Re: TV Glossary

From: Kimmo Loytana (kimmo.loytana@nokia.com)
Date: Wed, Sep 01 1999


Message-ID: <37CCD1F3.4FDEE702@nokia.com>
Date: Wed, 01 Sep 1999 10:12:51 +0300
From: Kimmo Loytana <kimmo.loytana@nokia.com>
To: "EXT Michael A. Dolan" <miked@tbt.com>
CC: WWW TV List <www-tv@w3.org>
Subject: Re: TV Glossary


It is useful to try to agree on a common terminology. However,
unfortunately the same words are already commonly used for
referring to different entities.

I have some comments to your list and would like to propose
slightly modified definitions in an attempt to avoid confusion.
Most of the terminology in my proposals is based on the terminology
used in the MPEG-2 standard.

> 
> Program - a collection of related video, audio and/or data in a
> single "file".  This is also generically called "content".

In MPEG-2 the word Program was a somewhat different meaning:

Program - a program is a collection of program elements. Program 
elements may be elementary streams. Program elements need not have
any defined time base; those that do, have a common time base 
and are intended for syncronized presentation.

This same entity is referred in DVB as a Service and my preference
is to use the word Service for this as this avoids confusion
with the other meanings of the much overloaded word Program.

Another commonly used related term is
Event / Program Event: a collection of program elements with a 
defined duration (and possibly start and end time). E.g. a football
game, an episode of a series, etc.

> Program Stream - as defined in ISO 13818-1 for MPEG - a streaming
> version of a Program.

Well, I have to point out that Program Stream has a very specific
meaning in MPEG-2. Namely, MPEG-2 has two very different multiplexing
schemes: the Program Stream and the Transport Stream. The Program 
Streams are mostly used in some storage applications, e.g. DVDs 
contain Program Streams. Broadcast usage commonly uses the Transport
Streams. 
It is very important to note that when you have a single Program, it 
does _not_ imply that the stream that carries the program would be 
a Program Stream, but in broadcast usage most commonly it is a 
so-called Single Program Transport Stream.

> Transport (Stream) - as defined by ISO 13818-1 for MPEG - a
> multiplexed collection of concatenated Program Streams without
> beginning or end.

As explained in above, a Transport Stream is _not_ a multiplexed
collection of Program Streams, far from that. A transport stream
is a multiplexed collection of Services (in MPEG terminology Programs, 
but I very much prefer the word Service).

> Channel - a Program Stream.  This is also known in DVB as a
> "service".

Again, a Channel is not a Program Stream. I would prefer to use 
the word channel for its original meaning: A physical frequency band
that carries the broadcast transmission, in the case of digital 
broadcast usually a Transport Stream.

ATSC has something called "virtual channel" that is synonyous
with Service.

> Broadcaster - company that emits radio waves (KABC).

With this definition, many companies that used to be broadcasters
(because they emitted their own analogue transmission radio waves)
would stop to be broadcasters when moving to digital as the
actual radio waves may be emitted by a company that multiplexes
many Services into a single Transport Stream that is emitted.

I would prefer a more consistent terminology of a:
- Content Provider  (provides content)
- Service Provider  (assembles content into Services)
- Network/Multiplex Operator  (multiplexes Services for transport and 
  emits the radio waves)

In this scheme, the common meaning of a Broadcaster is usually
equivalent to the Service Provider.

Best Regards,
Kimmo Loytana