university of surrey school of electronics and physical

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University of Surrey

School of Electronics and Physical Sciences

Department of Electronic Engineering

Undergraduate Level 1

Media Engineering Laboratories

http://www.ee.surrey.ac.uk/Teaching/Labs/MediaEngineering

ME2: Introduction to Video Engineering

ME2a - Basic Video

ME2b - Video Cameras

DATE OF ISSUE: January 2007

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Course resources

Recommended books

An Introduction to Video Measurement 2nd Edition

Peter Hodges

Focal Press 2001

ISBN 0-240-51641-9

Standard Handbook of Video and Television Engineering

J. Whitaker / B. Benson

McGraw-Hill

ISBN 0-07-069627-6

Basic TV Technology 2nd Edition

R.L. Hartwig

Focal Press 1995

ISBN 0-240-80-228-4

(UOS library)

Television video transmission measurements

L.E. Weaver

Marconi Instruments

Out of print

(UOS library)

Test equipment

Oscilloscope – Tektronix TDS 1002 or similar with line select and video triggering

Grey-scale chart

Video test pattern generator – derived from master SPG

Camera rostrum mount

Sony SSC-D58AP colour camera or similar with composite video output

Small trimming tool

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Preparation test questions for Level 1 ME2(a,b) laboratory exercises

Students should have read Chapters 3 and 4 of Hodges ‘An Introduction to Video Measurement’ (as provided),

and familiarised themselves with the requirements of the laboratory exercise. The following questions are

provided to assess that they have done sufficient preparation to undertake the laboratory exercise.

Questions should be completed and handed to the demonstrator at the start of the laboratory session.

Questions:

(1) How many lines are there in the PAL standard video format?

(2) What is the difference between a field and a frame?

(3) Why are PAL video signals interlaced?

(4) How many frames and fields are there per second in the PAL format?

(5) What is the voltage range of a PAL video signal?

(6) What voltage levels correspond to black and maximum white level in PAL?

(7) At what point is the synchronisation pulse transmitted for each line and what is the voltage level?

(8) What is the given to the signal immediately before and after the synchronisation pulse for each line.

(9) What is the aspect ratio of a PAL video image?

(10) How many pixels are there in a PAL image?

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UNIVERSITY OF SURREY


First Year Laboratory Experiment ME.2a

BASIC VIDEO

1. AIM OF EXPERIMENT

This experiment is intended to introduce you to the basic video waveform. It will demonstrate how picture and

synchronization information can be analysed using a cathode ray oscilloscope (CRO).

Identification of various connectors and cables used in professional video engineering is also included.

The experiment should also help familiarise yourself with the laboratory’s test-signal generating equipment.

2. INTRODUCTION

In order that different video equipment can interchange signals correctly, there exist certain standards relating

to voltage amplitudes and timings used. The ability to measure voltage and timing specifications is a

fundamental video engineering skill, this is followed closely by the ability to recognise and, if possible, correct

any picture impairments.

At its most simplistic level a video signal’s amplitude (in terms of voltage) is related proportionally to the

brightness of the picture. There are two specified reference points of the signal:-

White is represented by 1 Volt

Black is represented by 300mV

In order for a video display to scan the picture in time with the camera’s scan it is necessary to transmit

synchronising pulses. Rather than transmit two separate signals for picture and synchronising purposes it was

found that both could be added at the transmitter and decoded within the receiver.

Synchronising pulses are transmitted with the following specification:-

Top of synchronising pulse = 300mV

Bottom of synchronising pulse = 0mV

When added the complete signal is therefore 1 volt peak to peak – this is the standard reference level for the

analogue video signal.

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2.1 Bandwidth

The bandwidth of the video signal is related to the resolution of the scanned image; a finely detailed object can

generate a signal up to 5Mhz. Conversely a scene with low overall resolution e.g., a plain grey wall requires

less bandwidth.

2.2 Physical connections

Equipment manufacturers and broadcast organisations have endeavoured to standardise physical connectors

and cabling for common interchange purposes. Due to the bandwidth requirements referred to previously it

has been ascertained that coaxial type cable is the most efficient and economic medium to use.

All standardised video equipment and systems are based on a 75 ohm send and terminate principal,

therefore all cables and connectors must adhere to this characteristic impedance. The standard cables and

connectors are listed below:-

Cable – RG59 75Ω and similar for analogue signals

Connectors - BNC 75Ω

BNC plug

BNC socket BNC tee-piece

BNC inline adaptor BNC 75Ω terminator

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3. PREPARATION

3.1 For this experiment you will need:-

1 x 75 ohm BNC terminator

1 x BNC Tee adaptor

1 x BNC plug to BNC plug connecting lead

Oscilloscope with video triggering facility

Video test generator.

