Right, Chen, however, there is one very important difference between
Ed's operations and most of our operations.
1. He is generally DX so has a huge pileup on his station
2. He doesn't spend much time fighting for his frequency as those that
are calling him continuously during a contest (check his rate if you
doubt this) keep his QRG clearer.
We in the US are fighting to own a frequency, See Andy's chart of 20
Meters in the ARRL RTTY Roundup writeup on the ARRL page. There are
signals wall to wall and that is the general reason that narrow filters
work so well. Ed's points about the cascading and each radio being
different is also important. That is also why I still prefer my FT1000D
over many other radios even tho its 25 years old. To change to 500 Hz
from 250 Hz is just a quick push of a button.
Great points about 2Tone and its filters as well vs. MMTTY or hardware.
In point of fact I switch from 500 to 250 Hz filters as required during
a contest. Its amusing to hear some on this thread talk about filters
who clearly don't ever turn on a radio during a contest. In my opinion
its best to have the ability to use a couple of layers of filters, when
that wasn't possible like for my Icom 706 which I once bought I tested
250, 350, and 500 filters and settled on 350 as a compromise. Of course
its 350 is actually wider as Ed also points out.
To think we once used C-64's as computers, and Flesher Tu-170's or CP1's
as a terminal unit. How far we have come....
Jay WS7I
On 8/23/2013 11:34 PM, Kok Chen wrote:
On Aug 23, 2013, at 10:15 PM, Ed Muns wrote:
Fourth, when I tried opening up the bandwidth on difficult decoding
situations, many times decoding improved. When 2Tone came along I found
that a 500 Hz receiver bandwidth decoded much better than a narrower
bandwidth, even in high QRM and pile-up conditions, most of the time.
Ed hit the nail on the head!
The reason Ed has to widen the receiver filter (i.e., the modem sound card's
roofing filter) is because the filter in 2Tone is already optimal, and
therefore what I showed in Figure 2.2 in
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html applies.
In short, the filter in the receiver is doing nothing but adding intersymbol
interference (ISI) to the already optimized demodulation filter in G3YYD's
2Tone program.
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