided by the standard Apache webserver daemon. Asecond program was constructed, to act as a client, whichparses this stream, and compares the time–stamps in thestream to the local time. The CGI program was written tosend both the time–stamp and the data in a single operation,forcing the entirety of the information to arrive before thetime–stamp can be accessed. The clocks on the client andthe server were synchronised using NTP, and 5000 framesstreamed, using data sizes randomly chosen between 40 and250 kilobytes. Both client and server were connected tothe network via 100Mbit/s ethernet connections. The resultsfrom this test are shown in Figure 3. As expected, the latencyincreases with the size of the data.

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Webserver Latency for Different Amounts of Data

LocalRemote (Same Subnet)Remote (Different Subnets)

Figure 3. Latency measured when streaming data in the same manneras an MJPEG video stream. The server was hosted on the same machinemachine as the client, a machine on the same subnet and a machine on adifferent subnet, to generate these results.

E. Inherent Display and Capture Latency

In addition to the latency of encoding and sending thesignal, other components of the system can impact overalllatency. Many of these sources are easy to identify, although

some can be more difficult than others to evaluate accurately.For example, a common latency source in analogue anddigital systems will be the display device, which can beassumed to have a fixed refresh rate. Current LCD displaytechnology generally provides a refresh rate of 60Hz, leadingto a latency of ∼17 milliseconds. Regardless of the cameratechnology, this imposes a hard lower bound on the systemlatency.

In digital systems, the network itself is the main sourceof latency. Section II-D showed results for a simple setof network types, but real networks are often very largeand complicated, with bursty traffic behaviour that can leadto significant variations in system latency. Such behaviourcan be resolved through careful network management andbandwidth allocation. The size of the images also plays akey role in network latency, since sending a smaller amountof data will clearly take less time. Figure 4 shows the imagedata size as the quality settings on a particular camera arevaried. The next most important latency source is the timerequired to decompress a single frame. Figure 5 shows thetime required to decompress a single frame across a rangeof quality settings.

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Figure 4. Image size for various quality settings at 640x480 resolution,for an Axis 211 camera across the network.

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Figure 5. Image decompression time for various quality settings at640x480 resolution, for an Axis 211 camera across the network.

III. METHODS FOR DETERMINING CAMERA LATENCIES

The methods proposed here aim to measure the end–to–end latency of video surveillance cameras. That is, the timedifference between when events occur in front of a cameraand when they are observed on an appropriate monitor.

Such a measure will capture not only the latency withinthe camera, but also across the link medium, so that itbetter reflects the latencies that occur in real systems. Digitaland analogue cameras require different approaches to thisproblem, since the image from the analogue camera cannotbe directly analysed by a computer. The image could bedigitised, but that would simply adds an additional source oflatency. This section will describe a number of different teststhat were devised and implemented to probe various aspectsof the latency of digital and analogue video surveillancecameras.

A. Analogue Video Latency

The method for measuring analogue latency was based onthe structure of a pure analogue sytem. The analogue camerawas placed such that it was observing a scene that waschanging in a controlled manner. The output of this camerawas connected directly on a television screen for display. Athird camera, a digital SLR still–camera, was placed behindthe two so that it could observe the controlled scene and thetelevision screen. By capturing a series of images via theSLR, the difference between the scene and the screen couldbe observed, and quantified in terms of latency. Figure 6includes an image captured by this process.

The controlled scene was provided by displaying a seriesof squares on a laptop screen, arranged in a 40 × 25 grid.The squares were drawn left–to–right, top–to–bottom, witheach corresponding to one millisecond. The squares weredrawn either white or yellow depending on if the elapsedtime since the start of the test in seconds was even or odd.The difference between the image on the screen of the laptopand the television thus yielded the latency in the system.

