Re: WRKO thoughts on the 4th Wknd.

[Garrett Wollman <wollman>]

<<On Thu, 9 Jul 1998 11:23:03 -0400, mwaters@wesleyan.edu (Martin J. Waters) said:

>         The engineering department of the newsgroup may jump in on this,
> but my understanding is that there are two basic problems with audio
> quality on AM. One is the narrower bandwidth for the transmission (compared
> to FM), and the other is the low quality of most of the receivers.

Actually, before relatively recently, the bandwidth of an AM station
was effectively limited by what was happening on adjacent channels.
High-powered stations could and did stretch out to 10 or 15 kHz.
In recent times, the NRSC standards have reduced this significantly.

There are two reasons why receivers became so bad:

1) It's cheaper for manufacturers to use the same 3-kHz bandpass
filter as they put in their telephones, than it is to put in a narrow
notch at 10 kHz.

2) Since most tuner circuits are designed in Asia for the world
market, it is necessary for them to operate equally (poorly) in both
Region II (10-kHz spacing) and Regions I and III (9-kHz spacing).  In
order to simplify that task, all of the RI/III stations were moved up
1 kHz in 1978(?) so that their frequencies would be evenly divisible
by 9; the standard IF frequency was changed from 455 kHz to 450 kHz;
and manufacturers started putting brick-wall filters in to avoid the
need for tuning the notch from 9 to 10 kHz.

It didn't help that the FCC decided to prohibit AM stereo in an
intentional attempt to advantage FM broadcasters.  (XETRA in Tijuana
was broadcasting in ISB stereo in the mid-1960s according to a friend
of mine who lived there.  Other people say that it would have been
next to impossible for a traditional analog-design AM transmitter to
do ISB modulation and still meet the NRSC standards -- apparently
modulating two different signals 180 degrees apart causes lots of
splatter up and down the dial.  C-QUAM doesn't suffer from this
problem because it uses an FM-style L-R channel and only rotates it 90
degrees.  I may have these angles mixed up.)

Of course, since ``nobody listens to AM any more'', there's no reason for
receivers to get any better.

- -GAWollman

- --
Garrett A. Wollman   | O Siem / We are all family / O Siem / We're all the same
wollman@lcs.mit.edu  | O Siem / The fires of freedom 
Opinions not those of| Dance in the burning flame
MIT, LCS, CRS, or NSA|                     - Susan Aglukark and Chad Irschick

------------------------------