Mark Elphinstone-Hoadley 04131216
Waveform Monitors and Vectorscopes
Introduction
In this report, I will analyse the functions of a waveform monitor and a vectorscope, and examine their application in composite television signals. Firstly we will examine the waveform monitor. This device is a type of special test oscilloscope and is used to monitor the black and white (luminance) signal information. It can be used to analyse and improve this signal. A typical waveform monitor would be like that shown in fig. (a).
Fig. (a)
The waveform monitor displays the signal on measurements. The NTSC standard measurement is in IRE, which is named for the Institute of Radio Engineers. IRE values are based on the percentage of maximum picture. For PAL, the measurement is in volts. These are shown in table 1.
Table 1
Luminance / PAL (Volts) / NTSC (IRE)Minimum (black) / 0.3 / 7.5
Maximum (white) / 1.0 / 100
On a typical NTSC (IRE) based waveform monitor the values range from -40 up to 120. However with this signal you must note that anything out of the high and low range shown in table 1 lose all detail and aren’t visible so that if a light coloured face is at or above 100 units IRE, it will be washed out. If the range is below 7.5 units, there would be no visible detail. A proper exposure is considered to be in the +65 to +80 range. On the monitor you would see (without the arrows):
+120 ------
+7.5 ------
0 ------
-40 ------
When filtering what is seen on a WFM (waveform monitor) there are three main possibilities available. These are flat, low pass and chroma. Shown below are examples of these filters, displayed from the WFM using pictures of full field colour bars.
(Input controls on WFM)
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Mark Elphinstone-Hoadley 04131216
The flat position allows us to view luminance and chrominance information - the whole range of frequencies in the composite video signal.
The Low Pass (IRE) position passes only luminance information – all the chrominance information is stripped from the waveform.
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Mark Elphinstone-Hoadley 04131216
The Chroma position is the opposite of the Low Pass – the luminance information is missing, leaving just the chrominance.
These variations of the picture are so that different aspects of the video signal can be observed. You may want to see the absolute picture of a video signal, with all the chrominance and luminance, but this may obscure what you see in either the luminance or chrominance, and so the low pass or chroma filters come in handy.
A WFM can be used to calibrate television cameras, as a tool to assist in telecine (film-to-tape transfer), colour correction and other video production activities. You can also use it to monitor video signals to make sure neither colour gamut or analogue transmission limits are violated and in manufacturing test and research and development applications.
Introduction Ctd. - Vectorscopes
The vectorscope is also a type of special test oscilloscope and is used to sample the colour burst (chrominance) from a composite video signal and use that as a reference.
(Ref. 2) The scale of the vectorscope is a circle overlaid with the color amplitude and phase relationship of the three primary colors (red, green and blue). In the center of this circle graph is the luminance (black and white) value of the signal. Through this center point, three axes represent the primary colors.
An example of a vectorscope (fig. (b), ref. 1)
A vectorscope uses an overlaid circular reference display, or graticule, for visualising chrominance signals, which is the best way to refer to the QAM (quadrature amplitude modulation) scheme used to encode colour into a signal. The hue and colour amplitude information in the colour television system is carried on a single sub carrier frequency at 3.579545 MHz (NTSC) or 4.43 MHz (PAL). In modulated form this signal is called chrominance. The hue information is carried by the sub carrier’s phase, and is measurable as being anywhere between 0 and 359 degrees. The colour amplitude information is carried by how high a level the sub carrier has at any given point in the composite video signal. (ref. 1)
In fig. (c) you can see the where the hue matches up in relation to degrees, and how those parameters match up with the different points on the graticule of the vectorscope.
Fig. (c)
If you were to point your video camera at a white card and connect its signal to the vectorscope, a dot would appear in the vectorscope’s centre. If this dot is off centre, the white card would not be recorded as pure white, but with a tint of colour. To record the white as white, the camera operator must use the camera’s White Balance Control. The camera can also be adjusted internally with the red and blue gain control. On the vectorscope this would be adjusted until the signal on the scope were dead centre and not favouring red, blue or green.
In an editing session, the vectorscope determines the proper colours using colour bars (see fig. (d)). The picture you see below in fig. (d) is portrayed on the vectorscope as you would see in figure (c). Located around the middle of fig. (c) you can see there are targets associating with different hues of colour. The colour bars are of perfect colours (red, magenta, blue, etc) and are represented by conjoined dots in each of the applicable targets.
Fig. (d)
The following reference describes in detail the use of each ‘colour target’ and how the colour on the composite video signal may be adjusted to match the ‘targets’ in the vectorscope.
Each colour bar chrominance vector terminates in a system of targets in the form of two boxes - a small one inside a large one. The dimensions of the large box represent +/- 10 degrees centred on the exact chrominance phase, and +/- 20% of chrominance amplitude centred around 100% of standard amplitude. The smaller boxes represent +/- 2.5 degrees and +/- 2.5 IRE units. The idea, of course, is to get the vectors of each colour bar as closely as possible into the little boxes. This is done by adjusting the phase and chroma level controls of the device being monitored (VTR, camera encoder, etc.)
