On Oct 4, 2008, at 10/4 4:47 AM, Jay wrote:
> Its the usage of 3 or 2.4 Khz SSB filters that cause nearly 100% of
> the
> problems between RTTY operations and PSK31 operations.
Hi Jay,
My humble 2 cents (my understanding at this point in time):
This is not a universal problem. Some rigs have 90 dB to 100 dB of
dynamic range all the way to where you tap a sound card into. If
your sound card also has that kind of dynamic range, you will have no
trouble running 2.4 kHz passbands.
Good 16 bit sound cards have 96 dB of dynamic range and the best 24
bit codec that you can buy practically (the particular Asahi one that
is used in the FlexRadio 5000, for example) has 123 dB of dynamic range.
If your modem software had enough dynamic range (any software doing
all of its arithmetic in floating point will easily outperform the
dynamic range of the rig), then your software filters will also
handle the dynamic range -- i.e., if you don't clip your sound card,
and the sound card noise floor matches the noise floor of your rig,
the filters in your software should be able to perform the narrow
filtering for you.
Don't be so quick to condemn people like me who use wide bandwidths.
If you set up noise floors nicely and use sound cards with adequate
dynamic range, the limitation is at the receiver. You don't hear me
complaining about it and I have been using 2.4 kHz passbands for many
years now, and with the K3 plus LP-PAN combo, I plan to use a 100 kHz
passband once I get some software written for my non-Windows computer.
The FlexRadio, like many SDRs, taps the "sound card" right off the
mixer without any high gain I.F. stages that is often the source of
dynamic range woes of superhet receivers (even crystal roofing
filters can reduce the dynamic range of a superhet) You can think
of these SDRs as direct conversion rigs, and you achieve "single
signal" by phasing the unwanted sideband of a direct conversion rig
in software by taking the signal from a quadrature mixer.
Absence of other higher order effect, the blocking dynamic range of a
receiving system determines the largest and smallest signal that can
be copied simultaneously. But there is also the IM dynamic range of
a receiver which determines how much two strong signals can combine
to create a phantom signal that wipes out a third weak signal.
While the IM dynamic range of the best SDRs and the best supehets are
pretty much equal, the problem with SDRs (and any one else using
sound cards with digital modes, for that matter) is that, in addition
to the IM dynamic range of a sound card, the blocking dynamic range
of the receiving system is directly tied to the dynamic range of the
sound card. Some superhet rigs have blocking dynamic range of 140
dBm or better, while the blocking dynamic range of the Flex 5000A for
example, is limited by the 123 dB dynamic range of its sound card
(although this can be improved if you are willing to use a narrower
passband than 192 kHz).
The former (BDR) is probably important if you have a neighboring
ham. The latter (IMDR) is important in a contest or pileup situation
where there are lots of S9+40 dB signals in the passband, but no one
at S9+80 dB.
Until better codecs become available (and inexpensive enough), good
quadrature mixer based SDRs will have a limited BDR (in the range of
115 to 125 dB), with a very competitive IM dynamic range running in
the upper 90s to mid 100s dB.
One of the nice things about SDRs is that the IM dynamic range is a
constant whether you use a 250 Hz passband or a 192 kHz passband.
The K3 has one of the best BDR among amateur superhets today. The K3
has a BDR upwards of 140 dB and a IM dynamic range just at about 100 dB.
However, by applying decimation (digital filtering and then
downsampling), you can achieve better blocking dynamic range from an
SDR if you ware willing to use a narrower passband.
The decimation technique is in fact used by the so-called "direct
SDRs" to get enough BDR. The codecs in the "direct SDR" work at 70
MHz or higher and they are sampling the signals directly without an
analog mixer. However, they typically start with only 80 dB of
dynamic range. But by the time they decimate the RF down to a 2.4
KHz passband, the BDR will improve to better than 100 dB. Increasing
BDR this way does not usually improve IMDR, though.
During the next RTTY contest, take a note of the loudest signal which
you can find. Also, take a note of the weakest signal that you can
copy with a narrow I.F. filter. It wouldn't surprise me if most
people don't see a range of signals with more than 100 dB (the
difference between S1 to S9+52 dB). If so, you should be able to use
a 100 kHz waterfall in a good SDR.
