Scan Converters and Retrieving Digital Cctv Images

Scan Converters and RetrievingDigital CCTV Images

Publication No. 24/05

J Tighe

iScan Converters and Retrieving Digital CCTV Images

J Tighe



ii Publication No. 24/05

Scan Converters and Retrieving Digital CCTV Images

J Tighe


FIRST PUBLISHED 2005

CROWN COPYRIGHT 2005

The text of this publication may not be reproduced, nor may talks or lectures based onmaterial contained within the document be given, without the written consent of the Director,Home Office Scientific Development Branch.

Home Office Scientific Development BranchSandridge St AlbansAL4 9HQUnited Kingdom

Telephone: +44 (0)1727 816400Fax: +44 (0)1727 816233Email: [email protected]: www.hosdb.homeoffice.gov.uk

Management summary

This document is intended to give advice and guidance on the use of scan converters in theextraction of video from digital CCTV systems. If video cannot be removed from such asystem in its original digital format or via an analogue output, the only viable option is toconvert the signal intended for the systems monitor into a standard video format that canbe easily recorded. This process of scan conversion virtually always results in a loss ofinformation as the resolution and frame rate of a computer graphics signal usually exceedsthat of PAL video. However, the loss of useful information can be kept to a minimum bythe careful selection, installation and adjustment of appropriate equipment.

iiiPublication No. 24/05

1Publication No. 24/05

Contents

pageManagement summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2 Technical review of scan converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2.1 How scan converters work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.2 Other functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

3 Suggested procedures for operational use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

3.1 Selection of equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63.2 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93.3 Zoom to region of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103.4 Adjustment of video levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113.5 Output format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .123.6 Recording format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

4 Alternative methods to scan converting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

4.1 High quality frame grabber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134.2 Screen grab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Introduction

3

1 INTRODUCTION

The use of digital technology within the security industry has revolutionisedCCTV systems. Unfortunately, along with the undoubted benefits that thisdevelopment has introduced, police video technicians needing to extractinformation from such systems have experienced fresh difficulties beyond thoseencountered with analogue systems.

There is a hierarchy of preferred export methods currently employed by thepolice. At the top of this list is the removal of data in its original digital formatvia CD, DVD, hard disk drive, etc. However, the design of digital CCTVsystems has often not fully taken account of their role in crime investigation.Consequently, when police video technicians attempt to retrieve evidentialvideo in this form, they often find that it may not be possible to do so or thatextraction in this manner is prohibitively time consuming.

Many digital CCTV systems have a Video Out connection available, allowingan analogue signal to be output and subsequently recorded. Police videopractitioners will use this method if it is not possible to extract digital data,but the drawbacks of this form of retrieval are that there usually is a drop inquality, and metadata (e.g. time and date information) may be irrevocably lost.

Due to poor design, some digital CCTV systems neither allow the export ofdigital data nor provide an output of analogue video signals. In such cases, itis necessary to make use of the signal intended for the digital CCTV systemsmonitor by utilising a scan converter. This item of equipment provides ameans of transforming a computer graphics signal into a standard videosignal. The output video signal is usually of an analogue form such ascomposite or Y/C, but more expensive models sometimes also offer output in adigital format such as SDI. The use of scan converters is viewed very much asa last resort by police video technicians as it can be severe on quality and alsohas the potential for losing metadata.

This report is intended to provide guidance on the use of scan converters toextract video from digital CCTV systems. It is not a guide to the functionalityof individual models of scan converters and, as such, no references are madeto individual manufacturers or models. However, to gain an understanding ofthe technology used in scan converters, we have reviewed equipment rangingfrom that intended for domestic use (~300) up to that designed for use in abroadcast environment (~8,500).

