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Old 11th May 2004, 20:21   #1  |  Link
Doom9
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ISDB Basics (Japanese digital (HD)TV)

ISDB Basics

ISDB is the 3rd major standard for digital television. It is used in Japan and I wasn't able to find an as comprehensive documentation on it in English than for the other two digital TV standards. Nevertheless, I'll try to present what I could find. Similar to DVB, ISDB is comprised of 3 sub-standards: ISDB-S for digital TV via satellite, ISDB-C for cable and ISDB-T for terrestrial broadcasts. ISDB-T is still very young and has only been introduced commercially in 2003.

Similar to DVB and ATSC, ISDB uses MPEG-2 video and an MPEG-2 transport stream (TS for transmission). However, the audio is MPEG-2 AAC and marks the first widespread use of the AAC audio encoding standard. When transmitted over satellite, ISDB-S uses 8PSK for signal modulation and allows a bitrate of up to 52 Mbit/s when being broadcast by a standard 36 MHz transponder.

A TS packet is 188 bytes, to which an additional 16 bytes of redundant data is added (using a reed-solomon code). What seems to be unique in ISDB is that multiple TS can be combined on one transponder: 48 error coded 204 byte signal (known as a slot - each slot can have a different modulation scheme and contain a packet from a different TS) are grouped together to create a frame - the basic unit of channel transmission. Furthermore, 8 frames are grouped together as a superframe and serve as unit for interleaving and energy dispersal. The number of slots for a TS and the modulation scheme used is stores as a TMCC (no idea what that means) signal in every superframe. A receiver uses this signal to demodulate and decode the proper TS. In total, up to 8 TS can be transmitted on a single transmission channel.

ISDB-C uses QAM64 as modulation scheme, and there are two flavors, one that can handle only one TS per channel, and another one supporting multiple TS per transmission channel. On a 6 MHz channel, ISDB-C can transmit up to 29.162 MBit/s. As the bitrate on a satellite channel is twice as high, at least two cable channels must be used to transmit the information from a satellite channel. As in ISDB-S, TS packets are grouped together to a frame. 53 slots (packets) make up one frame. The header of each slot contains information about multiplexing and demultiplexing, allowing a receiver to extract the desired TS.

ISDB-T has a bandwidth of 5.6 MHz (though the standard also covers bandwidth of 6, 7 and 8 MHz) and divides the bandwidth into 13 OFDM segments (consequently OFDM is the modulation scheme used). The OFDM segments can be divided in up to three segment groups having different transmission parameters. Using segmentation, a hierarchical transmission is possible, enabling narrowband receivers to get a useful signal from receiving just a single layer. In other words, the same program can be broadcast in different resolutions, and a mobile receiver for instance would show you a standard resolution picture, whereas a more powerful stationary receiver would give you an HDTV picture.

Supported resolutions and framerates:

1920x1080 interlaced (and progressive, though no devices seem to be available for 1080p as of now) in 16:9
1280x730 progressive in 16:9 (aka 720p)
720x480 in both progressive and interlaced in 16:9 and 4:3 mode (4:3 only for interlaced content).

I did not find any notes on allowed framerates, but knowing that Japan uses NTSC I'd presume the same framerates as in ATSC are supported. I did also not find any bitrate limitations for either audio or video.

Additional notes: ISDB-S went live in December 2000, and ISDB-T in late 2003. Prior to this, Japan already had a digital TV standard, called CS and based on DVB. It supports only one TS per transport channel, and offers up to 42 MBit/s on a 27 MHz channel. The modulation scheme was QPSK, just as in DVB-S.

Sources:
http://www.nhk.or.jp/strl/publica/bt/en/pa0007.html
http://www.nhk.or.jp/strl/publica/bt...-set-le18.html
http://www.nhk.or.jp/strl/open99/de-2/shosai-e.html
http://www.dibeg.org/techp/Documents/Isdb-t_spec.PDF
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