The VoiceXML Forum MRCP Liaison Committee is currently evaluating the 
latest MRCP v2 draft to (a) evaluate the compatibility between MRCP v2 
and VoiceXML and (b) generate test assertions for MRCP v2 based VoiceXML 
browsers and MRCP v2 based media resources. We are currently examining 
the Speech Synthesis portion of the specification and have raised issues 
with the specification in prior emails to the SpeechSC list (See 
http://www.ietf.org/mail-archive/web/speechsc/current/msg01605.html
http://www.ietf.org/mail-archive/web/speechsc/current/msg01606.html
and http://www.ietf.org/mail-archive/web/speechsc/current/msg01607.html ).

These issues (and the responses to them) have been discussed by the MRCP 
Liaison Committee and we would like to make the following requests and 
suggestions:

1) The relationship between the Fetch Hint header and the Audio Fetch 
Hint header should be clarified. More specifically, it should be stated 
that, when specified, the Audio Fetch Hint header overrides the Fetch 
Hint header for audio files only.

2) It should be clarified that  SPEAK completion code 003 "uri-failure" 
only applies to  fetched SSML files and that  failure to fetch (or 
process) an audio file will not result in aborting the SPEAK request. 
This does mean, however, that there is no way to communicate the failure 
to fetch (or process) the audio file to the MRCP client. While SSML 
requires that the processor "notify the hosting environment" when such a 
failure occurs, the members of the committee agree that logging this 
event at the MRCP server is sufficient. It may be advisable for the MRCP 
specification to suggest that these events should be logged in some way. 
We would also like to suggest that future versions of MRCP consider 
adding an event (e.g. "Audio-Exception") to notify the MRCP client that 
such a failure has occurred without aborting the SPEAK request.

3) The definition of the Basic Synthesizer resources is a bit vague and 
should be clarified. Its not entirely clear from the description in the 
spec how it is supposed to work. The general consensus in the Committee 
is that this resource can be used for audio only prompts. It is supposed 
to accept a subset of SSML that only includes <speak><audio><say-as> and 
<mark>. What isn't clear, is how <say-as> is supposed to work in this 
case and if text strings are acceptable (you would think no if it wasn't 
for <say-as> being allowed). It may also be reasonable to make <mark> 
optional; a VoiceXML 2.0 browser certainly wouldn't need it anyway. We 
find that clarifications are needed in order to make any assertions on 
how a VoiceXML browser would use a basicsynth resource in an implementation.

A final issue worth noting is that the maxage and maxstale cache control 
headers are global in MRCP while VoiceXML breaks this down by resource 
type (e.g. audiomaxage, audiomaxstale, grammarmaxage, grammarmaxstale, 
etc.). This may be acceptable because the context of each request should 
govern the type of file to which these headers apply. i.e. in a SPEAK 
request the control audio file fetches and in RECOGNIZE requests they 
control grammar file fetches. As we continue to evaluate the spec we 
will keep our eyes open for scenarios where this does not hold. Thus, we 
are not requesting any changes related to this issue at this time.

Related to the above issue is the fact that the <audio> tag in VoiceXML 
extends the attributes defined in SSML by adding maxstale, maxage, 
fetchtimeout, and fetchhint (it also adds expr but that "evaluates away" 
to src). These fetch-related headers override their associated 
properties. Unfortunately, since MRCP is based on SSML, these attributes 
cannot be included in an MRCP request; instead, the associated headers 
would need to be set to control this behavior. This obviously introduces 
a problem if a request contains two <audio> tags that had these 
attributes set differently in the original VoiceXML document.

It would seem that one way to address this problem is to break apart an 
SSML prompt so that each audio file is sent in its own request. 
Unfortunately, Issue (2) from above prevents this solution from working. 
Consider a prompt with alternate audio files such as: <audio 
maxstale="A" src="A.wav"><audio maxstale="B" src="B.wav"/></audio> where 
maxstale values A and B are not the same. These files can't be sent as 
part of the same request due to their maxstale values. However, if they 
are sent as part of separate requests, the client would need to know if 
A.wav could not be fetched in order to decide if it should request for 
B.wav to be played. But as discussed above, there is no way for the 
client to know this. The MRCP Liaison Committee believes that the best 
way for this to be addressed is to make a request to the W3C Voice 
Browser Working Group to add these attributes to the audio tag in SSML. 
Again, we are not requesting any changes to MRCP related to this issue.

Regards,

Andrew Wahbe
VoiceXML Forum MRCP Liaison Committee

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