The use of scan conversion as an export method is not recommended; extractionof digital information or analogue video signals are preferred options. However,lack of alternatives sometimes means that it is the only viable extractionmethod. To address this problem, HOSDB is actively engaged with industry toencourage the production of digital CCTV systems that are compatible withpolice users requirements. A guidance document has been produced thatprovides manufacturers, resellers, installers and purchasers with informationregarding these requirements. Copies of PSDB Publication No. 09/05 UK PoliceRequirements for Digital CCTV Systems can be downloaded fromwww.hosdb.homeoffice.gov.uk



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2 TECHNICAL REVIEW OF SCANCONVERTERS

2.1 How scan converters workA description of scan converters is contained within PSDB Publication No. 20/02 Video Processing and Analysis Training Reference Manual but is also included here for completeness.

The most commonly encountered form of computer graphics signals is aprogressive scan format with separate red, green, blue, and horizontal andvertical synchronisation components. Such a signal is usually carried by adedicated cable and connectors (see Figure 1) linking the graphics card of thehost PC to the computer monitor. Typical figures for computer signalresolution are shown below in Table 1. These formats will usually update at aspecified refresh rate in the range 50200 Hz. While it may be strictly accuratefor VGA to refer to only a certain range of resolution, it is typical for the termto refer to computer graphics signals irrespective of resolution, a conventionthat is followed in this report.

Figure 1 VGA connector and pin-out diagram

Table 1 Common resolutions for computer images

Name Resolution (pixels)

VGA 640x400

640x480

SVGA 854x480

800x600

XGA 1024x768

1280x768

SXGA 1280x1024

UXGA 1600x1200


1. Red

2. Green

3. Blue

4. Monitor ID bit 2

5. Ground

6. Red ground

7. Green ground

8. Blue ground

9. No pin

10. Sync ground

11. Monitor ID bit 0


13. Horizontal sync

14. Vertical sync


Technical review of scan converters

5

PAL video signals have 625 lines and 25 frames (50 fields) per second in aninterlaced format. Analogue video can be transmitted in a number of differentformats such as composite, Y/C and component. For example, with compositevideo, the luminance, chrominance and synchronisation information iscombined within a single channel. Consequently, when a scan convertertransforms a computer graphics signal to a video signal, much processing mustbe undertaken. The two parameters that have to be adapted are the frame rateand resolution as illustrated in Figure 2. As both of these parameters generallyhave higher values in a computer signal than is the case with PAL video, theprocess of scan converting normally results in the loss of information. Inaddition, the format has to be changed from progressive to interlace.

Figure 2 Parameter adaptation required in scan conversion

Not all scan converters use the same technique to achieve the necessarytransformations. To adapt the line rate the choice is between:

omission lines from the VGA signal are dropped at regular intervals toobtain the required number; or

extrapolation the information from a number of lines from the VGAsignal is used to produce the necessary number of output lines.

Similarly, omission or extrapolation is used to obtain the correct frame ratewith VGA frames being dropped or combined as required.


Odd fields = BlueEven fields = Red

VGA refresh rate

VGA

PAL

VGA

VGA resolution 625 lines

PAL

i) Video frame conversion

ii) Video line conversion

50 fields per second


6

2.2 Other functionalityScan converters typically offer other functions in addition to those alreadymentioned. These are very model-specific but can include:

video level adjustment the ability to adjust brightness, contrast,chrominance gain, gamma value, etc;

zoom many scan converters offer the ability to zoom into an area ofinterest. The range of the zoom facility is model dependent but typicallyallows a magnification of 200% and it may be possible to zoom separately in horizontal and vertical directions;

flicker reduction a very noticeable problem with the output from scanconverters is that pictures may appear to flicker (also known as twitter). Thisarises from fine line detail that is present in one field of a frame but not theother. By averaging over lines, the effect of this flicker can be reduced butonly at the expense of detail in the vertical direction;

noise reduction a smoothing filter is applied in the horizontal direction which reduces the appearance of noise but has the side effect of removinghigh frequency detail;

remote control either by infra-red hand-held device or PC softwarecontrolled via an RS232 cable;

freeze the output of the scan converter is frozen to the image beingdisplayed when this function is selected;

genlock the ability to use an external reference signal to synchronise thescan converters output with other video equipment; and

test pattern generation one or more test patterns (such as standard colour bars,PLUGE or cross hatch) may be displayed which can be useful in optimising theperformance of other video equipment in the processing chain.

