In every numerical television broadcast
standard, the MPEG 2 norm is used, at least for video. In audio, things
are not that simple.
|Typical use||Terrestrial, cable||Satellite, cable, terrestrial|
|Video coding||MPEG2 Video||MPEG2 Video|
|Audio coding||AC3||MPEG2 Audio|
In the eighties, it was generally assumed that numerical broadcasting of television would not happen before the end of this century; because the bandwidth needed was very important (108 to 270 Mb/s for a display of 525 or 625 lines). Besides, Japan, Europe and USA thought that customers would need a higher quality for the image; so the standards IDTV and HTDV (1050 to 1250 lines) were developed. They would have required 1 Gbit/s. So the High Definition Systems proposed were analogue (MUSE in Japan, HD-MAC in Europe, and some propositions in USA) with a digital help.
At the end of the eighties, many things changed:
The MPEG 2 Norm allows compressing
video signal with very low quality loss. For example, for standard television,
a signal from 124 Mbits/s is compressed to between 4 and 8 Mbits/s.
The MPEG transport stream
Contrary to the MPEG 1 and MPEG 2 program
streams, which are dedicated to error free medium (CD-ROM, hard disks),
the MPEG transport streams is dedicated to error prone mediums (television).
So the length of the transport packets is fixed to 188 bytes. A transport
packet divides into:
An MPEG transport stream is divided
into several PES (packet elementary streams), which are divided into transport
packets. Each PES is identified by its PID (Packet IDentifier). We will
not describe here the different tables and information in MPEG norm, as
it would be too complex and not as good as that you can find elsewhere.
See  and .
In the video world, the colours are not stored in the same way in order to save some bandwidth. Our eyes are less sensitive to the actual colour information than to the luminance intensity, which characterises the brightness of pixels. So a possible way to compress the video signal is to allocate more bandwidth (i.e. spatial resolution) to the luminance information and less bandwidth to the chrominance information. The luminance Y and the chrominance values Cb and Cr are deduced from the RGB values by a lineal translation defined by the 3x3 matrix M:
Cb (respectively Cr) is usually the difference between the blue (respectively red) component and Y.
The MPEG2 norm also uses the YUV components. Here are the formulas:
Cb=0,564(B-Y) or U=0.493(B-Y)
1: Chrominance subsampling formats
|Format name||Horizontal subsampling||Vertical subsampling||Compression ratio||Application|
|4:4:4||1||1||1||High quality video|
|4:2:2||2||1||1.5||Standard TV format|
|4:2:0||2||2||2||Standard MPEG format|
In analogue television, interlacing is used to save some bandwidth. It consists in transmitting (and displaying) the even lines then the odd lines of each picture. So the used bandwidth is divided by two.
According to the MPEG vocabulary, a field is a picture where the vertical resolution has been divided by two. The top field contains the even lines (the first line being the line 0) and the bottom field contains the odd lines. A frame consists of a top field and a bottom field. Note that a frame is not necessary a coherent picture because the top and bottom fields are usually sampled at a different time.
The word picture refers to either
a field or a frame, depending on the context. In MPEG2, a picture corresponds
to the compressed data associated with a picture header. It can be a field
or a frame. An interlaced sequence is a sequence, which is field-based;
i.e. each picture displayed is a field. A progressive sequence is
a sequence where each picture displayed is a frame.
Table 2 - Standard TV formats
|Standard||Horizontal resolution||Vertical resolution||Colour format||Interlaced||Frame rate (Hz)|
This would be displayed in that order:
Inside the set top box, you generally find (here we take the example of the LSI Logic SDP-1000):
On such boards, you have many constraints: everything must be affordable for the customer. So it is like a small PC except that you do not have much memory, an hard disk, a very fast CPU and so on.
An MPEG sequence is usually decoded into a YCrCb colour space (see section 1.2.1), so we need an encoder to modulate NTSC, PAL or SECAM.