Recommended reading:-


Peter Hodges

Focal Press

ISBN 0-240-51641-9

3.2 Set up the measurement experiment as shown in Figure 1.

CH 1 CH 2

Video test generator

BNC Tee

adapter

BNC

75 Ω

termination

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Electrically this is equivalent to

4. EXPERIMENT

4.1 Basic video synchronization pulse waveforms.

4.1.1

Confirm that the BNC 75 ohm termination is in place

Ensure Trigger Mode is set to Video and ‘Select Line Number’.

Initially set Y gain to 200mV / cm

Initially set X timebase to 10µS / cm

Adjust trigger to display Line number 20

If necessary adjust the X shift to display two of the negative-going line synchronizing pulses.

Switch test-generator to ‘Black’

The oscilloscope is now showing a period of just over one television line:-

Line 20 is located in the Vertical Blanking Interval and contains no picture information.

Ignore ‘colour burst’ synchronising signal

75Ω

AC

sou

75Ω

Video signal source Termination and oscilloscope input

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4.2 Line synchronizing pulse timings and amplitude.

4.2.1 Measure the duration of one of the line synchronizing pulses at the half-amplitude duration (HAD)

points (the amplitude level halfway between the top and bottom of the synchronising pulse).

4.2.2 Measure the amplitude of the line synchronizing pulse.

4.2.3 Measure the risetime of the positive-going edge and calculate the bandwidth required to achieve this

figure.

4.2.4 Examine the line synchronizing pulse closely and notice that it has a trapezoid characteristic; consider

the circuit techniques used to achieve this.

4.2.5 Reset the X timebase to 10µS/cm and adjust the Y gain to once again display two negative-going line

synchronizing pulses.

4.2.6 Measure the time-period between the falling-edge of the first line synchronizing pulse and the falling-

edge of the second line synchronizing pulse.

This represents the overall period of one television line.

4.2.7 Calculate the frequency (in kHz) of this period.

4.3 Field/Frame pulse timings

To portray continuous movement there are 25 television frames displayed per second consisting of 625

lines.

In an interlaced system each frame subdivides into two separate fields which are interlaced.

Field 1 (even) consists of lines 1 to 312½

Field 2 (odd) consists of lines 312½ to 625

Casual references often interchange the two terms field and frame but this is technically incorrect.

4.4 Synchronisation

In order for the vertical scans to be in step at both the camera and the monitor it is necessary to transmit

synchronizing pulses in addition to the line pulses observed in the previous section.

In an elementary system all that would be necessary would be to transmit a long pulse once every field

as shown:-

4.4.1 Consider why this method is unsatisfactory.

not to scale

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4.4.2 Set X timebase to 100µS/cm

Adjust trigger to line number 2

The oscilloscope should now show the frame synchronizing period of the video waveform.

4.4.3 Expand X timebase to 25uS to display the centre group of broad pulses.

4.4.4 Measure the duration from the negative-going edge of the first broad pulse to the negative-going edge of

the second broad pulse.

4.4.5 Measure the duration of the positive-going pulses between the broad pulses. This is known as the broad

pulse separation period.

4.4.6 Reset the X timebase to 25µS.

Adjust trigger to line number 624

The 5 narrow negative-going pulses observed to the left of the broad pulses are

known as equalizing pulses.

4.4.7 Measure the duration of one equalizing pulse.

4.4.8 Measure the period between equalizing pulses.

4.5 Blanking

The television system is based on sequential scanning ie., horizontal scanning from left to right and

vertical scanning top to bottom. There must obviously be a finite period when the scanning mechanism

needs time to return to a starting position for the next scan (both vertically and horizontally). This time

period is known as the blanking interval and, like the synchronization pulses examined previously,

standard time periods have been specified.

The shorter of the blanking intervals as might be expected associated with the line synchronization

period.

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4.5.1 Line Blanking







Switch test-generator to ‘White Field’

The horizontal blanking interval is located either side of the line synchronizing pulse.

4.5.2 Adjust the oscilloscope to display one line synchronizing pulse.

The section left of the line synchronizing pulse is known as the front-porch.

It is always the same voltage level as black.