3 SUGGESTED PROCEDURES FOR OPERATIONAL USE

3.1 Selection of equipmentIt is important that before any equipment is purchased, the user clearly identifieswhat he/she requires of the scan converter. This operational requirement willdetermine which equipment is suitable and the user can choose accordingly. Thefunctionality of models varies greatly as does the price. The main issues that need tobe considered are listed below.

3.1.1 Frequency response

Higher quality scan converters typically offer more functions and support greaterranges of input resolution and rates. Perhaps more importantly, the main benefit


Suggested procedures for operational use

7

that is apparent with more expensive equipment is the improved response athigher frequencies. This high frequency information corresponds to detail inimages. The amount of information capable of being contained within a computermonitor picture typically exceeds that which can be displayed on a video device, soloss of information is virtually inevitable in scan conversion. However, lowerquality scan converters lose more information than is necessary in thistransformation. One of the reasons for this is that they do not sample in thehorizontal direction as frequently as their higher quality counterparts and socannot capture high frequency components in this direction. The quality of theoutput processing circuitry of the scan converter may also be a factor, with bettercircuitry capable of displaying wider frequency ranges. This loss of high frequencydata, evident as fine detail in the video sequence, may have an impact on theamount of useful information that can be extracted.

3.1.2 Acceptable scan rates

The manufacturers specification often quotes minimum and maximum allowablefigures for the input resolution and frame rate (also known as vertical scan rate)supported by the scan converter. Rather than specifying these separately, a figurefor maximum horizontal scan rate may sometimes be stated. This figure can becalculated by multiplying the frame rate of the computer signal by the number oflines in the vertical direction (including vertical blanking). The response of scanconverters to input resolutions outside of their acceptable range varies. Some willfail to produce an image at all while others may produce pictures that are distortedin some way, e.g. with an erratic frame rate.

3.1.3 Interpolation v. omission

Another issue lies with the way in which the scan converter achieves thenecessary reduction in frames. As described in 2.1 How scan converters work,the two methods employed are omission and extrapolation. Omission results inbetter quality individual frames, but when they are run in sequence motion mayappear to stutter a little. Interpolation, i.e., averaging over a number of inputframes to produce one output frame, allows motion to appear more natural.Individual frames may appear to have ghost images caused by this averagingwhich may affect the critical viewing of a sequence. The operational requirementshould be used for deciding which is the most suitable technology.

3.1.4 Pre-purchase testing

As the manufacturers literature may not contain all required information and thosedetails that are present need to be carefully scrutinised, it is strongly recommendedthat models are obtained on a loan basis and tested before a product is purchased.To get some idea of the quality of the scan converter, it may be advantageous to testin four areas: frequency response, grey scale reproduction, aspect ratio, and droppedframes. It is essential that before any assessment takes place, all equipment should beset up properly (see 3.2 Installation).

Frequency response can be assessed by scan converting some test patterns. Softwareis available that can display frequency charts on a PC at a variety of resolutions; anexample is the freeware Philips Test Pattern Generator (available to download fromwww.benchmarkhq.ru/english.html?/be_monitor.html) but the same functionality isalso present in other products. It is important to remember that VGA signals arecapable of displaying higher frequencies than video signals, so information may



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have to be lost on scan conversion irrespective of the quality of the equipment. Ifthe scan converter is not capable of displaying a frequency, it may either just displaygrey or aliasing may occur.

Figure 3 Suitable image for testing frequency response (Source: Philips Test Pattern Generator)

Grey scale ramp and staircase test patterns can be used in conjunction with awaveform monitor to investigate whether there is a linear luminance reproduction.

Figure 4 Suitable images for testing luminance reproduction (Source: Philips Test Pattern Generator)

The accuracy with which the equipment can maintain the correct aspect ratio can bedetermined by frame grabbing a scan converted image containing a circle or cross-hatch pattern. It is essential that the geometrical properties of the frame grabber areestablished by capturing an appropriate video test signal directly and measuringhorizontal and vertical dimensions using image analysis software. With the framegrabber distortion accounted for, measurements on images grabbed from the scanconverter will reveal whether the equipment is keeping the correct aspect ratio.

