[BC] Mono Expands on the FM Band

[Hershberger, Dave]

Rob Meuser writes:

> 
> Dave
> 
> You should discuss the vsb ssb FM stereo issue with Bob Orban 
> as I know he has 
> run the numbers and they are not really favorable.
> 
> R

Hi Rob,

I'm sorry, but I may not have been clear about what I was talking about. I
was suggesting that a very good but non-compatible analog FM stereo system
would place L-R on an USB subcarrier at 19 kHz. That way the L-R is farther
down on the triangular noise spectrum of FM, and the de-emphasis is going
the "right" direction.

I have run the numbers too and here they are:

Stereo penalty = 23.01 dB

This is the noise penalty for GE/Zenith FM stereo - the well known 23 dB
penalty.

What if only the LSB is demodulated for L-R? This makes sense because the
USB has a lot more noise in it than the LSB, even though the de-emphasis for
the LSB goes in the "wrong direction" because the spectrum is inverted.

Receive LSB ONLY Stereo penalty = 22.25  dB
DB improvement over conventional demodulation by using only LSB = .76  dB

These are the numbers for making a receiver that only uses the LSB at 53
kHz, and rejects the USB. There is less than a dB to be gained by doing so
in terms of white noise, although multipath may gain more. Not a big win.

Transmit LSB ONLY Stereo penalty with LSB only receiver = 16.33  dB
DB improvement over GE/Zenith with LSB signal and LSB receiver= 6.69  dB

These are the numbers for Bill Gillman's proposal of a few years ago - to
transmit LSB only at 38 kHz. This produces a compatible signal that works
with existing receivers. It uses up additional deviation by modulating the Q
channel at 38 kHz, increasing L-R energy by 3 dB, but that deviation is
somewhat "interleaved" with the I channel. For single tone modulation there
is no peak deviation penalty. There is no SNR advantage to existing
receivers but special LSB only receivers have SNR improved by almost 7 dB.
Now we're getting a noticeable improvement.

And here's what I mentioned - USB at 19 kHz, impractical only because it
wouldn't be compatible:

USB ONLY Stereo penalty with USB receiver = 12.49  dB
DB improvement with USB receiver= 10.52  dB

Over 10 dB improvement in SNR! Now we're talking. Too bad it isn't
compatible.

Bob would probably point out the difficulties of audio processing for SSB
(L-R) signals - Hilbert transforms are involved.

Dave Hershberger 
Principal Engineer 
AXCERA
Tel: 530-272-HDTV (4388)
Fax: 530-272-4505 
dhershberger at axcera.com <mailto:dhershberger at axcera.com> 
AXCERA. Axcessing the new era of digital communications. www.axcera.com 
960 McCourtney Road, Suite C, Grass Valley, CA 95949-7423