3 Introduction analogue video

3.1 Lines, frames, fields and PAL/NTSC

A TV signal consists of the even lines (which forms the even field) which are shown first, followed by the odd lines (which forms the odd field). These lines are called scan lines. When a pulse of a certain shape in the signal occurs, the TV knows a line has ended and that the beam should travel back to the other edge of the screen to start drawing a new one. This is called horizontal retrace. The fields are shown with a speed of 50 fields per second (PAL/SECAM) and 59.94 fields per second (NTSC). The fields are separated by vertical blanking interval lines (abbreviated: VBI lines) that are not drawn on your TV. These lines don't contain any picture data. Some are needed for synchronization of the TV set. Since it is known what is in these lines, there is no reason to store them and they are cropped of by the capture device and are recreated upon playback by your DVD/VCD player. A lines which contain picture data is called an active line. This way of showing video is called interlacing, and all analogue signals have it. Depending on the source you may not see it in a capture, but it is always present in the signal (see figures 1-3).
A single PAL line takes 64 µs (= microseconds = 1 / 1000000 second) to complete. Also in this case, not the whole line consists of picture information. The ITU-R BT470 specification states that the active part of a PAL line should be about 52 µs. For NTSC, it takes 63.556 µs with active part of about 52.6555 µs ⁽¹⁾. The rest of the line contains synchronization information on either side of the active part, known as horizontal blanking.
A set of two fields is called a frame. The terms frame and field are both used to describe a ‘raw’ set of scan lines including blanking and only the active part of it carrying picture information. PAL/SECAM has 25 frames per second (and 625 lines) and NTSC has 29.97 frames per second (and 525 lines).

figure 1: Interlaced scanning system (source: Composite/CVBS Interface)

The following two figures show two successive fields. In this example, you can clearly see that the white and red balls are moving, and the fields are snapshots from different points in time. This is what you will see on your television and it is called interlaced playback.

figure 2: odd field	figure 3: even field

However, the two fields are merged after capturing and the image is called interlaced (see figure 4).

figure 4: combined field (= frame)

In the sections 7.1 postprocessing with VirtualDub and 7.2 postprocessing with AviSynth will be discussed how to deal with this.

For those who are interested, a more detailed description of the workings of analogue video is given in section 9.

This subsection is concluded by a summary of the PAL/NTSC standards. Since TV transmission is analogue, there are no pixels, but only lines. A PAL system contains 625 lines which make the picture. Each of these lines take 64 µs to be drawn on your TV. NTSC contains 525 lines, and takes 63.555 µs each line. The active picture you see is only contained in a portion of this signal. The rest is for synchronization. (Therefore, capture cards only capture/sample a portion of this signal.)

The PAL/NTSC Standards Review

• PAL (Phase Alternate Line) -- A 625 line 50 interlaced fields per second analogue broadcast standard used in many parts of the world but not the U.S.A. It is named this way because the chroma is phase reversed on every other scan line to reduce picture artifacts. The PAL active picture is contained in about 52 µs of 576 active scan lines.
  
• NTSC (National Television System Committee) -- U.S. government and industry committee which defined the 525 line 60 (59.94) interlaced fields per second analogue broadcast TV standard over 50 years ago. The active NTSC picture is contained in about 52.6555 µs of about 486 active scan lines.

More information on these standards can be found in section 9.

References:
Display Devices:
Composite/CVBS Interface: Explains the workings of analogue video.
Temporal rate and its history: About interlacing and its origin.
ITU-R BT.470-5 recommendation: About the PAL/SECAM/NTSC standards.

3.2 Sample rate and size

A capture device digitizes the analogue signal. This is done by measuring the signal at a fixed time interval and recording the value as a binary number. The rate at which this happens is called the sample rate. Video is typically sampled many millions of times a second, or MHz (= megahertz, or 1000000 samples per second).
The capture card samples at a fixed sample rate and the driver resizes the picture afterwards. If you set a PAL capture size of 704x576 in VirtualVCR, the capture device will first digitize all lines at a high sample rate. It will then proceed to crop off all the VBI lines. Next it will crop off the horizontal blanking and the front porch, leaving 52 µs of samples per line which contains the video information. Then it will use the samples from this part of the line to calculate the required 704 pixels. The result is two fields of 704x288. Finally it will weave together the two fields and pass on the finished 704x576 picture to VirtualVCR.
You now have 52 µs of analogue signal, contained in 704 pixels. This makes your final sample rate 704 samples / 52 µs = 13.54 MHz. The more detail there is in the picture, the more pixels you need to see it. More pixels means a higher size, and so a higher sample rate is needed.

figure 5: the sampling process

source: Basic Signal Processing

References:
ITU-R BT.601-5 recommendation: The guideline for converting analogue signal into computer pixels.
An analysis of sampling and filtering: About why a pixel is not a little square, but just a point sample.

3.3 Vertical sampling and size

All analogue sources output 576 active lines per frame (PAL) or 480 active lines per frame (NTSC). Capture devices process line by line into pixels, so one line becomes one row of pixels. Because of this, only two vertical sizes are suitable for capturing: full and half: 576 and 288 (PAL) or 480 and 240 (NTSC). Any other size will cause the device to cap all lines anyway and resize them afterwards. Since vertical resizing by capping devices in general gives very ugly results you should not do this. When you cap at half vertical size, capping devices just discard every second field. (Note: the official NTSC vertical resolution is 486, but all capture devices crop this to 480 lines.)