Figure 5 Suitable image for testing geometrical properties(Source: Philips Test Pattern Generator)


Suggested procedures for operational use

9

If a sequence is running on the PC with the numbers 1 to 25 being displayed in order once every second, frame grabbing a sequence from the scan convertersoutput and subsequent analysis will confirm whether or not any frames in thesequence are being missed.

3.1.5 Other issues

Other practical issues that should be considered are:

does the product come with a warranty?

does the manufacturer provide technical support?

is all the necessary cabling provided?

is the scan converter rack mountable or easily portable?

what output formats are available? (See 3.5 Output format)

3.2 InstallationWhile the manufacturers instructions should always be followed when installingequipment, we use this section to make a number of general observations andrecommendations.

A scan converter is normally used in the configuration illustrated in Figure 6 withthe VGA signal looped through the scan converter. The scan converted signal isfed into an analogue video device (e.g. monitor, VTR) using cables appropriate tothe analogue video transmission format (e.g. a coaxial cable for compositesignals, three coaxial cables for component, S-video cable for Y/C). If SDI signalsare being used, the set-up is identical except a digital video device will be used.

Figure 6 Generic configuration for scan conversion

3.2.1 Impedance

A problem that has been observed with some equipment is the adverse effect onthe VGA signal when it is looped through the scan converter to the computermonitor. This is exacerbated if the impedance of the scan converter does notmatch that of the cabling/monitor. The impedance can usually be set either bynavigating the menu structure of the scan converter or by a manual switch locatedat the rear of the equipment.

PC

Analogue

video device

Scan converter

Computer

monitor

VGA cable

Coaxial cable(s)/S-video cable



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3.2.2 Optimisation

All equipment in the video processing chain should be optimised prior to its use to ensure the maximum amount of information can be obtained from the source material. More details of how to do this can be found in theEQUIPMENT chapter of PSDB Publication No. 20/02 Video Processing and Analysis Training and Reference Manual.

Some models of scan converter have an autoscan feature that should be usedwhenever the equipment is switched on or a new signal is input. Thisdetermines the parameters of the VGA signal and adapts the output signalaccordingly. While some models do this automatically whenever a change ininput signal is made, it may be necessary to restart other models before thenew parameters are detected. It is essential that the appropriate method for themodel in question is used otherwise the parameters used for conversion may beincorrect. The details of parameters used can normally be viewed either on anLCD in the unit or via an on-screen display.

3.2.3 Refresh rate

While the refresh rate of a PC graphics card is typically 60 Hz or above, thesignal usually carries a lot of redundant data as far as digital CCTV material isconcerned as this has almost always been captured at a maximum of 25 framesper second. It would be desirable for the scan converter to produce a videosignal retaining the useful information contained within the computer graphicssignal and discard the redundant part. Unfortunately, with a scan converterthat achieves its output refresh rate by omission, there is no guarantee that the25 frames per second it produces will replicate the CCTV material in itsentirety. For example, if the PC graphics card outputs at 60 Hz, the scanconverter will not be able to drop VGA frames at completely regular intervalsto achieve the standard video rate and so some frames of the CCTV materialmay be missed. If the operator has access to the host PC, an improvement maybe obtained by ensuring the graphics cards refresh rate is a multiple of 25 Hz. Iffor example a refresh rate of 75 Hz is chosen, then the scan converter can ignoretwo out of every three VGA images to obtain the correct PAL rate. Althoughthis may improve the situation it by no means guarantees success and usefulinformation may still be dropped. Furthermore the host PC usually will onlyoffer to display at certain rates and these may not include multiples of 25 Hz.

If it is essential that all frames are captured, it may be worth considering usingthe CCTV playback software to replay the material at less than full speed.With each CCTV image being displayed for longer, it is less likely that the scanconverter will miss any of these when producing its output video sequence.

3.3 Zoom to region of interestTo enable the maximum amount of useful information to be extracted, it isrecommended that the zoom function of the scan converter be used to display the region of interest (ROI) so that it takes up the entire output image.

