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High Definition Television (HDTV) - Frequently Asked Questions

1. What are the main advantages of HDTV over standard definition composite PAL or NTSC video?

2. What benefit does HDTV offer for underwater viewing applications?

3. Is there a single standard format for HDTV signals?

4. Which HDTV format is better, 720p or 1080i?

5. What is the effective resolution of HDTV in TV Lines?

6. What does YUV (Y-Pb-Pr or Y-Cb-Cr) mean on an HDTV specification?

7. What do the terms 4:2:2, 4:1:1 or 4:2:0 mean on an HDTV specification?

8. How do I transmit HDTV camera signals from underwater to the surface?

9. What are the advantages and/or disadvantages of using compressed or uncompressed HDTV signals?

10. Why would I want to transmit an uncompressed HDTV signal from the ROV to the surface, but then to record a compressed HDTV signal at the surface?

11. Which compression/decompression (CODEC) is best for HDTV signals?

12. Why are there different frame scanning frequencies used for the HDTV video formats in USA and Europe?

13. What are the optimum viewing distances for HDTV monitors?

14. Can I transmit live HDTV across an Ethernet network connection?


1. What are the main advantages of HDTV over standard definition composite PAL or NTSC video?

There are two main advantages. The first is the most obvious HDTV offers significantly better resolution (image definition) than conventional standard definition video signals. HDTV can offer twice the horizontal and twice the vertical resolution of conventional SDTV (four times the picture detail). The other significant advantage is the big improvement in colour fidelity, and the lack of colour interference that is such a problem with composite NTSC and PAL video images.

2. What benefit does HDTV offer for underwater viewing applications?

The much increased resolution available from HDTV means that high-quality video inspection and survey tasks can be performed faster and better. Full 1080i HDTV video offers the equivalent of a 2MPixel stills image on every single video frame. This allows video inspections to be more detailed, and potentially reduces the need for stills photographic inspection. For remote manipulator operations, the additional image definition provided by HDTV potentially allows complex manipulative tasks to be completed faster and more easily, with less operator fatigue.

3. Is there a single standard format for HDTV signals?

There are two HDTV formats in common use for broadcast TV applications. The formats are termed 720p and 1080i. The 720p format has an image resolution of 1280 horizontal pixels by 720 vertical pixels, and it uses a progressive scan to display 50 or 60 full video frames every second. The 1080i format has a higher image resolution of 1920 horizontal pixels by 1080 vertical pixels and it uses interlaced scanning to display 25 or 30 full video frames every second. In simple terms, 720p provides improved resolution (over conventional SDTV) with faster frame updates. 1080i provides much higher resolution than either SDTV or 720p, but at the same frame update rates as standard SDTV.

4. Which HDTV format is better, 720p or 1080i?

There is no simple answer to this question, although for broadcast programming it seems that broadcasters are increasingly tending towards using 1080i as the preferred HDTV format. Originally it was thought that 720p would be better suited to fast moving TV images (such as live sports), and that 1080i would be better suited to more-detailed slower moving images such as natural history documentary programming. With the significant recent advances in display technology and the sophisticated de-interlacing circuits built in to new HDTV monitors, it seems that 1080i is becoming the more dominant format, even for live sports events. 1080i obviously offers the greatest potential image definition. For underwater applications, the increased resolution available from the 1080i format offers the biggest potential improvement in inspection, manipulator and survey performance.

5. What is the effective resolution of HDTV in TV Lines?

Again this question does not have a simple answer, as it is difficult to equate the TV resolution measurements that were common for CRT (vacuum tube) based TV monitor displays to that of the discrete pixel based displays (e.g., LCD, Plasma, DLP) in common use for HDTV systems. It is safe to say that HDTV offers at least twice the vertical and horizontal resolution of Standard Definition TV, as well as much improved colour definition. In signal terms, HDTV requires around 6x the amount of information required for an SDTV signal. Although there are no definitive results, the table below provides some tentative comparisons for effective resolutions between SDTV and HDTV, quoted in the familiar TV Lines (per picture height).

 

Video Format
Scanning
Pixel resolution
Static Resolution (TV Lines/ph)
Dynamic Resolution (TV Lines/ph)
SDTV - NTSC Horizontal*
n/a
~350
~350
  Vertical
n/a
~340
~270
SDTV - PAL Horizontal*
n/a
~400
~400
  Vertical
n/a
~400
~320
HDTV - 720p Horizontal
1280
~500
~630
  Vertical
720
~500
~630
HDTV - 1080i Horizontal
1920
~750
~940
  Vertical
1080
~750
~620

Notes:

  • SDTV performance assumes 4:3 screen aspect ratio and CRT based display
  • HDTV performance assumes 16:9 screen aspect ratio
  • Kell Factor for all static resolution assumed ~0.7
  • Kell factor for SDTV/CRT based moving images ~0.55
  • Extended Kell Factor for HDTV based moving images ~0.875
  • Horizontal Resolution for PAL and NTSC assumes broadcast bandwidths

So what does all of this mean? It means that both 720p and 1080i HDTV formats offer significant increases in resolution performance in terms of TV Lines/ph over the composite SDTV formats. On static images, 1080i has the clear advantage, with almost twice the vertical and horizontal resolution of the best SDTV images. On dynamic images, the modern pixel based display technology provides a further advantage with 1080i offering almost twice the vertical resolution and greater than twice the horizontal resolution of the best SDTV images.

