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Understanding Resolution & TVL
This document is aimed at the more advanced reader. It provides an introduction to the terms TVL and usable video resolution (not to be confused with sampling resolution eg 4SIF). The document also provides details of a practical way to measure the TVL performance of IndigoVision products along with the results achieved.
In summary IndigoVision 8000 MPEG-4 rack mount products are capable of the following TVL performance
- 4SIF: 535 TVL
- 2SIF: 535 TVL
- SIF: 305 TVL
Contents
1 BACKGROUND
IndigoVision is often required to specify the achievable video resolution of our products in a term known as TVL or TV Lines.
This document gives a brief introduction to what the term TVL means and details how it can be easily measured.
2 INTRODUCTION
2.1 TVL
The term TVL or “TV lines” has been around since the invention of analog television. Its purpose was to provide a measure with which the usable resolution performance of a TV system could be assessed.
Today in the digital world we have become accustomed to using terms such as 4SIF and mega-pixels to specify resolution as the number of pixels samples used per image. Unfortunately this is both helpful and misleading. The number of pixel samples used only governs the theoretical upper limit of usable resolution. The actual usable resolution of a system cannot be directly inferred from the number of pixels.
Usable resolution is important to CCTV operators as it determines the level of fine detail that they will be able to see. A higher usable resolution will enable an operator to:
- Read a car number plate from a greater distance.
- Identify a face more clearly.
- Examine a portion of video in more detail.
TVL is in general terms a horizontal measurement of how fine a detail from a scene can be resolved on a display. This number relates the resolvable fine detail to an equivalent number of equally spaced black and white vertical lines across the width of the viewing screen*1.
A common misconception of the term TVL is that it relates to the number of Video Lines*2 on the screen, ie 625 for PAL and 525 for NTSC.
The use of the term bandwidth in this context must also be properly defined. Bandwidth refers to the frequency content in MHz of an analog or digital signal and should not be confused with the bit rate or network bandwidth in Mbps (or kbps) of a compressed video stream.
It is important to understand that the TVL of a system is governed by the lowest performance component. For example, if you connect a 200TVL camera to a 500TVL encoder then the achievable performance will never exceed 200TVL.
*1 Alternatively, the measure TVL per Picture Height (TVL/PH) can be used. This measure factors in the aspect ratio of the display such that the measure is performed over the largest possible square area of the screen. i.e. 3/4 and 9/16 of the screen width for 4:3 and 16:9 displays respectively.
2.2 Measuring TVL (of a device)
Traditionally the measurement of TVL was made using a test-card with a resolution wedge such as shown in Figure 1.
Figure 1: Resolution Wedge Image
Figure 2 Resolution Wedge Waveform
The measurement process is simple. Point a camera at the test card such that the image fills the field of view. Observe the point vertically in the wedge where the individual lines are no longer distinguishable from each other. Read off the number from the scale at that point.
Unfortunately this measure of resolution has a few drawbacks. It is a measure of the complete system including lens, camera and monitor, not any particular component in the system, except for the poorest. Also it is not independently repeatable because the measurement is subjective.
Figure 2 shows portions of the analog waveform that would be produced by an ideal camera when viewing the resolution wedge in Figure 1. The waveforms correspond to the Video Lines that pass through the scale markers on the test card. It can be seen that the further up the TVL scale you travel the higher the maximum frequency of your analog waveform.
Table 2 shows the well-known relationship between TVL and frequency / bandwidth, with notes showing typical equipment performance levels. [1]
TVL |
TVL/PH(4:3) |
Bandwidth MHz |
Note |
100 |
75 |
0.96 |
|
200 |
150 |
1.92 |
|
270 |
202 |
2.60 |
VHS |
300 |
225 |
2.88 |
|
400 |
300 |
3.84 |
S-VHS |
500 |
375 |
4.80 |
|
504 |
378 |
4.85 |
DVD, Broadcast PAL |
600 |
450 |
5.76 |
|
Table 1: TVL vs bandwidth
As Table 2 shows TVL is directly related to bandwidth. Therefore it can be seen that if you can measure the bandwidth capabilities of your system you can calculate the TVL performance.
2.3 A Simple Measurement Process
A simple and repeatable way to measure the bandwidth / TVL of a transmission system (i.e. camera and monitor removed) is to feed in a known signal from a test pattern generator (see Figure 3). This signal should contain a linear frequency sweep through the frequencies of interest. The output of the device under test can then be measured on an oscilloscope or waveform monitor to determine the usable bandwidth of the system.
Figure 3: Test Pattern Generator Frequency sweep output*3
The bandwidth measurement should be made as the point in the output waveform where either the frequency of the output signal no longer matches that of the input signal due to distortion or the amplitude of the output signal is half the voltage (or 6dB less) than this input*4.
*3 The six bold black to white transitions along the bottom of the image are scale markers marking 1MHz increments in the sweep from 1MHz to 6MHz.