The zoom function on some cheaper models of scan converter has just onesetting, typically allowing magnification to 200% of the original size. Higherquality scan converters allow the degree of zoom to be varied and it may bepossible to adjust the horizontal and vertical magnifications separately. The


Suggested Procedures for Operational Use

11

area of the computer monitor picture being scan converted that is beingenlarged can be adjusted so that the output of the process is the required ROI.

It has been observed that the aspect ratio of the scan converter output couldvary slightly on zooming. The extent to which this occurs varies according tomodel, input resolution, level of zoom, etc. However, such deformations arerelatively minor and may only be important if the intention is for the images tobe subjected to photogrammetry. Furthermore, the aspect ratio deformations caused by the scan converting could well be negligible compared to lens and recording distortions.

An added benefit of zooming is that fine detail in the 100% sized image withfrequencies affected by cross-luminance (see 3.5 Output format) will no longershow this phenomenon when displayed in zoom mode. Of course this refers tocross-luminance that occurs at the scan converting stage; there may already becross-luminance in the CCTV images arising at the capture stage.

If the ROI takes up most of the original screen size, it may not be practical touse the zoom option. This means that small detail (e.g. time and dateinformation) may not be apparent when scan converted. This problem can beaddressed by using something like the magnify facility available on Windowsplatforms (started by selecting Run from the Start menu, typing magnify inthe box and then the return key). A magnified view of the portion of screen towhich the mouse cursor is pointing will then appear at the top of the screen. Byplacing the mouse pointer over the time and date, selecting an appropriate levelof magnification, and then simultaneously pressing the alt and m keys tostop the magnifier following the mouse cursor, the time and date informationshould be displayed large enough to still be legible after scan conversion.

3.4 Adjustment of video levelsIf the maximum amount of information is to be retained, it is important thatvideo levels on the scan converter should be correctly adjusted. Suchadjustment is dependent on the image in question and is ideally performedusing a waveform monitor. For composite and Y/C format signals, theluminance component should be 0.7 V from black to peak white (see Figure 7).

Figure 7 The waveform of the luminance component of a PAL video line



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If this range is exceeded, this could result in clipping and loss of detail in someareas of the picture. On the other hand, if the full dynamic range is notutilised then the output image will appear to have poor contrast and noise willbe more perceptible.

A noticeable feature of scan converted images is how different colours can appearas compared to the same images displayed on a computer monitor. While theremay be some controls that allow adjustment of colour related parameters, such aschroma gain, it is almost impossible to ensure that the colours are reproducedwith complete accuracy. In addition to any errors arising in the conversionprocess, it is important to note that the colour gamut available to the computer isgenerally different to that of analogue video images, i.e. the PCs RGB is not thesame as that of PAL. Therefore it may not be possible to replicate some coloursof the original signal in the scan converters output. Furthermore, the bandwidthavailable for colour information in Y/C and composite video signals is muchsmaller as compared to RGB signals. Consequently, if extremely accurate colourmeasurements are required, care needs to be taken.

3.5 Output formatThe range of output formats provided by a scan converter is model specific.However, increased price usually reflects both a wider range of possible outputsand a higher quality. Of course the quality of scan converted digital CCTVmaterial is ultimately limited by that of the original recording. However, bychoosing an appropriate export method, the quality of the material is bestpreserved with the minimum amount of degradation. The most frequentlyencountered methods in approximate descending ranking of quality are:

Serial Digital Interface (SDI) this export format is available only on scanconverters designed for use within a broadcast environment. This format isintended to be of high quality with the only drawback to its use being thatonly expensive, broadcast-quality equipment has been designed to accept it;

component this method has three channels carrying either separate red,green and blue (RGB) signals or luminance and colour difference signals(YPBPR). Often, the scan converter is capable of exporting in eithercomponent form using the same connectors with the choice of whichformat is used set by navigating through the menu structure. Thesetransmission formats retain high quality in images as is evident whenviewed on a compatible monitor. However, the drawback with this methodis the general lack of recording devices that can accept component signals;

