Charles Eliot (charle@Exchange.Microsoft.com)
Mon, 11 Dec 2000 13:18:20 -0800
This is only true if the original message is forwarded as an embedded
message. VPIM also allows for the possibility of _concatanating_ audio
bodyparts, in which case you can get multiple generations of
coding/decoding.
-----Original Message-----
From: David McClintock [mailto:David.McClintock@connsys.com]
Sent: Monday, December 11, 2000 12:56 PM
To: 'Brian Cruickshank'; Glenn Parsons
Cc: 'IETF VPIM List'
Subject: RE: [VPIM] RE: 3GPP-T-WG3 codecs - A message is a message is a
message...
> Regarding your comment "There is also the matter of back-to-back VPIM
encodings. If MS-GSM is used, a signal that is forwarded via VPIM twice
will be encoded using MS-GSM twice, which will introduce more signal
quality degradation than if it was encoded using G.711 twice."
How is it that such a message would be encoded "twice" due to
forwarding? Typically when a message is forwarded, the MIME attachment
remains in-tact -- no "re-encoding" is required. This should be true
regarless of whether the forwarding entity is an email client or a voice
system. This is one of the key advantages of the single store paradigm
for messaging. A message is a message is a message...
Dave McClintock
Connected Systems
-----Original Message-----
From: Brian Cruickshank [mailto:cruicb@nortelnetworks.com]
Sent: Monday, December 11, 2000 12:29 PM
To: Glenn Parsons
Cc: 'IETF VPIM List'
Subject: RE: [VPIM] RE: 3GPP-T-WG3 codecs
Glenn, I understand that MS-GSM is currently viewed as the codec of
choice for VPIM but has this decision been finalized yet? I think a
good case can be made for adopting the 'audio / basic' G.711 codec as a
baseline VPIM codec. As well as being a MIME standard, G.711 is the
PSTN standard and preferred codec for Voice over IP on higher bandwidth
connections, is simple to implement (low complexity, low MIPs), and
offers toll-quality sound that is suitable for voice signals (single or
multiple talker) and non voice signals.
Another important consideration is voice quality degradation in an
evironment where multiple codecs are used in tandem. Because G.711 is a
waveform coder instead of a parametric "vocal tract" coder, it
introduces fewer transcoding artifacts than MS-GSM. This helps to keep
the intelligibility of voice messages high in situations where the
signal has already been encoded with one or more different encoding
technologies. You can easily come up with scenarios where the voice
message will go through several different transcodings on its way from
the caller to the called party and will be truly mangled in the process
(e.g. Cell Phone + Frame Relay + Voice Messaging System A + VPIM + Voice
Messaging System B + Satellite link + PSTN + Voice over IP gateway)
The fewer the number of different encodings, the better the voice
quality will be.
There is also the matter of back-to-back VPIM encodings. If MS-GSM is
used, a signal that is forwarded via VPIM twice will be encoded using
MS-GSM twice, which will introduce more signal quality degradation than
if it was encoded using G.711 twice.
The only drawback to G.711 that I can see is that it is a 64kbps codec
and is thus slower than real-time over a modem connection. This
bandwidth constraint is disappearing, however - the FCC says that there
are over 4 million high speed lines linking households and small
businesses to the Internet in the US, with growth at around 60% in the
last 6 months. DSL grew by 157% in the same time period. To address
short-term bandwidth concerns, perhaps a VPIM codec strategy that had
G.711 used as a baseline with a secondary codec used to support low
bandwidth connections would be appropriate. At least then the user
could decide whether they were more concerned about voice quality or
minimizing bandwidth utilization.
Regards,
Brian Cruickshank,
Nortel Networks
-----Original Message-----
From: Parsons, Glenn [NORSE:9T01:EXCH]
Sent: Thursday, December 07, 2000 9:19 PM
To: discuss@apps.ietf.org; www-smil@w3.org; 'Philipp Hoschka'
Cc: 'IETF VPIM List'
Subject: [VPIM] RE: 3GPP-T-WG3 codecs
Philipp, I'd be interested in the rational that made you pick
audio/basic
FWIW, there is a set of "recommended" codecs in the SMIL 2.0
draft of W3C, and I'm happy to explain why we chose those, if
needed:
http://www.w3.org/TR/2000/WD-smil20-20000921/smil20-profile.html#Baselin
eFormatsNS
> Widely Supported MIME Types
>
> This section is informative.
>
> The members of the W3C SYMM Working Group believe that the
following
> MIME types will be widely supported by SMIL players:
> * audio/basic [592][MIME-2]
> * image/png ([593][PNG-MIME], [594][PNG-REC])
> * image/jpeg ([595][MIME-2], [596][JFIF])
> Implementers of SMIL players should thus strive to provide support
for
> each of these types. Note, however, that this section is
> non-normative, and that support for these MIME types is not a
> precondition for conformance to this specification.
>
> Authors are encouraged to encode media objects using one of the
widely
> supported MIME types whenever possible. This will ensure that their
> SMIL documents can be played back by a wide range of SMIL players.
>
> If authors use a MIME type that is not in the list of widely
supported
> types, they should provide an alternative version encoded using a
> baseline format. This can be achieved by using a switch element as
> shown in the following example:
> <switch>
> <audio src="non-baseline-format-object" />
> <audio src="baseline-format-object" />
> </switch>
>
> In this example, a player that supports the non-baseline format
will
> play the first audio media object, and a player that does not
support
> the non-baseline format will play the second media object.
In general, I'm a bit confused about the request - why would the
IETF have to comment on the minimal set of codecs in a format
defined by another organisation ? This would make sense if the
goal is to define a minimal set of codecs that need to be supported
by MIME mail readers, but otherwise, I don't see the point - am
I missing something ?
I don't think the IETF _has_ to comment, we've just been asked..
This is more about the codecs available on various devices. Few if any
mail clients have audio codecs included.
Cheers,
Glenn.
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