6. What does YUV (Y-Pb-Pr or Y-Cb-Cr) mean on an HDTV specification?

The term YUV is a generic term used to describe the three components that go to make up the full HDTV video signal. All digital HDTV formats use component video signals. The most common component video format is RGB (Red, Green, Blue), and this is the component signal format used for all PC display monitors. For reasons explained later in this FAQ, it is more convenient and efficient to transmit colour video in YUV component format, where Y is the luminance or brightness channel and U and V are colour difference channels. The three channels, Y, U and V together do exactly the same job as RGB, but they allow this to be done with a smaller bandwidth or data rate (see later). YUV is a generic term for the three component video signals used in HDTV, the more correct technical terms are Y-Pb-Pr for analogue HDTV signals (e.g. connections to a TV monitor screen), or Y-Cb-Cr for digital HDTV signals (e.g., HD-SDI connection to a PC or to a digital HDTV recorder).

7. What do the terms 4:2:2, 4:1:1 or 4:2:0 mean on an HDTV specification?

These terms refer to the colour sampling scheme used in the HDTV signal. The human eye is much less sensitive to colour detail than it is to brightness detail, therefore to reduce the size or bandwidth of an HDTV signal it is possible to reduce the amount of colour information being transmitted in the signal without reducing the perceived quality of the image to the viewer. This is done by sampling the colour information (U and V colour difference channels) in the HDTV image less frequently than the luminance or brightness information (the Y channel). The luminance information is sampled for every single pixel in the image this sampling rate equates to the number 4 in the specification. So what do the other numbers mean?

4:2:2 This specification implies that the two colour difference channels (U and V) are only sampled every second horizontal pixel of the image, so the effective horizontal colour resolution is only half of the luminance or brightness resolution. The colour difference channels are still sampled on every vertical line of the image, so there is no loss in vertical colour resolution.

4:1:1 In a similar manner, this specification implies that the colour difference channels are only sampled every fourth horizontal pixel of the image. The effective horizontal colour resolution is a quarter that for the luminance or brightness resolution. Again, there is no loss in vertical colour resolution.

4:2:0 This is a slightly confusing nomenclature, but it means that the colour difference channels are sampled every second horizontal pixel and also every second vertical pixel. In this sampling scheme, both the horizontal and the vertical colour resolution is half that for the luminance or brightness channel.

RGB For an RGB component video signal the de-facto sampling scheme would have to be 4:4:4. All three colour component channels would have to be sampled on every pixel, as the luminance or brightness information is shared between all three colour component channels. This is why YUV component video can be more efficient than RGB as it allows some saving in bandwidth by reducing the colour information only, without affecting the luminance or brightness information.

Comparison between bandwidth requirements for YUV and RGB

  • YUV 4:2:2 requires 2/3 the bandwidth of RGB (4:4:4)
  • YUV 4:1:1 requires 1/2 the bandwidth of RGB (4:4:4)
  • YUV 4:2:0 requires 1/2 the bandwidth of RGB (4:4:4)

In subjective quality terms there is no difference in perceived image quality between YUV 4:2:2 and full bandwidth RGB. The perceived reduction in image quality using 4:1:1 or 4:2:0 colour sampling is very slight, with 4:2:0 offering perhaps a slightly better balanced performance both horizontally and vertically. For all practical purposes, 4:1:1 and 4:2:0 offer excellent image quality while taking up half the space of full bandwidth RGB video signals.

8. How do I transmit HDTV camera signals from underwater to the surface?

There are two issues to deal with firstly the increased resolution that HDTV provides, and second, the fact that all HDTV signals use three component video signals to make up the HDTV image. These two issues add up to a lot of information in an HDTV signal typically around 6x greater than that required for an SDTV signal. There are three alternative HDTV transmission methods:

  • Analogue component (x3) video signals
  • Digital serial signal / uncompressed (e.g., HD-SDI)
  • Digital serial signal / compressed (e.g., MPEG2, AVC, WMV-HD)

It is safe to say that the Analogue transmission option is only suitable for relatively short cable transmission distances (maybe several hundred metres with suitable long-line amplification), and requires three separate matched coaxials to be present in the cable. Each coaxial will carry one of the three video components (Y, U and V).