B. Digital Video Latency

The digital image stream provided by digital camerasallows for the implementation of an automatic processto measure end–to–end latency. The digital camera waspositioned to observe a changing, controlled scene, and thevideo was streamed to a computer for analysis, as shown inFigure 7. The same computer was used to display both thechanging scene and to detect changes in the video stream.This ensures accuracy in timing the delay between the scenechanging and the change being observed, and also allows formultiple automatic measurements to taken.

The scene initially consisted of a red screen, which isdisplayed for a short period of time to allow for the systemto settle. The screen then displayed a green square inthe middle of the screen and analysed each video framestreamed from the camera until the square was observed.The time between the image being display and detectedwas recorded for each repetition of the test. The sameprocess can be used to analyse a variety of configurationsincluding different camera models, different image sizes andcompression levels, where the camera supports these, anddifferent network configurations.

Figure 7. This figure shows the experimental setup for measuring digitallatency. A digital surveillance camera is shown pointing at a laptop display,whose image changes in a known fashion.

C. Digital Frame–Rate Consistency

In addition to the latency of the camera, the frame–rate ofthe video stream is also important. If the frame–rate is lowor changes significantly over time, then automated analysissoftware may strike difficulty, particularly if frame–to–frametracking is used. The frame–rate of an analogue cameras isconstant, unless the direct link is broken, in which case thevideo stream stops completely. The frame–rate was either25 or 30Hz, depending on whether the camera is using thePAL or NTSC standard. Digital surveillance cameras usuallyallow for different frame–rates but, importantly, may changeover time due in particular to the bandwidth constraints onthe network.

The digital frame rate and its consistency can be measuredby sending a video stream from the camera to a computer.The computer can then measure the time at which eachframe arrives, and compute the time deltas between con-secutive frames. By monitoring a stream over a period ofminutes, the frame–rate consistency can be determined.

D. Digital PTZ Command Latency

Command latency is the time difference between a PTZcommand being sent to the camera, and effect upon thecamera view being observed. This can be measured fordigital PTZ cameras in a similar manner to digital videolatency, but rather than detecting the green square, thesoftware waited for the square to move. Once movementwas observed, the camera was steered back to its originalposition, and the software then streamed a number of framesto allow the camera to settle. Once the camera had settled,the process was repeated.

IV. RESULTS

This section outlines the results of evaluating the surveil-lance cameras. The analogue results are based upon aSony EVI-D31 analogue camera. The digital camera modelsexamined were an Axis 206, an Axis 211M, an Axis 215,an Axis Q1755 and a Canon VB-C50i. Of these cam-eras, the Axis 215 and Canon VB-C50i models had PTZcapability and were evaluated for their command latency.The command latency of the analogue camera could notmeasured, since no automated measurement process exists.

(a) Analogue Experimental Setup (b) Cropped Results

(c) Firewire Results

Figure 6. Analogue latency results showing the difference between the observed scene, and the television screen. The left–most image shows the fullexperimental setup, whereas the middle shows two are hand-corrected crops of the two screens, placed side–by–side for clarity. The final image show theFire-i camera experiment.

The Axis 211M and Axis Q1755 have high resolutionmodes that were evaluated at 1280×1024 and 1920×1080respectively.

A. Analogue Latency Results

As discussed in Section III-A, the results presented hereare based on images of a television and a laptop screen.Figure 6 shows a series of images from this set. The shutterspeed of the SLR camera observing the scene was setdeliberately high, to ensure a crisp image of the differencebetween the two screens, free of motion blur from viewingrepeated scene changes. This high shutter speed allows thevertical scanning line of the television to be observed. Theresults clearly show that the latency in the system is definedby the speed of this line, as was the expected outcome.In cases where the scan–line appears before the currentlychanging row of squares, the latency appears high, whereaswhen the scan line is after, the latency appears low. Althoughthe latency of the system cannot be accurately measuredbeyond this, an upper bound of 40ms can be defined, derivedfrom the 25Hz update of a non–interlaced PAL system. Notethat most television systems are interlaced, however, whichyields lower image quality but a 50Hz, or 20ms, update rate.