So the encoder system encodes digital
YCrCb video data to an NTSC, PAL-CVBS or S-Video
signal and also RGB. The system has two separate digital video input channels
Digital video data input on the first channel is encoded into RGB format
while that on the second channel is encoded into PAL/NTSC-CVBS and S-Video
Table 3- Different TV formats and characteristics
|FORMAT||LINE/FIELD||BURST FREQ (MHz)||PIXELS|
|SECAM||625/50||4,250 and 4,406250||864|
In the oldest PAL, NTSC and SECAM standards,
some lines, which are not part of the active video, are left at blanking
levels; so further modifications of the standard allow you to add information
instead. This allows you to include teletext, widescreen and so on. See
188.8.131.52, 184.108.40.206 and 2.1.3. You can see in Table 4 and Table 5 , or in
Figure 4 what is in these lines.
Table 4- NTSC lines
|Lines||Length||Output from encoder|
|4-6||3||Broad vertical pulse|
|10-20||11||Normally at blanking level1|
|21||1||Field I closed-caption data or blanking level1|
|263||1||Hsync, burst, and equalization pulse|
|266||1||Half line equalization pulse, half line broad vertical pulse|
|267-268||2||Broad vertical pulse|
|269||1||Half line broad vertical pulse, half line equalization pulse|
|272||1||Half line equalization pulse, half line blanking level|
|284||1||Field II closed-caption data or blanking level*|
Table 5- PAL lines
|Lines||Length||Output from encoder|
|1-2||2||Broad vertical pulse|
|3||1||Half line broad vertical pulse, half line equalization pulse|
|6||1||Blanking level1; burst in odd frames only|
|7-21||15||Blanking level1 (or teletext)|
|22||1||Field I closed-caption data or blanking level1|
|23||1||Blanking level2 (or widescreen)|
|310||1||Active video3; burst in odd frames only|
|313||1||Half line equalization pulse, half line broad vertical pulse|
|314-315||2||Broad vertical pulse|
|318||1||Half line equalization pulse, half line blanking level|
|319||1||Blanking level1; burst in even frames only|
|320-334||15||Blanking level1 (or teletext)|
|335||1||Field II closed-caption data or blanking level1|
|622||1||Active video3; burst in odd frames only|
|623||1||Half line blanking level2, half line equalization pulse|
6- Different widescreen aspect ratio
|Number Ratio||Aspect ratio||Format||Position|
|WIDESCREEN_NO = 0x8||1 000||4:3||full||N/A|
|WIDESCREEN_001= 0x1||0 001||14:9||letterbox||centre|
|WIDESCREEN_010= 0x2||0 010||14:9||letterbox||top|
|WIDESCREEN_011= 0xB||1 011||16:9||letterbox||centre|
|WIDESCREEN_100= 0x4||0 100||16:9||letterbox||top|
|WIDESCREEN_101= 0xD||1 101||>16:9||letterbox||centre|
|WIDESCREEN_110= 0xE||1 110||14:9||full||centre|
|WIDESCREEN_111= 0x7||0 111||16:9||full||N/A|
This part aims at describing what are the requirements for teletext. Some of the registers concerned cannot be changed at any time, because the teletext information is given during blanking periods (VBI) and if you change the parameters during this time you will have troubles. The registers concerned with that are the registers to enable it and to say which are the begin and end lines in odd and even fields. These parameters may change during the sequence. So we need to be able to write them at each image displayed, but not during the VBI interval.
As a reference, we have been using
the  made by the European Broadcasting Union in October
96. Teletext data are conveyed in Packetized Elementary Streams (PES).
Each PES data field has several data_field
and each data_field has the line_offset
which the field shall be displayed (from 7 to 22 for field 1 and 320 to
335 for field 2). The toggling of the parity_field
a new field.
7 - Meaning of 14 widescreen bits
|3..0||Aspect ratio label, letterbox and position code.|
|4||Camera mode or film mode.|
|7..5||reserved, set to 0.|
|8||subtitles within teletext bit|
|10..9||mode of subtitle|
|13..11||reserved, set to 0|
8 -Aspect ratio for widescreen
|Number Ratio||Aspect ratio||Format||Position|
The values of bits 3 to 0 are rather complex and may depend on the MPEG sequences. The basic idea is that a 16:9 widescreen TV ( as opposed to an older one 4:3) needs to know sometimes some extra information: if it receives for example an image with 16:9 ratio inside a 4:3 broadcast frame (see  or ), you will have black bands of bottom and top, like in figure below What happens is that in television broadcasting, you may for example convey. Therefore it’s useful to know that for the receiver: if the receiver is 4:3, it will display it with black horizontal bands; if it is a 16:9 receiver, it will adapt the image thanks to the widescreen information.