For longer transmission distances, the only practical option is to use a fibre-optic cable. A fibre-optic transmission system lends itself to using a single serial HDTV streaming signal format and the choice is between an uncompressed serial digital signal (e.g., HD-SDI), or a compressed serial digital signal (e.g., MPEG2, AVC, WMV-HD).

9. What are the advantages and/or disadvantages of using compressed or uncompressed HDTV signals?

Uncompressed The main benefit of using an uncompressed serial digital signal (such as HD-SDI) for HDTV transmission is that the video images reaching the surface are effectively in real-time. There is no significant processing delay between what the camera sees and what the operator views on the surface. The main disadvantage of using an uncompressed signal is the enormous data rate that is required. The HD-SDI interface has a transmission bit-rate of approximately 1.5Gbps (1.5 Gigabits per second). This is a formidable bit-rate, and requires special fibre-optic transmission system components, and the very careful choice of interconnecting cables, connectors and circuit wiring.

Compressed The principle benefit of using a compressed serial digital signal (such as MPEG2, AVC or WMV-HD) is that they require a much smaller digital bit-rate than an uncompressed signal. This means that they can be transmitted using less sophisticated fibre-optic transmission techniques, and they are much more suitable for direct recording and storage purposes because they take up less storage space. The main disadvantage of compressed HDTV signals is that they are not real-time. The compression (and decompression) process involves a significant delay (typically several seconds). This means that the image viewed by the operator on the surface is delayed compared to what the camera sees. This obviously means that compressed signal transmission is not suitable for any viewing task requiring real-time performance such as ROV navigation, manipulator operation or certain survey tasks. Another potential disadvantage is that dependant on the amount of compression being used, there is always the possibility of compression artefacts reducing the perceived image quality of the HDTV signal compared to the original.

10. Why would I want to transmit an uncompressed HDTV signal from the ROV to the surface, but then to record a compressed HDTV signal at the surface?

For viewing applications requiring real-time video performance (e.g., navigation, manipulator applications, pipeline survey) the only practical option is to transmit an uncompressed HDTV signal from the ROV to the surface. This ensures that there are minimal processing delays through the transmission chain and the HDTV images at the surface can be viewed in real-time. The standard signal interface format for uncompressed HDTV is HD-SDI. Because of the very high bit-rate of the HD-SDI signal (~1.5Gbps) it is impractical to consider recording this uncompressed signal. Recording the HD-SDI signal would require 2TB (Terrabytes) of digital storage space for a single 3-hour video recording interval.

For storage and archiving purposes it is more practical to consider using one of the compressed HDTV signal formats. All current broadcast HDTV programming transmissions use the ubiquitous and well-proven MPEG2 compression method and typical transmission bit-rates are between 20Mbps and 25Mbps for broadcast quality video. At a bit rate of 25Mbps, the digital storage space required for 3-hours of HDTV video would be approximately 34GB (Gigabytes). This is a much more practical storage requirement than the uncompressed HD-SDI signal. The use of one of the more advanced compression systems such as AVC or WMV-HD might allow the storage requirement to be further reduced to half that required for MPEG2.

11. Which compression/decompression (CODEC) is best for HDTV signals?

There are currently three main classes of video CODEC used for HDTV signals. Each of the CODECs has their own particular advantages and disadvantages.

  • MPEG2 / HDV
  • AVC / MPEG4 Part 10 / H.264 / AVC-HD / Apple Quicktime 7
  • WMV-HD

MPEG2 has been a well established video compression standard for many years, and it is the de-facto compression used on all commercial digital and HDTV network broadcasting systems across the world, for terrestrial, satellite and cable transmissions, as well as on DVD movies. Efficient and cost-effective MPEG2 coding and decoding hardware is readily available and there are many software editing solutions available for PC video manipulation, editing and recording.

AVC (Advanced Video CODEC) is an International ITU approved CODEC standard also commonly referred to as MPEG4 Part 10, or H.264. It is a more advanced video CODEC than MPEG2. This means that it has the potential to maintain the same video quality at a much lower bit-rate. Typically an AVC compressed HDTV signal will occupy around half the storage space of an equivalent MPEG2 coded signal while maintaining the same perceived image quality. The disadvantage with AVC is that it is a more complex CODEC and requires more powerful processing, both for coding and decoding. Hardware coders and decoders are only now becoming available. They are significantly more expensive, and software editing applications are also only just becoming available. AVC requires more powerful PC's for video manipulation and editing purposes.

