> 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.
The problem is this is not universal and those who are using an
original IC-706, FT-100, TS-140 IC-746, etc. with only a 20 KHz
first IF filter simply do not understand the concept of BDR and
"AGC blocking." Combine AGC thresholds well into the BDR and
2 KHz IMD DR < 70 dB and most entry level radios simply are not
suited to using wideband receive techniques.
Unfortunately, the less technically astute operators who own
the radios not capable of handling wideband operation are also
the same operators not capable of understanding either the
limitation of their radio or the benefit of using narrow band
reception techniques.
73,
... Joe, W4TV
> -----Original Message-----
> From: rtty-bounces@contesting.com
> [mailto:rtty-bounces@contesting.com] On Behalf Of Kok Chen
> Sent: Saturday, October 04, 2008 2:28 PM
> To: RTTY Reflector
> Cc: Jay Townsend
> Subject: Re: [RTTY] rtty
>
>
> 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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