Note that some modern 4:3 televisions are able to decode the widescreen information, too.
In MPEG sequence, we will have to take into account two parameters: the full screen size and the active region, which can be different; so the widescreen will depend on this information.
This is an only the simplest case that we showed here. See , , , and  for complete explanation.
Camera mode/film mode
The film mode should not be very interesting as far digital television is concerned. In this mode (contrary to the default one), the 2 fields of one frame come from the same image; in television, the 2 fields of one frame have been taken at different times, and therefore are independent of each other.
There is a possibility of having subtitles;
you can choose whether to put into active image area or out (i.e. in this
case in the black bottom band). See , .
This part deals with teletext.
So if we look more precisely for one
And for a frame:
The PES teletext data field has this
|NB of bits|
|data_identifier= 0x10, EBU|
|data_unit_id: can be 0x02(EBU teletext), 0x03 (EBU subtitle) and 0xFF (stuffing).||8|
|data_field()||352 (for EBU)|
N is determined by the PES_packet_length (equal to N*184-6).
I do not think that we should consider another value for the data_identifier instead of 0x10.
Each data_field is like that:
(there is one data_field for each line sent to the encoder)
|Description||Number of bits|
|line_offset (0x00, or 0x07 to 0x16).||5|
We do not care about the framing_code, magazine_and_packet_address and data_block; they are just sent to the encoder.
For the data_field with a
data_unit_id of 0x02 (EBU Teletext for non-subtitle data), we have a line_offset
between 0x7 and 0x16. This offset must be indicated to the encoder, which
needs to know the start and end line. It means that we will need to analyse
a little the data. This corresponds to the line during VBI blanking into
which the information is sent (so this is not surprising to find such a
There can be subtitle if there is a data_unit_id of 0x03. I think that we should send it to the encoder. However, the data_unit_id is not transmitted to the encoder, so I wonder how the encoder makes the difference between subtitle and no subtitle. I wonder too if the 64108 makes any difference.
There is also the case of stuffing data. We think that we should get rid of it.
These stuffing data have a data_unit_id of 0xFF. We need to know that the 64108 does with that one.
Data sent to the encoder
Basically, you have in the
MPEG sequence for one line:
|reserved_future_use/ field_parity/ line_offset (0x00, or 0x07 to 0x16).||1||skipped|
|framing_code||1||sent to encoder|
|magazine_and_packet_address||2||sent to encoder|
|data_block||40||sent to encoder|
According to the ,
page 18, for each teletext packet, there is 45 bytes sent to the television,
|clock run in, used for synchronisation, (1010101010101010, and 0xAAAA).||2||added by 64108|
|framing_code||1||comes from MPEG|
|magazine_and_packet_address||2||comes from MPEG|
|data_block||40||comes from MPEG|
LSI Logic documentation
Encoder User Specification, LSI Logic, June 2, 1998
 ITU-R Recommendations, BT-R 1118, Enhanced compatible widescreen television based on conventional television systems.
 ITU-R Recommendations Wide Screen Signalling (WSS) encoding, BT-R 1119.
BT.653-2, on Teletext system-B encoding
 [Philips datasheet for SAA7182A, Digital video encoder, 1996 Sep 11, http://www-semiconductors.philips.com/acrobat/datasheets/SAA7182A_83A_2.pdf
Application Note, AN96055, programming tables for SAA7111, SAA7182/83 and
General explaining documentation
 Video demystified, Second Edition, Keith Jack, HighText, see http://www.video-demystified.com
télévision numérique: MPEG-1, MPEG-2 et système
européen DVB Application, 2ème édition,
Hervé Benoit, Dunod.
European Telecommunication Standard (ETS) documentation
 ETS 300 468, on Service Information in DVB, from European Telecommunication Standard.
 ETS 300 472, Specification for conveying ITU-R System B Teletext in DVB bitstreams, from European Telecommunication Standard, October 1996.
300 706, on Enhanced Teletext Specification, from European Telecommunication
 Digital Terrestrial Television, Requirements for interoperability, DTG, June 1997
- Closed Caption encoding