[Elecraft] Ideal RTTY filter.

[Joe Subich, W4TV]

> 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.

Chen has written extensively on RTTY filtering both on his web site
as well as in his postings on the RTTY list.  In addition, Chen has
provided links to work by others (Hoff, Nyquist, etc.) in the field
- one can't take a single link in QST as the sum total of Chen's work.

 > It would be nice to someday finally nail this whole RTTY filter issue
 > down.

Read *all* of Chen's work and check out some of the bibliographical
information as well.  The information will make the concepts quite
clear (although the mathematics is a whole different matter).  One
does need to understand that filtering in the receiver and filtering
in the demodulator are two *entirely different issues* although they
must be considered together along with transmit filtering - because
all three "filters" are connected in series and any group delays
(filter induced multipath) additive.

 > 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.

"Key clicks" in RTTY are a result of improper FSK (or AFSK) generation
and the lack of appropriate transmit filtering/waveshaping as described
in Chen's link on transmit filters.  AGC has no effect on them.

73,

    ... Joe, W4TV


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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