[TenTec] Question regarding Omni VI+ on SSB

[Jim Brown]

On Thu, 25 Nov 2004 22:01:27 -0800, Adam Farson wrote:

>The ITU-R AF response mask for SSB transmitters in the maritime radio
>service is 350 to 2700 Hz at the -6 dB points (Recommendation M.1173). 
That
>is a good starting point for SSB radiotelephone transmitter setup.

VERY Interesting, Adam. Many thanks for posting this. This is the missing 
piece of the puzzle as to why our radios are built this way!  But it is a really 
poor recommendation, because it requires the use of special microphones. 

It is, indeed, good to limit the bandwidth of SSB transmitters, but the -6 dB 
point ought to be higher.  With a standard like this, you MUST use a mic that 
has a big response peak at 2.7 kHz if you want decent speech intelligibility, 
and if you want to sound good!  And nearly all mics sold as communications 
mics have such a peak. For example, look at a data sheet for the Shure 444. 

I sit on an international standards committee (for audio), so I can understand 
how this happens -- manufacturers control most of these committees, and the 
practices of the majority, good or bad, get cast in stone. 

We have an similar "de facto" standard in the pro audio world. Thirty years 
ago, the big HF drivers used for sound reinforcement at concerts had a big 
rolloff beginning at about 6 kHz. It wasn't intentional -- it was the best that they 
could do then and still get high output to fill a stadium. Shure built a mic, the 
SM-58, with a broad  peak at 6 kHz that compensated for the rolloff, and it 
became quite successful.  30 years later, many systems are still tuned for 
mics with this kind of peak (called a "presence" peak), and far more accurate 
mics that don't have it are heard as dull sounding.  

But the peak causes another big problem called sibilence, which is the 
unpleasant emphasis of "s" sounds and sounds in that part of the spectrum. 
So for 20 years, system users must use products called "de-essers" that 
remove this sibilence. 


Jim Brown  K9YC