3.4 Bandwidth and detail (for advanced users)

If you are new to the capturing process, you are advised to skip the following subsections and proceed with section 4. However if you know how to capture, and you want to know more about bandwidth, you should read this subsection. In the analogue world, there are pixels nor sizes. The term bandwidth describes the theoretical maximum amount of detail that can be reproduced for a given medium. If you have a VHS VCR connected to your TV set, you will probably have noticed the recording never looks as sharp as the original broadcast. The cause has to do with bandwidth: a VHS tape has a lower bandwidth than a TV signal, so you loose some of the fine detail. Likewise a SVHS tape has a lower bandwidth than a TV signal, but higher that a VHS tape. Like sample rate, bandwidth is also given in MHz, or millions of changes per second. The difference in bandwidth between TV, SVHS and VHS can easily be seen in the following screenshots:

figure 6: source: 712x480 cap from TV

figure 7: source: 712x480 cap from SVHS

figure 8: source: 712x480 cap from VHS

As can be seen, the screenshot from VHS also contains some cross color artefacts (chroma noise in the text for example). More about this in the Removal of chroma artefacts section. Btw, have a look at the vertical lines in the girls collar on the TV cap (which are not present in the SVHS and VHS screenshots).

The more fine detail there is in a picture, the more rapidly the video signal has to change to describe it. Suppose you are looking at a close up of a black and white pinstripe shirt with 500 black and white lines in the shot. The video signal has to change very fast between black and white to describe this. You have 250 black/white cycles in your 52 µs video line or 250 / 52 µs = 4.8 MHz.
PAL/SECAM Television broadcast typically has a bandwidth of 5 - 5.5 MHz depending on your country. The shirt shot would not pose any problems, and if you watched it on TV you would be able to count the single lines. But if you taped it to ordinary VHS tape, which has a bandwidth of 3-3.4 MHz, the detail would be gone and on playback you would see a solid gray shirt. You might still recognize the buttons, but the fine stripes would be blurred. Now suppose the camera zooms in very closely on the same shirt until you have two vertical black and white lines filling the whole screen. A single video line now has to change twice from white to black to white and to black again in 52 µs (PAL) to describe this. The bandwidth needed for this low amount of detail is only 2 / 52 µs = 0.04 MHz. Taping this shot on VHS would not pose a problem.

So, the finer the detail in your picture, the higher the bandwidth you need to see it without blurring.

The bandwidth of an analogue source can be found in the manual of your source. Sometimes measure called lines of (horizontal) resolution is given. It is defined by the number of side by side dots that can be reproduced within a scan line. To get the bandwidth, divide the number of lines by 78 ⁽²⁾ for PAL (79 ⁽²⁾ for NTSC) and you have the bandwidth in MHz. For example: NTSC broadcast has 330 lines of resolution, which equals 330/79 = 4.2 MHz.

3.5 A few words about the amount of detail and bandwidth in various analogue sources

However, this is the theoretical minimum. In practice, there are some issues which imply that you need about three times the bandwidth (this is explained in more detail in section 5). Thus, for you VHS source, you need about 3 x 3.0 x 52 = 468 pixels per line (say 480x576) to keep all detail.

Either bandwidth or "lines of resolution" can be found in the specifications of various video equipment. The table below gives the "lines of resolution", bandwidth, minimal sample rate and minimal horizontal resolution needed for various analogue sources. The sizes are rounded to the next multiple of 8 and can be used for PAL, SECAM and NTSC targets.

As you can see CVD (352x576 / 352x480) and VCD (352x288 / 352x240) have a to low horizontal size (besides the reduced vertical size for VCD) to do justice to most analogue sources. Some may get by on S-VCD, but many require full DVD resolution to loose no detail.

References:
ITU-R BT.470-5 recommendation: If you register, you can download three ITU recommendations for free!
PAL Site - The home of the PAL videosystem
An introduction to VCRs
Sencore TechTip 189: Comparison of tape formats
Bandwidth: Horizontal and vertical resolution for television and video.
Leopold's Home Video Formats Page: Some information on various video formats.
TV and Video Resolution: Horizontal and vertical resolution for television and video.

Footnotes:
(1) There are actually several NTSC standards with 52.6555 µs being one of them (ITU-R BT.470-6, EIA RS-170A). Two others are 53.07 µs (FCC 73.699) and 52.86 µs (SMPTE 170M-1999). Notice that ITU and EIA give the same number, but measure it on a different point. The difference is roughly equal to the build-up time. We don't how to find out which standard is used. Therefore the standards with an active NTSC picture is assumed in this guide.
(2) The number of lines you can put in 1 MHz of bandwidth: for PAL: 2 * 1 MHz * 52 µs / 1.3333 = 78 lines of resolution, and for NTSC: 2 * 1 MHz * 52.6555 µs / 1.3333 = 79 lines of resolution.

Next: Capture sizes for newbies or Capture sizes for advanced users.

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English version last edited on: 06/13/2004 | First release: n/a | Authors: Version4Team | Content by Doom9.org