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Date: 18 December 1998
From: tholden@TORONTO.CBC.CA
Subject: Re: DAB Receivers compatibility vs MOT

Francois Gauthier asked:

>Why does none of the actual receiver displays PAD and DLS in the same
>way?

I agree with his observations, from some limited work I have done with
automated PAD programming. DLS displays may be 8, 16 or 32 characters
wide, 1 or multiline, have a buffer of some unknown but non-uniform
size, and have various practical character throughput rates. The
narrower displays scroll through the buffer content at a rate
uncontrolled by the sender while multiline displays can show a page of
perhaps 256 or more characters at a rate limited by the transmission
rate, i.e. much faster.

PAD graphics and HTML content supported by the MOT spec are displayed
on graphics screens (which may double as the multiline DLS) of
different sizes - 320 x 200 pixels for a Bosch Balupunkt receiver we
have seen to perhaps a size limited only by the graphics adapter of the
computer to which a DAB receiver may be attached. Moreover, because the
HTML specs do not really control page format, DAB receivers are likely
to have dissimilarities akin to those of the various HTML browsers we
use on the Internet. Internet Explorer does not display an HTML page
identically to Netscape Navigator which is different from Spyglass;
there are variances among versions of even the same browser family.
Moreover, the HTML specifications are periodically revised and
Microsoft and Netscape add unique extensions supported by their
respective publishing tools. These extensions MAY be supported by the
other's browser at some later stage.

Settling on the MOST COMMON DENOMINATOR of DAB receiver capabilities
will be a problem for DAB datacasters. It is futile to program to the
highest standard if the receivers that support it are rare while aiming
for the lowest may not be very impressive. I do not think PAD/MOT
currently supports receiver type specific addressing of content.

Moreover, it is desirable to multi-purpose the publishing effort so
content can be used both for the Web and for DAB. The disparities
between these are even greater than perhaps among receivers alone. For
example, in the automated PAD content trial, I found that the HTML
<TABLE> element, which is widely used to organise HTML pages, was not
supported by the Bosch/Blaupunkt receiver we had. Hence, a page that
displayed beautifully on a WWW browser was scrambled on the receiver
display. A DAB receiver compatible version of the page was designed and
now that version is sent to the DAB transmitter while the nicer one is
sent to the Web server.

>Right now, none of the PAD inserter softwares in combination with any
>of the DAB receivers that I was able to see, were giving may any
>confidence that second-based synchronization will be possible in a
>near future. I think that this issue will become crucial when the
>firsts full-DAB services are going to be broadcasted. If any one of
>you have some experience with synchronization of PAD (successful or
>not), it will be great to share it with everybody.

Nothing really to add to this in the way of experience except that the
concept of Triggering the display of Pre-Loaded content is covered in
the MOT spec. Certainly, 'live' transmission of content will be subject
to delays varying according to file size, transmission channel rate and
reliability and is therefore inconstant and varying for different
receiver locations and data detection and error correction performance.
Viewing the WWW through a 28 kbps modem is likely a very good
indication of the best response time that one can expect for content
delivery via DAB PAD.

Tom Holden
Manager, Systems Engineering (Radio)        Phone: +1 416-205-3081
Broadcast Engineering                       FAX: +1 416-205-8500
Canadian Broadcasting Corporation Internet: tholden@toronto.cbc.ca

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