[opendtv] Re: MPEG Transport Streams
- From: "Gary Hughes" <ghughes@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 22 Jun 2005 09:21:21 -0400
> [mailto:opendtv-bounce@xxxxxxxxxxxxx] On Behalf Of=20
> dgrimes@xxxxxxxxxxxxxxxxx
> Sent: Tuesday, June 21, 2005 7:49 PM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: MPEG Transport Streams
>=20
>=20
> "All transport streams in digital broadcasting are MPEG-2."
>=20
> So what makes a transport stream MPEG-2? Are the block size,=20
> headers, and timing defined in the MPEG-2 standard while=20
> defined differently in the MPEG-4 standard?
>=20
The MPEG transport format was defined as part of the MPEG-2 Systems
standard (ISO 13818-1). It was designed from outset to carry payloads
of various formats and provides mechanisms to carrytime synchronised
streams along with a reference clock for each program (a group of
related
streams with a common time reference). Defined payload types include
MPEG-1 video and audio, MPEG-2 video and audio, MPEG-4 Part 2 video,
MPEG-4 Part 1 system streams, H.264 video. Private data formats are
used for things like cable and satellite system program guides and
proprietary
codecs such as AC3 or DTS. It is based on small fixed length packets to
allow
it to operate in various switched network environments. A single stream
may carry multiple programs and the spec defines how to identify
programs
and their components.
It works. It is very flexible (if you use a cable modem, the downstream
IP
data sent to the modem is carried in transport streams). There was no
need to reinvent it. There is a lot of gear that knows how to handle
MPEG=20
transport without caring about the payload.
MPEG-4 does have a system spec, which deals with combining and
synchronising
the rich variety of formats that comprise the full MPEG-4 system. It
does not supplant MPEG transport (indeed, it can be carried in MPEG
transport).
I prefer to use terminology like "MPEG-2 video in MPEG transport" or
"H.264
in MPEG transport" to avoid arguments over how many transport packets
can
fit on the head of a pin...
gary
Gary Hughes
Director of Media Engineering
Broadbus Technologies, Inc.
ghughes@xxxxxxxxxxxx
v: 978 264 7919
f: 978 264 9108
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