Digital TV Basics

Digital Broadcasting
A Glossary of Terms

Terms:	Topics:
ADC or A/D Converter Binary Numbers Bits and Bytes Bit Error Rate (BER) Codec Concatenation DAC or D/A Converter Delay HDTV Interlace Modem Pixel Progressive Scanning SDTV	Component Video/Standard Component Video High Definition Television (HDTV) Sample (Clock) Frequency Serial Digital Interface (SDI)

Binary numbers
In digits, binary numbers are used where numbers represented to the base 2
(i.e. 1 or 0). Each 1 & 0 is referred to as a bit.

Bits and Bytes
1 Byte (B) = 8 Bits (b) or 256 discrete values
Note: Telecoms & broadcast engineers talk about 'bits' (b) whilst IT people talk about 'Bytes' (B).

Bit Error Rate (BER)
The ratio of the number of bits lost, compared to the total.

Pixel
A pixel, a shorted version of Picture Cell, or Picture Element.
It is one sample or the smallest piece of picture information.

ADC or A/D converter
An Analogue to Digital converter

DAC or D/A converter
A Digital to Analogue converter

Codec
A coder/ decoder combination

Modem
A Modulator/ demodulator combination

Concatenation
Concatenation is the name given to the linking of digital systems and compressions in tandem. Each compression encoding pass removes information and the compatibility of the final concatenated system can cause concern.

Delay
Delay is measured in frames or time. One frame is 40 ms

SD or SDTV
Standard Definition Television:

	525 lines/60 Hz (USA, Japan, etc.)
	625 lines/50 Hz (Europe, etc.)

Horizontal resolution: 720 pixels x 576 active lines

HD or HDTV
High Definition Television:

	1125 lines/60 Hz (USA, Japan, etc.)
	1250 lines/50 Hz (Europe, etc.)

HD resolution: 1920 pixels x 1080 lines

Composite Video
In Composite Video the luminance and chrominance elements or components of the picture are combined and presented as a serial signal. (eg PAL, NTSC, SECAM.
See also SDI).

Component Video & Standard Component Video
In Component Video the luminance and chrominance elements or components of the picture are digitised and presented as a parallel signal.

Standard Component Video is the standard for Standard Definition Television.
The standard is known as ITU-R BT.601 or REC 601 or CCIR 601
The components are Red, Green, Blue (RGB), represented as:

	Luminance signal:	Y
	Colour difference signals:	(R-Y) or (u), (B-Y) or (v) when analogue Cr & Cb when digitised

Standard component video is also referred to as: 4:2:2
The numbers relate to Luminance & the two Chrominance signals respectively.

The '4' is a random reference relating to Luminance resolution and corresponds to 13.5 MHz sampling and 720 pixels
The '2' is the proportion of the '4' and relates to the Chrominance resolution, and to 6.75 MHz sampling and 360 pixels
(The colour difference signals contain less information since the eye is better at resolving luminance than chrominance.)

Coding levels can be either 8 bit or 10 bit

	8 bit coding provides 256 discret coding levels (level 16 for black, 235 for white, a total active of 219 levels
	10 bit coding provides 1024 discret coding levels (level 64 for black, 940 for white, total of 876 levels

Other standards commonly used

4:2:0	One line is sampled 4:0:0 (ie luminance only), the next 4:2:2 ie vertical chroma resolution is halved.
4:1:1	The colour difference signals are sampled at 3.375 MHz and 180 pixels ie horizontal chroma resolution is halved.
3:1:1	Chroma as above, with luminance sampled at 10.125 MHz

Sample (Clock) Frequency
Nyquist says clock must be significantly above 2x max frequency
ie 2 x 5.5 = 11 MHz

The sampling frequency has to produce a static pattern on both 525/60 & 625/50 and therefore needs to be a whole multiple of both line lengths. The lowest common frequency to provide static sampling on both standards calculates out to be 2.25 MHz.

11.25 MHz (5 x 2.25) is not sufficiently above 11.0 MHz so 13.5 MHz (6 x 2.25) is the chosen sample frequency

For 4:2:2, the Sample Frequencies are:

	Luminance (Y) 13.5 MHz
	Chrominance (Cr & CB) 6.75 MHz

Serial Digital Interface (SDI)
Is the internationally accepted standard is for an uncompressed 4:2:2, 625/50, 10 bit coded digital signal. With SDI the Transfer Rate is 270 Mb/s. This comprises:

	720 pixels x 576 active lines (Y signal) 2 x 360 pixels x 576 active lines (Cr & CB signals) 25 frames/second 10 bit coding (720 x 576 + 2 x 360 x 576) x 25 x 10 = 207 Mb/s (active picture)
	This becomes 270 Mb/s with audio,data, and overheads Coding rate 10 bit Embedded audio Four groups of four channels

For 8 bit coding this reduces to 166 Mb/s, total active picture, 216 Mb/s full rate.

Interlace
Interlacing is the method of scanning used in television whereby each frame (individual picture) comprises two interlaced fields. One field displays odd lines, and the next even lines. TV formats have odd numbers of lines eg 625 so that the lines of one field 'interlace' between the lines of the next.

Interlacing improves the portrayal of motion and reduces flicker, without increasing picture rate, bandwidth or data rate. The disadvantages of interlacing is that it reduces vertical bandwidth by about 30%, can cause dither, and makes further prossessing difficult eg in DVEs, freeze frames.
See also Progressive Scanning

Progressive Scanning
Progressive scanning simply scans all the lines in one vertical scan, from top to bottom. It is used in computer displays and some HD systems. With progressive scanning, each frame (individual picture) is self-contained, as with 16/35mm film

There is much debate on the relative advantages of Interlaced and Progressive scanning.
See also Interlace

High Definition Television (HDTV)

HDTV Standards
ITU-R BT.709 defines two standards:

	1125 lines/60 Hz (USA, Japan, etc.)
	1250 lines/50 Hz (Europe, etc.)

At 4:2:2, the sampling frequency for high definition (HD) is 5.5 times that for standard definition (SD)
i.e. Y = 74.25 MHz (SD is 13.5); Cb/Cr = 37.125 (SD is 6.75).

ITU-R BT.709-4 adds 1080 active lines for both 1125 & 1250 standards.

HD Resolution:	1920 pixels x 1080 lines (SD is 720 x 576)
HD Aspect Ratio:	16 x 9 (Widescreen)

HD-SDI to SMPTE-292M is 1.485 Gbits (SD is 270 Mbits)
(10bit, 4:2:2 component video/audio/ancillary data)

HD Contribution Standard:	45 Mbits DS3 (no simple SD equivalent)
HD Distribution Standard:	19.39 Mbits SMPTE310M (no simple SD equivalent)

A Guide to Digital Microwave Technology:

Digital Compression Techniques
Digital Modulation Techniques
Digital RF Transmission
A Glossary of Terms

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