WMV-HD is a proprietary CODEC developed by Microsoft as part of the Windows Media software suite. It is generally considered to be very similar in performance to AVC in terms of coding efficiency. The main disadvantage again is that it is a complex CODEC and requires significant processing power, although notably less processing power for decoding than AVC. The other potential disadvantage is that it is proprietary to Microsoft. On the plus side, the WMV-HD decoding technology will be built in to the ubiquitous Windows Media Player an integral part of the Windows operating system, and therefore it will have widespread distribution across the world.

12. Why are there different frame scanning frequencies used for the HDTV video formats in USA and Europe?

This is a good question, as at first view it appears that there is little advantage in using different frame rates for HDTV on different continents. The answer appears to lie in the history of the NTSC and PAL broadcast TV systems used on these different continents, and in maintaining compatibility of newly recorded HDTV program material with these older standard definition TV systems.

For the USA, the frame scanning rates for HDTV are either 30Hz for interlaced scanning or 60Hz for progressive scanning. This makes for easy compatibility with the 30Hz interlaced scanning used in the NTSC TV system.

For Europe, the frame scanning rates for HDTV are either 25Hz for interlaced scanning or 50Hz for progressive scanning. This makes for easy compatibility with the 25Hz interlaced scanning used in the PAL TV system used in most of Europe.

The HDTV monitor specifications normally also allow for a frame rate of 24Hz, which provides direct compatibility with movie films (normally recorded at 24 frames per second).

13. What are the optimum viewing distances for HDTV monitors?

Much closer than you would think! Based on the performance of the human eye and brain to resolve image detail, the Society of Motion Pictures and Television Engineers (SMPTE) have recommended an optimum viewing distance of 3x picture height for HDTV displays. This compares with the SMPTE recommended viewing distance of 6x picture height for SDTV displays.

The SMPTE recommended viewing distances are calculated by considering the visual acuity of the human eye / brain. It is generally accepted that the eye can resolve detail down to approximately 1 minute of arc (1/60th of a degree of arc). Theoretical calculations based on the physical properties of the perfect eye indicate a visual acuity better than this, maybe down to 0.5 minute of arc, but 1 minute of arc is generally accepted as the standard for 20/20 vision. Using this as the baseline performance for the human eye, the SMPTE made recommendations for viewing distances that would allow the viewer to exploit the full resolution performance of the TV system without exaggerating the line scanning structure (CRT) or pixel structure (LCD, Plasma) for the TV display.

Shown below is a table showing the optimum viewing distances for various standard TV screen sizes, at SDTV resolution and both common HDTV resolutions.

 

TV Format Screen Size
Screen Aspect Ratio
Recommended Viewing Distance (feet)
SDTV (PAL or NTSC)
21"
4:3
6.5'
28"
4:3
9'
32"
4:3
10'
36"
4:3
11.5'
HDTV 720p
26"
16:9
5'
32"
16:9
6'
42"
16:9
8'
60"
16:9
11.5'
HDTV 1080i
26"
16:9
3'
32"
16:9
4'
42"
16:9
5'
60"
16:9
7.5'

 

As can be seen, the recommended viewing distances for optimal HDTV viewing are much closer than for SDTV. The principal advantage of HDTV is that it allows a larger TV image to be viewed much closer to maximise resolution potential for the viewer.

Note that if the viewer is positioned much further away than the recommended viewing distance then it is unlikely that they will be viewing the full resolution potential of the HDTV signal. If the viewer is much closer than the recommended viewing distance then it is likely that the scanning structure of the TV display pixels or phosphors will become apparent, and this will reduce the overall perceived quality of the viewed image.

 

14. Can I transmit live HDTV across an Ethernet network connection?

It is unlikely that this will be a practical option in the near future. There are two common Ethernet network systems currently in widespread use.

  • 100BaseT (Fast Ethernet)
  • 1000BaseT (Gigabit Ethernet)

The maximum theoretical data transmission bit-rates for these systems is 100Mbps and 1000Mbps (1 Gbps) respectively. Under real network operating conditions, because Ethernet uses a packet switching and collision detection system for data transmission, the actual data throughputs can be considerably less than these figures.

It is immediately obvious that neither Ethernet system has sufficient data bandwidth to be able to stream uncompressed HDTV video with a data bandwidth of 1.5Gbps. It is possible to consider transmitting live compressed HDTV video over an Ethernet network, but this would likely involve very heavy compression of the HDTV signal, and a consequent big reduction in image quality. If there are other bandwidth intensive applications running over the network at the same time then it is likely that there will be glitches or breaks in the video, even at lower transmission bit-rates. In summary, Ethernet is not a good transmission system protocol for high-bandwidth streaming HDTV video signals.

Of course it is perfectly straightforward to transfer an already recorded HDTV video data file across an Ethernet network as this does not rely on real-time network performance. Because of the size of recorded HDTV files, transfer over a network may take some time, and it may use up a significant proportion of the available network bandwidth.

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