4.5.3 Measure the time period of the front-porch.

The section right of the line synchronizing pulse is known as the back-porch.

It is always the same voltage level as black. (As previously ignore the colour burst).

4.5.4 Measure the time period of the back-porch.

4.5.5 Carefully identify where the picture information starts and ends either side of line synchronization pulse.

4.5.6 Measure the time period between the end of picture information and the start of picture information -

this will give you the horizontal blanking period.

4.5.7 Field blanking


Switch test-generator to ‘White Field’




4.5.8 Adjust trigger to display line number 23, this is the first active line of field 1.

Compare with line number 336, the first active line of field 2.

Note any differences.

4.5.9 Adjust trigger to display line number 623, this is the last active line of field 1.

Compare with line number 310, the last active line of field 2.

Note any differences.

Examine the lines immediately before the active picture areas on either field.

With our test generator they are set to black level, in practice these lines are often

used for test-signals and various text-based data such as teletext and timecode information.

Being located within the field-blanking period they are not therefore visible to the viewer.

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4.6 Video levels

As mentioned in the introduction, video signals are defined by standardised agreement.

4.6.1 Confirm that the BNC 75 ohm termination is in place






Switch test-generator to ‘White field’

4.6.2 Measure the relative peak-to-peak amplitude of the line synchronization pulse.

4.6.3 Measure the relative peak-to-peak amplitude from black-level to the top of the waveform.

4.6.4 Remove the BNC 75 ohm termination. Repeat steps 4.6.2 and 4.6.3 noting any differences.

Notice any impairments to the signal eg., overshoots etc,

Replace the BNC 75 ohm termination

The characteristic impedance of video interconnections is always 75 ohms and must be terminated

correctly at the receiving equipment.

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UNIVERSITY OF SURREY


First Year Laboratory Experiment ME.2b

VIDEO CAMERAS

1. AIMS OF EXPERIMENT

To create a simple test chart used to assess the performance of a television camera and it’s lens.

Understand the imaging geometry of pin-hole camera model.

Understand relationship of spatial image resolution to video signal frequency.

2. INTRODUCTION

Test charts have traditionally been used to quantify the performance of video equipment since the earliest days

of television. When testing display monitors, recording media or transmission paths it is preferable to generate

the test pattern electronically. However because a television camera is an opto-electronic device it is necessary

to create a test pattern which represents a ‘real-world’ object.

The most important camera tests are:-

Resolution

All cameras have a theoretical upper limit to their ability to resolve the finest details of a picture, this is defined

by the number of television lines used. It will in practice however be limited by other factors including the

camera’s lens performance.

Scanning area

This defines the borders of the picture as seen by the camera. The lens must always present the optimum image

area to the camera’s pickup device (previously this was a thermionic tube; Charge Coupled Devices (CCD) are

now used instead). If the image focused in the CCD target is too small then it is wasteful in terms of light

sensitivity performance.

Grey-scale

It is important that the camera’s tonal reproduction is as accurate as possible. For a given light input to the

pickup device there should be a proportional voltage output from camera. If this function is wrong then

lowlights such as shadows will be black-crushed ie., appear far too dark. Non-linearity at highlights can cause

white-crushing where objects of different high brightness levels appear to merge into one.

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3. PREPARATION

3.1 For this experiment you will need:-

1 x 75 ohm BNC terminator

1 x BNC Tee adaptor

1 x BNC plug to BNC plug connecting lead

Oscilloscope with video triggering facility

Picture monitor

Sony SSC-D58AP colour camera or similar with composite video output

Camera rostrum stand

Computer drawing facility

Recommended reading:-


Peter Hodges

Focal Press

ISBN 0-240-51641-9

Standard Handbook of Video and Television Engineering

J. Whitaker / B. Benson

McGraw-Hill

ISBN 0-07-069627-6

3.2 Obtain and read any technical / operational documentation on the camera you will use for this

experiment.

3.3 Ensure you understand the basic operation of the camera.

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3.4 Mount the camera on the rostrum stand as shown:-

3.5 Your test chart will be created using A4 sized paper. Features can either be printed using an accurate

computer graphics program or traditional drawing methods.

3.6 Connect camera to oscilloscope and picture monitor as shown below:-

Baseboard

Vertical support

Camera

Lens

BNC

75 Ω Termination

(See below)

BNC Tee

adapter

BNC

Video

out

Video input Loop

Picture monitor

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Many picture monitors have a ‘loop’ output which is essentially the video input signal in parallel with

the equipment’s input; this enables several inputs to be fed from one single output without the need for a

distribution amplifier..