The same test was also carried out using a Uni-Brain Fire-i video camera, the closest digital equivalent to an analoguecamera, in terms of direct connectivity. In this case, thetest yielded a latency of approximately 97ms, based onfour measurements. This correlates well with prior work[7].Figure 6 shows examples image from this test.

B. Digital Latency Results

This section explores the latency results obtained fromtesting a variety of digital camera models. These cameraswere all tested at a resolution of 640×480, the most com-monly used size in practice, with two models tested at high-definition resolutions as well. Each test was carried out for 5different quality settings, from 20% through to 100%. Thecameras were all connected via a dedicated switch to thehost computer, unless otherwise specified. Figure 8 showsthe video latency results for each of the camera types.

Axis 212Axis 206

Axis 215

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Figure 8. Digital latency results over a dedicated switch for a variety ofcamera models. At 100% quality the following latencies are off the graph:Axis 211M at 1.20s, Axis 211M HD at 1.62s, and Axis 212 at 0.594s

A number of interesting points are illustrated by the re-sults. The high–definition cameras exhibit very high latencyas the quality increases, which cannot be explained exceptby inadequacies in the camera hardware. The specificationfor the Axis 211M, for instance, allows for up to 12 framesper second at maximum resolution, but 30 frames per secondat 640×480. The manufacturers do not specify under whatconditions these rates are achievable, however.

Another interesting point is that the best performingcameras, the Axis 206 and the Canon VB–C50i, are the onlydedicated MJPEG cameras. All the others are ALSO capableof MPEG-4 or H.264 encoding. In modern cameras, a singlechip is used to do all video encoding. This result impliesthat by adding these additional codecs, MJPEG performancesuffers, either due to technical limitations or design focus onthe newer codecs.

A final note is the difference between the Axis 215 andAxis Q1755 cameras in HD mode. Although both camerasare from the same manufacturer, the Axis Q1755 has muchbetter performance.

The latency figure can be analysed for a particular setting.As an example, the Axis 211 has a latency of approximately

150ms at 640×480 with 80% quality, corresponding to avery common used configuration. Table I shows a specula-tive breakdown of the latency, based on the measurementspresented so far. If it is assumed that the latency of thefirewire camera, approximately 97ms, represents an approx-imation of the latency inherent to a digital sensor, then thetotal latency of the camera can be accounted for.

Table IAXIS 211 LATENCY BREAKDOWN

Network Latency 7msDecompression Time 9msLaptop Display Refresh Rate 16msCamera Frame Rate 33msTotal 65ms

C. Frame–rate Consistency ResultsFrame–rate consistency was again measured for all the

digital surveillance cameras. Figure 9 shows the result ofthis experiment, and a number of interesting points areapparent. The Axis 215, the Axis Q1755 and the CanonVB-C50i all appear limited to 25Hz. Both the 215 andthe VB-C50i are digital PTZ cameras. The most commonmethod of construction for such a camera is to mount ananalogue sensor on the PTZ head, to reduce the amount ofcabling between the head and the main–board of the camera.The Axis Q1755 is an interesting camera, appearing to bea normal HD video camera complete with component videoconnectors, with a digitiser embedded within it. It showsvery impressive performance across a range of compressionsettings, dropping performance only when at very highquality settings and video size.

Axis 212Axis 206

Axis 215

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Figure 9. Average frame-rates for a variety of camera models across adedicated switch

The Axis 206 and Axis 211 are both static cameras, andare thus likely using digital sensors directly, and are not

limited by traditional frame–rates. The Axis 211M and theAxis 212 both performed poorly in this test. The reasonsfor the 211M’s poor performance are unclear, but the Axis212 is a virtual PTZ camera (a high resolution omni–directional camera which sends sub–regions of the video toemulate PTZ), which must sub–sample and undistort everyvideo frame that leaves the camera, making it likely that itsperformance will be lower than the other cameras. The Axis211M also displays a consistent frame–rate fall–off acrossquality levels and images size, suggesting internal resourcelimitations.

