Another delay story?

Scott Fybush
Tue Oct 10 10:21:14 EDT 2006

Peter Q. George wrote:
> More than likely, Comcast is receiving the HD signal
> of WHDH now (and all of Boston's TV stations).  If
> you're using a (hybrid) digital box, tack on a few
> more seconds of delay as well. I've noticed the delay
> factor between the over-the-air signal of Boston's TV
> stations and what Comcast provides, myself.  

It's actually even more complicated than that.

My Time Warner system here in Rochester is doing the same thing Comcast
is doing in Boston, and I'm delighted about it.

The issue is "ingress" - that nasty problem wherein the over-the-air
analog signal on channel 2, 4, 5 or 7 will be strong enough within a TV
set to interfere with the cable signal on the same channel. Because the
cable signal is delayed slightly against the OTA signal (purely by
virtue of all the cable it's traveling through to get to you), you end
up with a big ugly bar across the picture. Those of us who live near TV
transmitters (I'm 4300 feet from our TV site in Rochester) know this
problem all too well.

I know that at least some of the Comcast systems very close to the
Boston TV stations - Needham, Newton, etc. - "remap" the local channels,
putting 4 on 23, 5 on 24, etc. That wastes channel capacity, though,
since all you can then use 4, 5 and 7 for are "low-value" signals like
community bulletin boards and such. (I believe WYDN 48 is on cable
channel 5 in Newton, where it's literally unwatchable.)

So what Comcast is doing is taking the standard-definition signals from
WBZ, WCVB, and WHDH and retransmitting them over its compressed digital
cable system. The upside - and it's a huge upside - is clean signals
with no ingress problems.

The downside is that there's inevitable delay involved at several stages
of the process. The feeds from the stations to the Comcast headend are
probably coming over digital fiber, and the compression and
uncompression involved in that transmission adds at least a bit of
delay. And then the compression involved in creating the digital cable
signal creates most of that 7-second delay that Joe noted. (For what
it's worth, the delay on the Time Warner system here is somewhat
smaller, in the 2-second range.)

Delays like this have nothing to do with Janet Jackson - they're just
part of the technology that allows cable to deliver 300 channels of
video, all kinds of on-demand stuff, plus internet and phone service all
over the same pipeline. Peter is quite correct that ALL analog cable
service will vanish at some point in the future, though probably well
after the 2009 deadline for analog OTA service to go away.

(Peter is also quite correct, by the way, that some cable systems now
take in a DTV signal even to feed their analog subscribers. This is more
common in places like the Berkshires, where the systems still depend on
receiving their semi-distant signals over the air rather than via direct
feeds from the stations. A good example is the legacy carriage of
Springfield's WWLP 22 in the Great Barrington area. The arrival of new
co-channel DTV signals on 22 has made analog reception of WWLP iffy in
the Berkshires, so the systems there now receive WWLP-DT 11 instead.
Some stations are even providing a standard-definition, 4:3 program
stream over one of their DTV subchannels specifically to feed cable
systems, so there's no worry about the cable systems screwing up the
downconversion from 16:9 HD material to feed viewers still watching
analog cable.)


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