*4 Some companies may use –20dB as the measurement point [1].
3 Measurement
3.1 Test Set-up
To demonstrate and measure the end-to-end bandwidth and TVL performance of IndigoVision products. A simple process of using a frequency sweep was used. Figure 4 shows the test set-up.
Figure 4: Test setup
The Test Equipment and products used were as follows:
- Tektronix TSG95 signal generator. Set-up to output a 60% amplitude linear frequency sweep. Composite video PAL.
- Tektronix TDS3014 DPO oscilloscope with TDS-3VID option, external 75Ω termination.
- IndigoVision 8000 MPEG-4 rack mount transmitter.
- IndigoVision 8000 MPEG-4 rack mount receiver.
- IndigoVision Control Center viewing software.
The system was set up so that the 8000 transmitter was streaming to both the 8000 receiver and the PC viewer using UDP multicast.
The three test points in Figure 4 are defined as follows:
1. Oscilloscope measurement of the input signal, generated by the TSG95 signal generator.
2. Oscilloscope measurement of the analog video output from an 8000 receiver.
3. Visual measurement of the output video bandwidth using a software decoder from IndigoVision’s Control Center viewer application.
3.2 Results
The highest resolution / bandwidth results were achieved by simply configuring the 8000 transmitter to compress 4SIF at a bit rate of 4096kbps.
The input signal at test point 1 was measured and is shown in Figure 5.
The bandwidth at test point 2 (the analog output) was measured and is shown as follows:
- Figure 6: Full waveform with frequency markers*5 overlaid.
- Figure 7: Expanded waveform view showing 3MHz to 6MHz frequency markers.
- Figure 8: Expanded waveform view showing only 5MHz frequency marker.
The image captured from the software decoder at measurement point 3 is shown in Figure 9. Figure 10 is an expanded view showing only the 5MHz marker.
*5 Some companies may use –20dB as the measurement point [1].
3.3 Conclusion
The results of measurements performed clearly show that the IndigoVision products tested are capable of achieving a bandwidth of about 5.15MHz. This equates to a usable resolution of 535TVL*6 .
Note the testing performed and results achieved in this paper are for 625 Line PAL. Results for 525 Line NTSC should be comparable to within 1%.
Finally there are many factors which can have an effect on the achievable TVL performance of any system, including IndigoVision products. The following list gives some examples:
- A poor quality or badly set-up camera.
- A poor quality or poorly focussed lens.
- Poor quality video cables. These can cause distortion of the high frequency analog signals.
- Pre-filtering the video to reduce the amount of high frequency it contains. This is commonly performed to aid compression efficiency and network bit rate, but it has the downside of reducing the usable bandwidth.
- The number of pixel samples taken from the image (eg SIF, 2SIF or 4SIF). As noted earlier in this document the number of samples taken from a signal bounds the upper limit of usable bandwidth.
- The network bit rate used to transmit the video. If the bit rate is too low then TVL will be reduced.
- Product quality. Poor design of the analog to digital conversion electronics can greatly reduce TVL.
Figure 5: Test pattern generator output
Note the two horizontal measurement bars in Figure 5 (arrowed). These bars show the peak-to-peak amplitude of the frequency sweep to be 423mV.
Figure 6: 8000 receiver output
Note the attenuation of the signal at the higher frequencies in Figure 6. This image clearly shows not only the bandwidth of the system but also its frequency response.
Figure 7: 8000 receiver output Expanded view showing 4MHz to 6MHz
Note the absence of distortion in the signal in Figure 7 until well after the 5MHz marker.
Figure 8: 8000 receiver output Expanded view showing 5MHz
Note the two horizontal measurement bars in Figure 8 (arrowed). These bars are set to half the input signal level at 212mV (423mV/2) The vertical line in the image shows the point where the signal level drops below the 212mV level. The frequency at this point is 5.15Mhz.
Figure 9: Control Center display of frequency sweep
Figure 10: Control Center display, expanded view showing 5MHz marker
Note in Figure 10 the vertical lines are clearly distinguishable well beyond the 5MHz marker.
References
[1] “A story of wiggly amps, or making sense of TV Lines” Colin Greene CCTV –Image, Winter 2004. CCTV User group.
Terminology
Term |
Definition |
2SIF |
704x288 PAL, 704x240 NTSC |
4SIF |
704x576 PAL, 704x480 NTSC |
D1 |
720x576 PAL, 720x480 NTSC |
DVD |
Digital Versatile Disk (formerly Digital Video Disk) |
NTSC |
National Television Systems Committee (America) |
PAL |
Phase Alternate Line |
SIF |
352x288 PAL, 352x240 NTSC |
S-VHS |
Super Video Home System |
TV |
Television |
TVL |
TV Lines |
TVL/PH |
TV Lines per picture height |
UDP |
User Datagram Protocol |
VHS |
Video Home System |
Table 2: Terminology
