[To SA people who don't know him, Mike is the
real expert on the MP4 file format and access to
MPEG-4 data as stored in fixed files. At one point,
he was also in charge within MPEG of integrating
all of the different kinds of software -- audio,
video, SA, etc -- into a single reference package.
Not an enviable task!]
Just to make things as clear as possible:
1. When Mike refers to MP4, he is talking about one
very specific thing. In MPEG language, MP4 is the file
format that lets you hold MPEG-4 data in a nice format
so that you can find segments, stream it out, cut and
paste it together, and so on. None of the current
SA implementations support MP4 yet. The "MP4" format
that is produced by sfront encoder and saenc is a
simpler format that just pastes bitstream data together
in a single file. It's bad to have these names crash,
perhaps we should agree to use something else for SA-
specific files -- maybe '.sa'?
Put another way, you don't need to use the official
MP4 format in a streaming MPEG-4 application, since
you're just sending and receiving chunks of data and don't
need 'files'. But once you want to write all those chunks
of data in the same place, MP4 is recommended to promote
maximum interoperability between applications.
MP4 is based on Quicktime and is very similar to
that format.
2. I don't think any particular concern with the
technical content of MP4 or MPEG-4 systems was being
raised, only the fact that non-MPEG people can't yet
see the specifications for them. To my knowledge,
MPEG-4 Systems and MP4 do support all of the features
SA needs them to. However, I haven't been tracking
the MP4 work closely recently.
I hope this prevents confusion rather than causing
it! These are tricky points since there's a lot of
names that are very similar and different functions
that are enabled here.
Best,
-- Eric
-----Original Message-----
From: Mike Coleman <mc@fivebats.com>
To: saol-dev@media.mit.edu <saol-dev@media.mit.edu>
Date: Tuesday, November 30, 1999 5:55 PM
Subject: Re: sfront encoding-order
>At 9:12 AM -0800 11/30/99, John Lazzaro wrote:
>>SA_access_unit's are in sfront's future, although handling
>>these without the context of knowing how SA_access_unit's are
>>supposed to be integrated into the MPEG 4 file format as a whole
>>is going to be tricky
>
>If you think that there is something missing from mp4 that would make
>this easier please let me know. It is possible that there is
>something unique about structured audio that wasn't considered when
>we worked on mp4.
>
>I have some experience with the mp4 format, so I may be able to
>answer questions and help you understand it better.
>
>-mc
>--
>Mike Coleman <mc@fivebats.com>
>Five Bats Research, Portland Oregon
>Phone +1 503 245-8177 / Fax +1 707 922-1088
>
>
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