Questions on VoiceXML 2.0
this monthly column, an industry expert will answer
common questions about VoiceXML and related technologies.
Readers are encouraged to submit questions about VoiceXML,
including development, voice-user interface design,
and speech technology in general, or how VoiceXML is
being used commercially in the marketplace. If you have
a question about VoiceXML, e-mail it to firstname.lastname@example.org
and be sure to read future issues of VoiceXML Review
for the answer.
While testing my VoiceXML 2.0 application, I noticed
there are sometimes large gaps of dead air when I transition
from page to page. What can I do about that?
You're wise to recognize that gaps of silence are one
of a voice application developer's worst enemies. Since
audio is the only sensory input a user receives when
using a voice application, dead air is to be avoided,
and there are several things you can do.
between VoiceXML documents involve zero or more HTTP
requests between the VoiceXML interpreter and the HTTP
server that hosts your voice application. Regardless
of whether your VoiceXML documents are static or dynamic,
you should investigate how your HTTP server facilitates
By specifying a future expires time on your VoiceXML
documents, you're granting the interpreter the right
to hang onto a copy of the content until that expiration
time elapses. Until that time, the interpreter need
make an HTTP request for the document because expiry
data that specifies a future date and time indicates
that the interpreter has the latest version. VoiceXML
interpreters may further optimize by storing a parsed
version of the document in its cache.
don't need to set the expiration time very far in the
future to see immediate performance gains. Realize,
however, that setting an expiry on a document limits
your ability to make changes to the document and guarantee
that a VoiceXML interpreter will pick up those changes,
so be careful!
the document to which you're transitioning pulls data
from a backend database and cannot be cached, you should
verify that your Web infrastructure is designed to handle
the load. This means making sure your HTTP servers have
an adequate connection to the Internet. You'd also be
wise to provide one or more
additional HTTP servers to handle the load and to handle
the possibility of failure.
applications will make HTTP requests to a lengthy process
such as one that connects to a flight reservation system
to search for available flights based upon several criteria
such as date, time, origin, and destination. No amount
of caching or load balancing is going to alleviate that
latency. In that case,
it's time to take advantage of the fetchaudio attribute.
The fetchaudio attribute allows you to fill in the gap
of silence while a resource is being requested with
an audio clip. According to the VoiceXML 2.0 specification,
this attribute is supported on the following elements:
set the value of the fetchaudio attribute to the URI
of an audio clip to be played by the interpreter while
it is fetching the resource bound to the element. For
example, let's say you use the submit element to make
a request to a CGI that performs a time-consuming process
on the backend. The submit
might look like the following:
namelist="origin_city dest_city start_date return_date"
interpreter submits the four variables in the namelist
to agent.cgi while simultaneously playing the audio
clip associated with the fetchaudio attribute.
that the interpreter does not loop the audio associated
with the fetchaudio attribute, so make sure the clip
is at least as long as the fetchtimeout value. The default
fetchtimeout is platform dependent, but you can
set the attribute explicitly on any of the aforementioned
tags. You'll also want to check with the vendor of the
VoiceXML interpreter you're using to see if they support
streaming media. If not, the interpreter won't start
playing the audio associated with the fetchaudio attribute
until it has completely fetched the audio file. If you
use fetchaudio, you should definitely configure your
Web server to allow the interpreter to cache this audio
to the top
© 2001-2002 VoiceXML Forum. All rights reserved.
The VoiceXML Forum is a program of the
Industry Standards and Technology Organization (IEEE-ISTO).