This month our speaker will be from Andrews and the topic will be tower
loading. A comparison will be made between the historic standards and the
current standards.
As usual our luncheon will be at Sinbad's. Sinbad's is
just south of the Ferry Building on the Embarcadero near the foot of Mission
Street. Please RSVP to Karen Prasek at Zack's: 408-324-0551 x126 as we've
been running out of tables and chairs. We meet at 11:30 and are seated at
12:30.
The chapter 40 web page is at http://www.lns.com/sbe.
The keeper of the chapter 40 email address list, Warren Reese his address is
We will be having a joint meeting with SMPTE on the evening of July 24th. No
details at this time. Check our web page and use the link to SMPTE for
information.
The news in the latest SBE newsletter is that all the operators and
engineers, who took the Television Operator's certification test, passed. This
was certainly the largest group I've given a cert test to in all the years I've
been doing it. Congratulations!
When I'm stuck on the highway I'm just as likely to listen to Spanish radio
as gringo radio. One station has a logo they promote all the time and I just
don't get it. It sounds like "La Seta" which I translate as "the mushroom". If
that ain't it, what is it?
Digital shares a problem with poets and botanists. It can't describe a rose
in a few words. In fact its ability to turn a burp into a thousand bits is
pretty much its undoing. We've spent years working on throwing as much of the
signal away as we dare just to get it down practical communications pipes. Once
we've run it through the MPEG trash compactor it's pretty much untouchable.
Trying to break into the bit stream is like getting into Fort Knox.
I knew that somehow they had managed to get the 19.39 MHz into the
transmitter via a device called an 8VSB modulator but it was only last week that
I figured out how. It's an analog system.
We haven't heard from a few of you about renewing your membership! It's
almost time when we are required to drop you from the member list. If you can't
locate your membership renewal form, call Teresa Ransdell at the SBE National
Office at (317) 253-1640 or e-mail Teresa at transdell@sbe.org, as soon as possible.
Payments can be by check, VISA, Mastercard or American Express.
July 30th - Fiber Optic Fundamentals will be presented by Rick
Cabalka of ADC Telecommunications.
When you
consider that a HDTV camera can put out a bit stream of 1.2 to 1.6 gigabits and
that somehow we get that down to 19.39 megabits (including audio) to plug it
into the transmitter, it makes you wonder if we just couldnt cut to the chase
and start out with a much more economic source. Frankly it looks like no one
seriously contemplates replacing their baloney with prime rib any time soon.
There are to damn many ones and
zeros to deal with so the first step is to abandon binary. Each possible
combination of two bits is used to generate a four level code. 00 = 0, 01=1,
10=2, 11=3. This has the advantage of cutting the frequency in half as one 4
level symbol is created in half the time required for two bits.The code passes
through a device called a trellis coder whose function is to make the signal as
goof proof as possible. Its output has 8 levels: -7, -5, -3, -1, 1, 3, 5, 7. The
bit stream is split with one symbol taking the inphase path and the other the
quadrature. By this time they have been converted to analogs of the 8 level
signals and are used to modulate a suppressed carrier. The result of their
modulation is a vector whose amplitude and angle define a particular value in a
constellation chart. Once again weve cut the frequency rate in half.
The key
factor in transmission is linearity. If we go for a 5 and overshoot to a 5.3 or
undershoot to a 4.7 weve eaten up some of the margin the consumer needs for his
decoder. It would be like reducing transmitter power.