Y/C another popular format of fairly high quality is by Y/C which hasseparate channels for luminance and chrominance. This is available as aninput on many video recording devices and some video laboratories are setup to transport video in this format;

composite almost all scan converters allow export in this format. As thesignal contains luminance, chrominance, blanking and synchronisationinformation in one channel, it can be carried via one BNC cable. The maindrawback associated with its use concerns the quality of images obtained.As compared with RGB, the colour information in composite signals issub-sampled. In addition, the 4.43 MHz colour subcarrier is contained


Alternative methods to scan converting

13

within the 5.5 MHz luminance signal. This leads to a phenomenon knowas cross-luminance, where high frequency detail is misinterpreted as colourinformation. In scan conversions this is particularly noticeable with finedetail such as screen text which can become illegible;

RF signal only found on the very cheapest converters, this is a UHF signalintended to be input to domestic television sets via aerial sockets. Thisproduces low quality images and is extremely susceptible to noise and cross-channel interference. Consequently its use should be avoided if at all possible.

3.6 Recording formatThe appropriate recording format to use will depend on the quality of theoutput from the scan converter and which formats are used at other stages inthe evidential chain. At all stages in this processing chain it is important toretain the maximum amount of information. Therefore, VHS videotape maynot be the most suitable medium to record an SDI signal as the drop in qualitycan be substantial. By recording low quality video onto high quality media, allthe benefits of using such media may not be realised, but what detail is presentcan be better preserved and the signal-to-noise ratio kept as high as possible.

4 ALTERNATIVE METHODS TO SCAN CONVERTING

4.1 High quality frame grabberBy using a high quality frame grabber it is possible to capture VGA signalsonto another PC and store in an appropriate digital format. The benefit thishas over scan converting is that the recorded image does not have to conform tocomposite analogue video standards regarding resolutions or frame rate, i.e. itcan be done at the native resolution and rate of the VGA signal. However,working at high resolutions and rates is very computer intensive and generatesvery large files and so a specialist frame grabber and high specification PC isrequired. Saving to a compressed format and/or reducing recorded frame ratewill reduce file sizes but can adversely affect quality. Work is currently beingundertaken at HOSDB to ascertain the viability of this approach.

4.2 Screen grabSoftware is available to grab the contents of a window and to store it in asuitable video format (e.g. AVI). However, this software would normally beinstalled on a separate computer and used in scenarios where it has alreadybeen possible to extract the data and replay program from the original CCTVrecorder. It may not be practical to install it on the CCTV recorder itself andwould not be worthwhile if the system then did not have the facility to exportthe recorded sequence. If such an export method exists, it is recommended thatit be used to extract the original data and player, so that the processing can becarried out elsewhere.



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5 SUMMARY

While the use of scan converters is not a preferred option for extracting videoevidence from digital CCTV systems, it is often the only viable approach.While it is virtually inevitable that scan converting will result in the loss ofinformation, this loss can be kept to a minimum by appropriate selection,installation and adjustment of equipment.

Before settling on a particular model of scan converter it is important that thepurchaser clearly define what he/she desires in terms of frequency response,acceptable input parameters and how the scan converter achieves the necessaryframe rate. To determine whether the equipment in question meets thespecification, pre-purchase testing should be performed.

In addition to following manufacturers instructions for installing the scanconverter, some other issues may need to be addressed including optimisationof the video processing chain and how to minimise the possibility of missingCCTV frames. To extract the maximum amount of relevant information froma digital CCTV system via scan conversion, it is recommended that the scanconverter be zoomed into the region of interest. By choosing appropriateoutput and recording formats, and correctly adjusting the levels of the scanconverters output signal, as much of this information as possible is retainedfor future processing and analysis.


Home Office Scientific Development BranchSandridgeSt AlbansAL4 9HQUnited Kingdom

Tel: +44 (0)1727 816400Fax: +44 (0)1727 816233Email: [email protected]: www.hosdb.homeoffice.gov.uk

Documents

Scan Converters and Retrieving Digital Cctv Images