Subject: Fw: VBI group response

From: " Harry Davis" <[email-masked]>
Date: Thu, 23 Sep 1999 11:58:43 -0700

-----Original Message-----
From: Harry Davis <[email-masked]>
To: [email-masked] <[email-masked]>
Cc: [email-masked] <[email-masked]>; [email-masked] <[email-masked]>
Date: Tuesday, September 21, 1999 5:57 PM
Subject: Fw: VBI group response

It was recently brought to my attention that a time of 60 seconds may be proposed as the time between transmissions of a group header address in VBI NABST format files.  If I understand correctly what is being proposed, in my opinion, this wait time is far too long, and the memory required far too large.
 
For example, using 11 available lines in the VBI (NTSC format), one would have to wait for 39,600 lines or 1,306,000 Bytes of information to be transmitted before a new group header and/or a retransmit would occur.  For full frame,(253 lines in NTSC) these number would increase to over 900,000 lines or 30+MB of memory!!
 
Thus we would like to propose that "lines" or "packets" be considered as the measure between headers rather than "time".......and the number of packets should be either 256 or 512.
 
With 256 or 512 packets being the measure, there would be a repeat of the group header address:
(a) every 2.133 or 4.266 seconds for services using 2 lines in the VBI.  (If I were providing a service on 2 lines, I would prefer the shorter time of close to 2 seconds)
(b) every 0.5 or 1 second for services using the full available VBI, and
(c) every 60th or 30th of a second for Full Frame
 
The hold memory requirements at the receiver/decoder would now be limited to 512 packets x 33 Bytes/packet (NABST) = 16,896 Bytes .................for header and data retransmits, for error detection and correction, which seems very reasonable.
 
These figures are ideal for an NTSC signal and would also likely be acceptable for a PAL system, although they could be increased slightly to get the best performance in PAL.
 
I hope this is useful for the discussion, and I would appreciate your comment.
 
Harry k. Davis, P.Eng..
Consulting Engineer

---

From: Ruston Panabaker <[email-masked]>
Date: Thu, 23 Sep 1999 14:20:08 -0700

Harry, 
I believe there is some confusion regarding this restriction.  This rule
applies to the server to ensure that the reuse of group addresses does not
result in corrupt data on the client.  It was made sufficiently long to
ensure that an error condition does not persist.  Careful reading of the new
spec will show that there are no requirements on the client to cache data,
and in fact this restriction on the server lowers the overhead on the client
because it allows the client to ignore data that has not been qualified
after the 60 second limit (if the client implementation so desires).
 
By the way.  It is interesting to note that you can reuse a group address by
sending all IP packets with full headers for the 60 second duration.
 
Ruston

-----Original Message-----
From: Harry Davis [mailto:[email-masked]]
Sent: Thursday, September 23, 1999 11:59 AM
To: [email-masked]
Subject: Fw: VBI group response

-----Original Message-----
From: Harry Davis < [email-masked] <mailto:[email-masked]> >
To: [email-masked] <mailto:[email-masked]>  < [email-masked]
<mailto:[email-masked]> >
Cc: [email-masked] <mailto:[email-masked]>  <
[email-masked] <mailto:[email-masked]> >; [email-masked]
<mailto:[email-masked]>  < [email-masked]
<mailto:[email-masked]> >
Date: Tuesday, September 21, 1999 5:57 PM
Subject: Fw: VBI group response
It was recently brought to my attention that a time of 60 seconds may be
proposed as the time between transmissions of a group header address in VBI
NABST format files.  If I understand correctly what is being proposed, in my
opinion, this wait time is far too long, and the memory required far too
large.
 
For example, using 11 available lines in the VBI (NTSC format), one would
have to wait for 39,600 lines or 1,306,000 Bytes of information to be
transmitted before a new group header and/or a retransmit would occur.  For
full frame,(253 lines in NTSC) these number would increase to over 900,000
lines or 30+MB of memory!!
 
Thus we would like to propose that "lines" or "packets" be considered as the
measure between headers rather than "time".......and the number of packets
should be either 256 or 512.
 
With 256 or 512 packets being the measure, there would be a repeat of the
group header address:
(a) every 2.133 or 4.266 seconds for services using 2 lines in the VBI.  (If
I were providing a service on 2 lines, I would prefer the shorter time of
close to 2 seconds)
(b) every 0.5 or 1 second for services using the full available VBI, and
(c) every 60th or 30th of a second for Full Frame
 
The hold memory requirements at the receiver/decoder would now be limited to
512 packets x 33 Bytes/packet (NABST) = 16,896 Bytes .................for
header and data retransmits, for error detection and correction, which seems
very reasonable.
 
These figures are ideal for an NTSC signal and would also likely be
acceptable for a PAL system, although they could be increased slightly to
get the best performance in PAL.
 
I hope this is useful for the discussion, and I would appreciate your
comment.
 
Harry k. Davis, P.Eng..
Consulting Engineer