What is Digital TV

Introduced in the 90s, digital television refers to an advanced broadcasting technology of images and sound by means of discrete signals (ones and zeros).

Also known as DTV, Digital TV is more flexible and efficient than analog television in that it has the potential to deliver higher-quality images and sound, more programming options, and new digital interactive services.

However Digital Television does not necessarily imply a better quality TV. The potential is there - but it all depends on the compression levels used by the television broadcasting stations in their aim to fit in more content. This brings about a corresponding expense in mage quality, depending on the level of compression.

For the consumer, DTV refers to a TV set with a built-in ATSC tuner. ATSC stands for Advanced Television Systems Committee; this is the regulating body in the United States that governs digital TV standards.

In the light of the digital TV transition completed in the US by end June 2009, the Federal Communications Commission (the US government interstate communications regulating body) had ruled in its digital tuner rule that all new TVs sold after March 1, 2007, had to include a digital (ATSC) tuner. Furthermore, TVs sold ex-stock after March 2007 that did not include a digital tuner, had to include a 'Consumer Alert' as mandated by the FCC. This consumer alert had to indicate clearly that the said TV would require a converter box to receive over-the-air broadcasts with an antenna after the DTV transition is complete.

Often, TV sets with an ATSC tuner are referred to as HDTVs in that all HDTVs are digital TVs. But care should be taken because as we will explain further on in this article, not all DTV sets are capable of displaying high definition content.

Why Digital TV: What are the reasons for change?

The main primary reason for this change - also referred to as digital TV transition or DTV transition - is one of freeing valuable spectrum space. While the available spectrum has remained the same, yet the available TV broadcasts and wireless services have increased to a point that urgent action became mandatory to manage properly this scare resource.

Digital television is more efficient in delivering TV content - meaning that it requires less bandwidth due to compression techniques that are not possible with analog transmission. As a result, DTV can deliver more TV programs in significantly less spectrum space.

Hence, it is possible to free up a good portion of the spectrum allocated for TV transmission, for other services.

But not only, digital television has the potential to deliver a clearer picture, and new interactive video and wireless data services that are not possible with traditional analog technology.

And there is an equally important reason for the he DTV transition. This digital TV transition is also about money!

New technology is always good for the economy, but not only. It is also good for the Federal Communications Commission; the FCC aim has always been to auction the freed spectrum space as a result of the DTV transition. In the process, it expected to raise some fifty billion dollars!

DTV Transition: What about Cable and Satellite TV Subscribers?

The digital television transition has nothing to do with cable or satellite TV. Digital terrestrial television services affects only TV sets hooked up to an over-the-air antenna. In other words, unless your TV has a digital TV tuner, then you will not be able to receive DTV services unless you invest in a suitable converter box and an appropriate on-the-roof TV antenna.

Digital TV Formats and Bandwidth Issues

While the misnomer is that DTV is HDTV, yet Digital TV comprises various TV formats and not just HDTV. The most common are:

	Standard Definition Television (SDTV) - this is the basic level of quality and is the digital equivalent of analog NTSC.
It comes in interlaced format and carries a picture resolution of 640 x 480 pixels in either traditional 4:3 aspect ratio or in widescreen 16:9 format.
	Enhanced Definition TV (EDTV) - this is the next level and represents a step-up from analog television and DTV.
It still has the same picture resolution and comes in either aspect ratio but contrary to SDTV - which is interlaced in nature, EDTV comes in progressive 480p (progressive) format only. For an explanation of the differences between interlaced and progressive formats, please refer to our 1080p HDTV article.
	High Definition TV (HDTV) - with its array of different formats, represents the highest resolution and picture quality of all digital television broadcast formats.

Discussing the different HDTV formats is a topic on its own, yet the most commonly used HDTV formats in the TV broadcast industry are 720p and 1080i. Available only in 16:9 widescreen format, 720p carries a resolution of 1280 x 720 pixels in progressive scan mode while the 1080i interlaced format has a picture resolution of 1920 x 1080 pixels.

There is the misconception here that 1080i is better than 720p. This needs not necessarily be the case; for an understanding of this issue, please refer to our HDTV guide.

Interesting is that with the coming of digital TV, each broadcast channel still has an analog bandwidth of 6 MHz (8 MHz in most European countries), but then the measured capacity becomes digital. In this respect, each TV channel can carry a digital bitstream having a maximum data rate of up to 19.4 megabit-per-second or 2.375 megabytes-per-second. This is the bitstream required for full HD content.