> Since PSK31 only
> requires a very narrow bandwidth as does RTTY then it makes PSK31
> ops angry
> when the RTTY operator (contesting and using 250 Hz fitlers doesn't
> hear
> nor is within say 500 Hz of the PSK31 Operator that all of hate and
> disconent messages get sent and answered.
The AGC of the rig and the dynamic range of the waterfall display are
usually the culprits here. A large signal often makes the waterfall
go "clean" and you can no longer see a weak signal (i.e., although
the signal is available to be decoded, you can see it visually in the
waterfall when a strong signal comes up. However, you will notice
that unless the large signal has clipped the sound card, good
demodulators will continue to copy a signal which you are already
tuned to. cocoaModem has a waterfall dynamic range selector for such
occasions. Using false colors in a waterfall can also extend the
visual range, but they tend to produce a "busy" display -- hence I
ended up allowing the op to change the dynamic range of the waterfall.
A wideband system does work (otherwise none of the Flex-5000 ops will
ever be able to get on the air in a contest) Neal K3NC and Bill W9OL
are two RTTY contesters I know who use Flex rigs; you can ask them if
large signals bother them in a contest. My guess is that they can
copy S1 signals in the midst of a few S9+40 to S9+50 signals.
> What SNR is needed with PSK31 using 250 Hz filters?
Lets see... for white noise, a 250 Hz noise bandwidth is about 10.8
dB down from a reference 3 kHz noise bandwidth. So a signal that
appears at 0 dB SNR with a 3000 Hz passband will appear as +10.8 dB
SNR with a 250 Hz passband (assuming that your signal is narrower
than 250 Hz).
(When noise power is evenly distributed, a 3000 Hz passband will have
12 times more power than a 250 Hz chunk that is taken from it.)
Listening with a 250 Hz filter, a PSK31 has to be quite loud (+11 dB
SNR) relative to the noise before it can print.
Typical HF Channel Simulators can be set up for 1.24 kHz, 3 kHz and 4
kHz noise bandwidths. Specifying modems using a 3 kHz bandwidth is
common in the amateur practice since the primary Windows based HF
Channel simulator (PathSim) uses a 3 kHz noise bandwidth. My
cocoaPath program for the Mac allows the three noise bandwidths
above, but I still mostly use 3 kHz in my experiments just so that I
can compare the numbers from my experiments with other amateur articles.
http://homepage.mac.com/chen/w7ay/cocoaPath/Contents/users.html
A real world "250 Hz filter" could have a noise bandwidth anywhere
between 200 Hz and 300 Hz (or worse :-). You'll need to sweep a
particular filter to really know. However, as a rough guess, you can
say that with a 250 Hz filter, a SNR of 11 dB is needed to get "good
print" with BPSK31.
The error rate curve can be very sharp -- in the theoretical Additive
White Gaussian Noise (AWGN) case that you find in textbooks (i.e., no
Doppler spreading or Doppler shifting), both PSK31 and RTTY can go
between pristine print to complete garbage when you change the SNR by
just 6 dB.
As you introduce slow QSB, you need to maintain this SNR even when
the QSB has dropped the strength of your signal -- hence the need to
use more power as there is more QSB. As you further introduce
further deterioration (such as multipath, or Doppler spreading)
however, PSK31 drops out completely while RTTY will work if you have
enough power to handle the depths of the QSB and flutter.
Under "CCIR Good Conditions" (also known as the "Mid-latitude Quiet
Conditions" in the ITU 1487 standard), the range of SNR between
pristine print and mostly garbled print for PSK31 increases to around
15 dB.
Propagation will eventually kill RTTY too -- for example when flutter
starts spreading a data bit. If you look on a scope, you can often
see the Space signal overlapping well into the Mark signal of a
character as propagation gets worse. The ST-8000 has a circuit (just
a couple of gates) that makes sure mark and space don't overlap.
Good software can actually "equalize" the mark-space delay and they
handle the overlap better than the good old workhorse.
73
Chen, W7AY
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