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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