The savings in bandwidth comes as DTV does not require the full 19.4 MBits/sec. It is all a matter of what the stations put in their allocated 19.4 Mbps bitstream. For example, a TV station may broadcast its HD at 15.5 Mbps or less, to simultaneously broadcast one or two SD channels, plus any other non-video datacasting services. How many channels a broadcaster may fit depends on the adopted quality and compression levels.

This ability to provide either a single full HD feed or multiple lower resolution mixes is often referred to in digital TV as multicasting. This means that say on channel 21, you could watch the main program channel 21-1 in high definition, plus subchannels 21-2 or 21-3 with local information in standard definition. Alternatively, stations may swap between HDTV and SDTV during different times of the day on the same program channel.

Allocated Channels

At present, channels allocated for TV broadcast in North America, range from Channel 2 to 13 in the VHF (Bands 1 and 3), and Channels 14 to 36 and 38 to 69 in the UHF (Bands 4 and 5); channel 37 is considered a silent channel allocated for radio astronomy research and may not be used by any TV station.

However, from the above, it is clear that a DTV broadcaster does not need multiple broadcast channels to broadcast multiple program feeds at the same time. This means that not all of the originally allocated channels for TV broadcast are required now that the digital TV transition is complete.

As a result, Channels 52 through 69 will be reallocated for other purposes - thus freeing 108 MHz of valuable spectrum space thanks to this transition to digital television broadcasting.

Advantageous and Disadvantageous of DTV

More Content: We have already touched upon the main advantages of digital TV - namely reduced spectrum space, more content choice through multicasting, provide high-definition television services, and support of non-television services such as multimedia, interactivity, and program information.

The latter means that a digital TV receiver is able to display information about a show, or even call an electronic program guide (EPG grid). This is similar to cable or satellite TV boxes, listing upcoming shows arranged by channel, time of day, and day of the week.

Improved AV Quality: DTV supports 5.1 surround sound, and as expressed in the previous section, it also has the potential to deliver higher quality picture. In particular, from the digital TV reception aspect, the digital picture will be free from the 'ghosting' and 'snow' that can affect analog transmissions.

And what are the disadvantageous of digital TV?

The most obvious of all disadvantageous associated with digital television arise by way of the DTV transition process itself, more specifically, the conversion from analog to digital television.

DTV Transition: Switching off from analog to digital means that all non-digital televisions have been made obsolete unless as further explained in the next section, these are connected to an external digital tuner box. It is true that this is no big deal in that these boxes are relatively cheap, but having a set-top box connected to your TV renders your TV nothing more than a monitor. In other words, you would not be able to use your TV remote to change channels as this is something that is handled by the converter box. And ending with two remotes is not the most convenient way to use your TV.

But there are a few others disadvantageous as well that are specific to digital TV...

Digital TV Picture Artifacts: Despite that DTV has the potential to deliver better images and sound than its analog counterpart, DTV images have some picture defects that are not present on analog television system. Artifacts such as 'quantization  noise' and 'blockiness' are quite common in DTV pictures.

Quantization noise is the resulting error in the displayed picture signal between the actual 'analog' value and the discrete digital representation; this is inherent in any digital system. Blockiness arises when the picture breaks down when trying to display scenes with complexity. At times, a picture may also dissolve into a stuttering motion, or even a brief of total freeze when trying to display fast movement, or both.

This mainly happens with unconverted program content to be displayed in HD. But it can also happen with SDTV content as well and is the result of excessive compression levels used by the TV broadcaster to fit in the available content while reducing the required bandwidth. You see, digital TV is not a miracle technology - you get more content feeds from the same TV broadcaster thanks to compression techniques (MPEG-2) at the expense of a varying reduced quality.

Buffering: Another issue that may arise only with digital TV is what is referred to as preload delay till the set buffer enough content to begin showing the broadcast stream. This delay varies between different TV sets and leads to a significant delay - in comparison to the almost instant response associated with an analog TV - when changing channels. This delay can make 'channel surfing' on digital television sets more difficult.

Effects of Poor Reception: While with analog television, it is still possible to display some form of degraded picture in difficult reception areas, with DTV reception, things are completely different.

In reality, this represents both an advantage and a disadvantage. An advantage because while with analog TV reception, you may experience a gradually degraded picture quality as reception conditions worsen, with digital television, you would still continue enjoying a perfect picture. However, this continues till the system reaches what is referred to as the 'digital cliff'. At that point, your DTV would not be able to generate a picture at all and would simply display nothing. In contrast, an analog TV would still continue to display some form of degraded image - no matter how worse it is.