D. PTZ Command Latency Results

The final test measured command latency in the twomodels of digital PTZ camera: the Axis 215 and the CanonVB-C50i. Figure 10 shows the results of this test. In manyways, this test is the most critical as low command latencyis important for live PTZ control. Achieving a latencyof around 200ms would bring it within average humanreaction time [7]. Both models of camera come very closeto this limit, with the Canon VB-C50i better for low qualitysettings.

These PTZ latency results are difficult to interpret, how-ever, due to the construction of the test. The test looks fora single column of pixels along the side of the square tochange, as the first point when motion can be detected.Low quality settings would likely smear the edge, leadingto inflated times. Conversely, a human is not likely to noticesuch a small change in the imagery. The Axis 215 also hadto have rate–limiting enabled, to allow the test to succeed,which may also inflate its latency figures.

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Figure 10. PTZ command latency across a dedicated switch

V. CONCLUSION

The end–to–end latency within video surveillance systemshas received little attention in the literature, but is a commonpoint of discussion within the wider video surveillance

community. The common perception is that the latency ofdigital surveillance cameras is too high to allow live steeringby a human operator – which in turn is slowing the spreadof digital surveillance cameras and live, intelligent analysistechniques.

This paper has demonstrated a number of attributes ofdigital surveillance cameras which are not immediatelyapparent, such as constrained frames rates and reducedperformance across different size and quality settings. A setof methods designed to evaluate the latency of a digital oranalogue video surveillance cameras were also presented.The methods were then used to compare the latency of anumber of common surveillance camera models at variousquality settings and image sizes.

The results of these experiments demonstrate that thedigital encoding of video streams, which is the basis ofthe flexibility of digital surveillance systems, is higher thanthat of analogue systems. The lowest latency displayed bya digital camera in any test was around 120ms, far higherthan the upper bound of 40ms on an analogue camera. Theresults also showed a wide range of latencies across modelsand quality settings, meaning that the selection of both isimportant in the design of surveillance system where PTZcameras will be steered by human operators. Note, however,that this figure is lower than the commonly quoted 200mstypical human reaction time [7].

This level of latency should not have a significant impactupon the practical operation of a surveillance system, oran operators ability to react to events. Indeed, the addedflexibility of networked digital cameras, including theirability to be directly analysed by software, far outweighsany perceived penalty induced by this latency. Such flex-ibility allows for more effective monitoring of the systemby the available operators, through an increased ability tofocus their attention on important areas of change. This isespecially important in large surveillance networks where itis impossible to display and watch all the video streams atthe same time.

The command latency of digital cameras was shown tobe high, with a number of qualifications. Measuring theperceived latency would be a difficult, but useful, experi-ment. Even with the qualifications provided, the Canon VB-C50i was able to meet the 200ms threshold, showing thatdigital surveillance cameras provide low–latency operation.Most current generation digital PTZ cameras side–step thelatency issue by providing new, more effective, methods ofcontrol. For instance, many allow the user to click on, orselect regions of the view which should be focussed upon.The user then has a less active, and certainly less continuous,role in steering the camera, which will in turn minimise theeffect of any latency in the system.

These latency results demonstrate that although digitalcamera have higher latency than their analogue counterparts,it is low enough in most cases that it will not impinge uponday–to–day operations of real surveillance systems. Theadditional benefits of digital cameras are numerous, such ashigher resolution images, the flexibility of digital encodingand compression and the ability to use analytical software,overcome the latency considerations. It is likely that in thenear future, intelligent PTZ steering algorithms will becomeavailable, which will remove the need for a human operatorto directly interfere with the system during routine operation.A range of work in the literature is already moving downthis path [1], [2]. With new developments in image encoding,such as the multiple pipelined encoding process proposed byPureVu [8], video compression encoding latency may alsobe reduced to a less below the threshold of perceptibility inthe near future.

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