Analysing and Adjusting an Image Using a Waveform Monitor
Aim
To import a picture file into video editing software, and use the YC waveform facility to analyse the image. Describe the waveform of this image and what information can be obtained by looking at the waveform. Use the brightness and contrast effects within the video editing software to get the correct black and white levels. In the report describe how these effects change the waveform display.
Apparatus
Computer software: Adobe Premiere Pro 2.0
Method
First of all I had to import the jpeg image across into the video editing software and insert it into the timeline of the video track. Then I changed the workspace view type to the colour correction option, so I could see the colour correction facilities with three windows; one for the original source image, one for the adjusted image and one to monitor the waveform. I changed that last window to the YC waveform property and analysed that window, which shows me the following results.
Fig. (e)
Using previous knowledge of waveform data, I know that a proper exposure of white (or full luminance) would be at the range of +80 IRE, and that the lowest range for the lowest level of luminance (black) for any visible detail is at +7.5 IRE, so I need to aim to adjust the brightness and contrast to fit these levels according to the EIA. The EIA standard chart we have been given is a chart to measure the standard levels of luminance balance using different squares of levels of luminance. The brightest square is the highest and the darkest square in the absolute centre is the lowest.
Fig (f)
On inspection of the waveform image from left to right, there are an elevating range of steps from low to high luminosity and high to low luminosity, with a straight line through the middle which represents the midtone around the two charts. The levels stepping from low to high and vice versa represent the 2 charts parallel to each other, and the single line right at the bottom in the middle represents the black square in the centre of the picture (see comparisons of fig. (e) and fig. (f)). Looking at the first 3 different levels of luminosity and comparing them to the original, it was noticeable that the highest corresponds to that of the white square on the far left of the bottom chart (pt. (a), fig (e)). This was measured at around +70 IRE. The bottom line parallel to the lowest luminosity level on the left therefore corresponds to the dark square on the far left on the chart (pt. (b), fig (e)). This was measured at around +20 IRE. This gives an obvious idea as to what we’re seeing on the waveform monitor in comparison to the original picture in fig. (f).
It is noted that, considering this chart is expected to be in black and white and considering the adjustments to be made only affect the luminance, we do not need to see the chroma levels, and so this feature is not visible on the waveform monitor.
From a closer analysis of this image, it is evident that the highest and lowest luminance levels from this chart are not entirely in level with the IRE standards of the best possible luminance. The highest luminosity on the top left is only at +70 IRE and the lowest level (the black centre square) is at around +15 IRE. These need to match the standard low and high exposures of +7.5 and +100 IRE. First I needed to turn up the gain in order to put the levels of luminosity approximately in the centre between the levels ‘+7.5 IRE’ and ‘+100 IRE’. This was done by turning the brightness up to an extra 34%, which amplifies the luminance and puts the highest and lowest levels at +19 IRE and +74 IRE respectively. By adjusting the contrast we affect the luminance levels by their highest and lowest points, which in turn amplifies the higher IRE levels and fades the lower IRE levels. This was adjusted by 41%. It is noted that the software’s peak seemed to be at just below +100 IRE. Now the levels of the monitor in fig. (e) are at their proper levels of +7.5 IRE for black and +100 for white.
Fig. (g)
Conclusion
From the original waveform it was noted about the different IRE levels we could see in correspondence to the lighter and darker colours on the waveform monitor. Different levels of IRE displayed on the waveform indicated different levels of luminosity on the composite video signal, so you could detect what you were seeing in the signal. Using the brightness and contrast settings you could change the different levels of luminosity in the image and therefore stretch and move different levels on the video signal’s waveform.
Analysing and Adjusting an Image Using a Vectorscope
Aim
To assess whether the provided image is monochrome or if there is a colour cast, using the vectorscope to assess this. Determine whether the waveform monitor is a useful tool to help determine colour cast. Use video editing software colour correction tools to obtain a true monochrome image.
Apparatus
Computer software: Adobe Premiere Pro 2.0
Method
First, keeping the original data from the previous experiment loaded in the video editing software, and changed the YC waveform monitor to a vectorscope. If the image was monochrome, I would expect to see a single dot in the middle of the vectorscope where there is no saturation in any of the colours around the luminance.
Fig. (h)
According to fig. (h) though, the image definitely contains levels of saturated hues. The only way to make the picture truly monochrome is to desaturate the hue. To do this I applied an effect from the video editing software called “fast colour corrector”. Within this effect was an array of different functions, including one to desaturate the image. The image’s saturation level was reduced by 100%, showing me only a single dot in the centre of the vectorscope. This indicates that the image is now truly monochrome (fig. (i), fig. (j))
Fig. (i) Fig. (j)
This same experiment can be monitored using the YC waveform monitor. I brought the saturation back up to 100% back to its original colour cast, and analysed this picture using the waveform monitor. Looking at the waveform of the video signal, I can see bright blue areas surrounding the original area of the waveform (compare fig. (g) and fig. (k)). Upon removing the saturation, this field of blue decreases to almost match the green (fig. (l)). This clearly indicates the lack of chrominance within the composite video output, due to the lack of any visible chrominance (blue area). The final image I can see of the composite video signal matches that of fig. (j).
Fig. (k) Fig. (l)