Mar 25
DIGITAL TV by Alex J. Socorro  |  Posted in Articles  |  on Wed, Mar 25, 2009

On June 12, 2009, the United States will change over to DTV (Digital Television). As previously scheduled in February of this year, analog broadcast of TV signals will be totally stopped.

There are about 1,800 full power stations in the USA which are covered in the impending switch. This is the culmination of the 1996 authorization of the US Congress which allowed the distribution of additional broadcast channel so that they could simultaneously broadcast in digital and analog.

The Netherlands was the first country to switch to digital over-the-air broadcasting in 2006. It was followed by Finland and Sweden the following year. In the other parts of the globe, the movement is gaining ground. Canada is ready by 2011 and the whole of United Kingdom will switch by 2012.

In contrast to analog signals, DTV is the sending and receiving of moving images and sounds via digital signals. This can be likened to the difference of the phonograph disc as against the CD of today. The phonograph disc conveys analog sounds via the grooves while the CD stores sound in digital format. Another comparison is that of the volume control of a TV remote control versus the olden volume dial.

For some technical jargon, the Digital signal supports many picture formats in combination of size, aspect ratio and interlacing. HDTV (high definition tv) and SDTV (standard definition tv) are just two of the definitions that arise from the switch to DTV broadcasting.

Just like the computer format, HDTV uses 1280 x 720 pixels in progressive scan mode or the 1920 x 1080 pixels in interlace mode. This is the same convention being used by late model HD cameras. However, HDTV mode has the advantage that it can be transmitted over the current analog channels.

SDTV on the other hand, can use different aspect ratio formats depending on the technology being used in the country of broadcast. For NTSC countries (like the Philippines), the 640 x 480 ratio is the standard while for PAL countries it is the 720 x 576 dimension which is rescaled into 768 x 576.

The aerial antenna is the most common and oldest way of receiving TV signals. Channels are, however, limited to what the antenna can pick up and the quality of reception is affected by the strength of the signal that is received.

Other ways of receiving TV signals are via digital cable and digital satellite. There is also the IPTV (Internet Protocol TV) which enables a viewer to watch programs in the ordinary tv channels. With the proliferation of the open internet (unrestricted by license), a lot of P2P Internet Television software are in the free market. This method converts the computer monitor into a tv set.

For the time being, while the analog signal is in the process of being phased out, co-existence is maintained thru a table of signal-to-noise and signal-to-interference ratios. This table is the controlling factor of power levels of the tv stations. Fewer channels are needed to carry an all-digital set because Digital TV is more tolerant of interference than analog signals.

Technically speaking, the most signifact advantage of DTV over analog TV is that it takes up less bandwidth. The bandwidth need of DTV is consonant to the image quality and also depending on the level of compression. Translated into layman’s terms, digital broadcasters can provide more digital channels given the same space.

Some other technical advantages of DTV are:
1. It permits such services as multiplexing which enables a viewer to watch more than one program in a single tv channel.
2. It allows additional language feature whether spoken or subtitled and the use of electronic program guides.

These no-television services may provide additional sources of income for the broadcaster.

For the viewers, the big difference is the clarity of video and audio. Digital signals don’t suffer from ghosting or noise since numeric (digital) information is, unlike analog, precise. On the other hand, the use of compression such as MPEG-2 tends to cause digital degradation in some cases:
1. Quantization Noise, Incorrect Color
2. Blockiness
3. Blurred, Shimmering Haze

The Quantization Noise is caused by the uneven distribution of noise hence making it more noticeable especially in motion that is fast, random and complex. Other degradations is partly caused by the unexact decompression.

Another disadvantage of DTV is the significant delay when changing channels. This delay is caused by the so-called preload time which is actually the process of decompression. Some converters may show damaged picture and some may manifest the “cliff effect” where no picture is displayed.

The switch-off ruling of the analog tv would render non-digital television obsolete. That means old tv sets running on pure analog will not be serviceable anymore come the switch-off-date. However, an external converter box can be purchased so that analog tv can still be used. But when a converter box is used, the TV’s remote control is rendered inutile. Like in the US and other countries, it takes the enactment of a law to make the switch to DTV broadcasting. And since the Philippine congress is preoccupied with the coming elections next year, the switch to DTV in the country is definitely far at hand.

Comments to this article can be sent to ajsocorro@yahoo.com


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