Back in ye olde days of RTTY when we used mechanical printers, the
thinking was that the minimum bandwidth required was that sufficient to
pass the 3rd keying sideband without too much attenuation or time shift.
(This was way before measuring group delay was something that could be
done other than in a well equipped lab, but if you looked at the output
of the modem detector after the low pass filter and before the slicer,
an oscilloscope clearly showed the changes resulting from changing the
tone filter and low pass filter bandwidths.
60 WPM Baudot RTTY has a data rate of 22 Hz*, so the 3rd keying
sidebands would be ±66 Hz from the tone. With 170 Hz shift, and a single
passband filter, the outer (upper and lower) keying sidebands would be
at 66 Hz above and below. Hence the target filter bandwidth would be 302
Hz.
A more technical explanation is that the tone filters, post-detection
low pass filter and slicer work to restore the transmitted waveform and
that waveform reconstruction becomes more difficult and less accurate
the more keying sidebands are removed. A Fourier analysis of a square
wave will show this as you increase the number of terms (harmonics of
the keying waveform) in the reconstruction.
If you want to tinker a bit, there's an on-line Fourier simulator at
http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Freq/Freq4FourierSeriesSimulators.htm
-- select the square wave and run it with 1, 3, 5, 7 etc. harmonics and
you will see that passing just the 3rd harmonic yields a not that bad
appearing square wave.
Where things get a bit more complicated is that these simple rules and
the Fourier simulator assume the harmonics are passed without
significant time (or phase if you prefer to think of it that way) shift.
Depending on the tone filter design, there may well be significant time
shift between the tone frequency and the keying sidebands. A filter with
uniform time delay, such as a Bessel will be much better in this regard
than the same order Chebyshev, for example, but for the same filter
order and -3 dB bandwidth, the Bessel will demonstrate much wider
skirts. At least in the days when we built filters from 88mH loading
coils, there was always tension between designing a filter with a
picture perfect square sided response but with gross time distortion and
one with a rounded nose and gentle flank selectivity and low time
distortion. A Butterworth filter was used in most modem designs of that
era as a compromise between time distortion and flank response and also
the ability to design and implement the filter. DSP based filters are a
huge improvement over those LC filters and permit good time delay
performance and skirt selectivity.
Jack K8ZOA ... my first piece of RTTY gear was a model 15 page printer
acquired surplus in the late 1960's from Michigan Bell Telephone through
their ham radio - RTTY program.
* 60 WPM Baudo = 368 operations per minute, 7.42 length code = 45.5
baud, or 22.7 Hz.
On 5/18/2013 8:18 AM, Brian Alsop wrote:
Ed et al,
As I said in my posting the receive filter info came from a quote
attributed to Chen in the QST article. I pointed out that the link
supplied by QST was not for receive.
So we either have to accept the quote of Chen on the receive side
(additional data exists that Chen has?) or the QST author got it wrong.
Nothing at all was said about dual peak filtering which is used by
many of us in conjunction with a 400 or "250" filter.
It would be nice to someday finally nail this whole RTTY filter issue
down. Also it would be nice to find a set of optimum AGC settings for
RTTY. I suspect there are parameters or a formulation that would
produce less spurious clicks. AGC off is definitely not a practical
solution.
73 de Brian/K3KO
On 5/18/2013 04:17, Ed Muns wrote:
Brian K3KO wrote:
This comes from June 2013 QST page 59.
First of all, Chen's article is about transmit filtering which is not
directly translatable to optimal receive filtering. Second, the cascade
effect of the K3 crystal filter and DSP filter must be considered in
determining the net receive bandwidth. So very different net receiver
bandwidths result depending on what DSP bandwidth is used with the
engaged
crystal filter bandwidth, e.g., KFLA250 which is really a 370 Hz filter.
Third, the ideal receive bandwidth for optimal decoding is not the
same as
the transmit bandwidth for minimum QRM. Depending on the decoder, a
receiver bandwidth of around 400 Hz is optimum ... unless there is
such a
heavy QRM situation that a better overall system trade-off is
obtained with
narrower, e.g., 250 Hz, net IF bandwidth. A transmit filter of 280
Hz is an
optimum trade-off between minimizing QRM to neighboring QSOs and
maintaining
signal integrity for the intended receiver. Finally, this transmit
filter
can be implemented in either the radio or the encoder. MMTTY, for
example,
provides a number of transmit filter bandwidths and the default
48-tap TX
bandwidth for AFSK meets Chen's proposal.
Ed W0YK
According to W7AY:
The ideal RTTY filter is 280 Hz wide. Narrowing it further by 60 Hz
doubles the error rate.
The article references:
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
Which doesn't come out and say the above! It's talking about transmit
filters. W7AY doesn't like uneven power in transmit tones either.
Anyhow this may confirm what has been said on this reflector. The 350 Hz
(AKA 250 Hz) filter is probably the narrowest practical choice for RTTY.
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