Re: Lectro D4 system

Hi Billy,
Intelligent comments as usual. You are absolutely correct that the D4
is in a different direction. We wanted a link that didn't use up any
of the current spectrum space. Also when we started the project, no
one was sure what future spectrum space would even be and what the FCC
was going to do for licensing. Once we decided to play around in the
unlicensed 900 MHz band, the design bandwidth was then set by FCC
requirements. The FCC specifies that you can only have a small amount
of power in any frequency interval. Basically the rules are reversed.
Instead of forcing you to put all your carrier in a narrow bandwidth
(200 kHz in Part 74) they require power to be spread over a wide
frequency of 500 kHz or more. The more power you radiate, the wider
the bandwidth required. The bandwidth we finally ended up with is much
wider than necessary, in order to eliminate the Zone of Death I
mentioned before. The wide bandwidth significantly increases the
useful range, does a little for frequency response, and does allow
multiple, high quality audio channels. However, it really makes a
difference for close in multipath which greatly increases its link
reliability. We never saw this system as a substitute for standard
wireless with requirements for many channels but as a very reliable
multi channel audio link of a few (four) audio channels.
Best Regards,
Larry Fisher
Lectrosonics



On Sun, 21 Jun 2009 19:40:57 -0700 (PDT), Billy
<williamsarokin@xxxxxxxxx> wrote:

I've been waiting for Lectro to jump into the fully digital game.
With their huge customer base and reputation for quality design and
engineering I'd been hoping for something that would expand the
playing field and increase competiton for digital radio mics. But on
first look I don't think the D4 is it. It does go in a different
design direction than both the Sony and Zaxcom digitals but it seems
to need 10 times the bandwidth to give only slightly improved
performance (2-4 mHz bandwidth as opposed to 200kHz). The wide
bandwidth limits the system to the 900 mHz band and if I read the spec
sheet correctly would allow only 4 independent systems to operate in
that band. That would be fine in most cases as a digital camera link,
but could get very problematic in a large venue. I don't question
their claims of increased range and a 20-20K flat freq response
(which bests both the Zaxcom and Sony). I'm very interested in their
use of spread spectrum to increase the range (I assume by limiting
narrow band rfi and multi-path rfi). I wonder if that could be
combined with current digital radio technology to allow the best of
both worlds - a narrow bandwidth type accepted digital radio that
could be operated on current UHF freqs with the expanded range the SS
seems to allow. (Larry, I know this is a contradiction, Spread
Spectrum is by definition wide band but I'm wondering if there is a
way to start with a narrower digital signal so it could be type
accepted for the UHF bands).
Anyway, now I'm starting to confuse myself. btw, here's a bit of a
spread spectrum primer..
http://sss-mag.com/ss.html
All the best,
Billy Sarokin
.

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