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.
If you had tried the same experiment on the version of fldigi that was released
in the last 6 or 9 months, you will find the same thing to also be true. It is
well known that wider receive filters are better for RITTY and cocoaModem.
This applies doubly to the so called twin peak filters (what was called a Comb
filter by Frank Gaude back in the RTTY Bulletins back in the 1960s).
Use a "twin peak" filter in front of 2Tone, and you will find that the ISI from
then will cause even more harm to 2Tone than narrow roofing filters.
The reason narrower filters (and twin peak filters) work better with MMTTY is
because the filter inside MMTTY is not optimal for 45.45 baud RTTY. Narrowing
the receiver's crystal filter probably improved the SNR for MMTTY, and that
took precedence over the increased ISI
On Ed's second point regarding QRM and wide filters with 2Tone: the reason the
widened filter will not cause any harm at all to 2Tone in the presence of QRM
is that its already the narrowest filter you can build for RTTY without causing
ISI. This holds true until the point where a very strong QRM causes the
sound card to clip.
If you have a sound card that has better dynamic range than the receiver, then
you never have to worry about clipping the sound card before the receiver
itself folds. If you don't have a good sound card, then you would need to ride
the RF gain control of the receiver so the sound card stops clipping, and
everything will again work fine as long as the sound card's dynamic range is no
smaller than the difference between the QRM and the weaker signal you are
trying to copy.
If the QRM or its keyclicks is overlapping and louder than the weak signal,
then all bets are off. The only thing that will save that situation is
possibly diversity RTTY, but that is a different topic altogether.
-----
On a slight tangent, while we are on the subject of optimal RTTY filters, the
optimal Raised Cosine filter is all good when there is no multipath. Andy K0SM
and I were just discussing this aspect offline earlier.
If you look at the impulse response of the Raised Cosine (especially when beta
is less than 1), it rings like crazy. Yet the ringing does not cause it to
wipe out the next bit. The reason is that while Nyquist filters ring, they
always pass precisely through zero at the middle of the bits that follows it,
and that is where the next bits are sampled -- and thus no ISI.
However, there are also two cases that cause these "perfect" filters to break
down -- the first is if you use some bit period that is not precisely 22ms.
This can be the result of not using exactly 45.4545 baud, or when you use
hardware bit-banging to perform FSK, where the bit periods will jitter from bit
to bit.
The second case where the Raised Cosine will no longer be optimal for RTTY
decoding is when there is multipath (I am told that area of 2Tone still needs
improvement). Multipath will cause the Mark and Space bits to advance or
retard relative to one another. ITU HF Simulation models have the bits
advancing and retarding by up to an average of 7 ms (with a Gaussian
probability density function) during severe high latitude flutter and during
poor NVIS conditions.
So how much harm does multipath and wrong baud rates contribute to ISI? Turns
out to be quite a bit.
If you go look at Figure 8 here
http://w7ay.net/site/Technical/Extended%20Nyquist%20Filters/index.html , you
will find a red curve for the impulse response of a raised cosine filter, there
is ISI when the red curve does not pass exactly through zero at the middle of
the "next" bit(s).
Notice that the red curve rises steeply when the distance between the two
successive bits are shortened -- so much so that if the multipath is 11 ms
long, the ISI is only 6 dB below the signal itself -- i.e., you have lost 6 dB
of SNR due to ISI -- equivalent to reducing your transmit power by four.
The moral of the story are
(1) the Raised Cosine is optimal only during good conditions, just don't bet on
it being very good when there is multipath, a good modem will need other
mechanisms to counter multipath -- perhaps a bank of different voting filters
or an adaptive equalizer that changes the relative delays between mark and
space demodulators (cocoaModem uses multiple delays that goes through a voting
mechanism, for example).
(2) if you cannot get precisely 45.45 baud (e.g., the MicroHam microkeyers),
choose a baud rate that is *slower* than 45.45 baud. I.e, if you can choose
between 45.0 or 46.0 baud, choose 45.0, since it will fall on the "shallower"
side of the Raised Cosine's ISI curve. Do not choose the higher baud rate to
shave a few milliseconds from a contest exchange. You can send your exchange
faster with faster baud rate, but you will also be asked for more repeats since
the other end cannot copy you as well. To optimized your transmit signal for
the good modems at the other end, always use 45.45 baud precisely and never,
ever use bit banging techniques.
Finally, a hint for those interested in copying through polar flutter... of all
Nyquist filters, a Matched Filter is the one that is the most immune to
multipath ISI since its impulse response is a rectangle. K6STI was no fool
when he chose a Matched Filter for RITTY.
73
Chen, W7AY
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