3.7 Initial oscilloscope setting





Adjust trigger to display line number 150


4. EXPERIMENT

Scanning area

4.1 Set the distance from the baseboard surface to the camera focal-plane to 48cm.

4.1.1 Draw a box which just fits inside the A4 paper using both the image-size of the camera’s CCD sensor

and the focal-length of the lens to calculate the width.

4.1.2 Calculate the height required.

At the centre points of each side draw small arrows ensuring that the arrowhead touches

the line exactly. The chart should now look like this:-

4.1.3 Place chart under camera and focus lens as necessary. The chart’s edges, as indicated by the arrows,

should now correspond to the upper, lower, left, and right extremes of the transmitted picture.

Important!

It is essential that the signal is always terminated correctly at the end of the chain.

This is achieved either by

a) Placing a tee-adaptor and 75Ω resistor at the final input or

b) The manufacturer may have a built-in terminator switchable between

LOOP (high impedance) or TERMINATE (75 Ω)

Ascertain which configuration applies to your monitor

Erroneous double or non-termination will cause gross signal level errors and distortion

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4.2 Resolution

Using another A4 sheet create| a pattern approximately 4cm x 4cm consisting of alternating black and

white vertical lines. When placed under the camera this pattern will correspond to one particular

frequency within the video bandwidth - the higher the frequency, the higher the resolution.

4.2.1 Check that camera to baseboard distance is still set to 48 cm.

4.2.2 Determine the dimension S to create a pattern which corresponds to 1Mhz as measured

on the oscilloscope.

4.2.3 Place the resolution grating at the centre of the A4 sheet.

Ensure that the black and white lines are as parallel to the vertical sides of the baseboard as possible.

A waveform representing the plain white sheet with the resolution grating at the centre

should now be visible.

4.2.4 Measure the period of the duration corresponding to distance S.

Check that the grating corresponds approximately 1Mhz.

S

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4.2.5 Create four copies of the 1 Mhz pattern, cut and paste one in each corner and one at the centre of the test

chart as shown:-

4.3 Corner/edge horizontal resolution

4.3.1 Visually observe the detail and resolution of the gratings at each corner.

4.3.2 Use the oscilloscope’s line trigger facility to monitor the amplitude of the video signal.

4.3.3 Compare with the resolution grating at the centre.

4.3.4 Consider the theoretical voltage waveform produced by the above resolution gratings and compare with

your results.

4.4 Measurements using a grey-scale chart

4.4.1 Remove the above resolution pattern and replace with grey-scale chart.

Adjust camera height until grey-scale steps fill the entire visible screen area.

4.4.2 Display one line of video on the oscilloscope and adjust the camera’s iris control until the chart’s

white bar represents 1V.

4.4.3 Measure and note the voltage values for each of the remaining bars.

4.4.4 The black bar should represent 300mV – if this is not the case suggest why.

4.4.5 Consider any factors which affect the accuracy of the above measurements.

not to scale

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4.5 Camera synchronization and system timing

When multiple cameras are used within a studio it is necessary that all scanning circuitry (generally known as

timebases) are synchronised together therefore each camera has another input specifically for synchronizing

pulses only. Due to propagation delays however it is possible for each camera’s video output to arrive back at

the common point (usually a vision-mixer) at fractionally different times. Most cameras offer a horizontal

phase adjustment to compensate for these timing errors.

4.5.1 Reconfigure the previous setup as follows:-

4.5.2 Initial oscilloscope settings

Channel 1


Set Y gain to 100mV / cm

Channel 2

Set Y gain to 100mV / cm

Both channels




If necessary adjust the X/Y shifts to display one negative-going line synchronizing pulse.

It is very likely that the two synchronizing pulses will have a timing offset relative to each.

BNC Tee

adapter

BNC

VS in

BNC Video

out

CH 1 CH 2

Mixed synchronization

pulses (MS) from

Synchronisation Pulse

Generator

(SPG)

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4.5.3 Using a suitable trimming tool adjust the PHASE control on the side of the camera until the falling edge

of one synchronizing pulse is in vertical alignment with the leading edge of the other.

It is useful to expand the X timebase for greater precision of this adjustment.

4.5.4 Attach more than one camera to this setup and repeating the phase adjustment on each camera.

4.5.5 Consider the effects on the picture of multiple cameras that have not been phased correctly before sending

to the vision-mixer.

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