Thanks Chen for the comments.
I wonder if the filter implementation realized in MMTTY is known to someone
in the group?
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Kok Chen
Sent: Friday, February 24, 2012 4:40 PM
To: Jeff Blaine
Cc: RTTY Reflector
Subject: Re: [RTTY] LMS Notch filter
On Feb 24, 2012, at 1:46 PM, Jeff Blaine wrote:
> Is there a benefit when using a twin peak filter (of whatever
> implementation) - assuming there is only one signal within the passband?
An arbitrary twin peak filter can actually be detrimental if the software
modem or TU already includes an optimal filter (a Matched Filter or Raised
Cosine filter that are designed for the specific baud rate).
My own modus-operandi is to use as wide a receiver filter that I can get
away with (i.e., without having QRM clipping the receiver chain and the
sound card). This presents a signal with the flattest response, and with
the least group delay, to the modem.
Then feed that wide signal to a software modem that is specifically designed
for the baud rate which you are receiving.
The Raised Cosine filter is the narrowest filter than can be used. For a
45.45 baud signal, the Raised Cosine basically has -6 dB points at 23 Hz
from the center of the mark and space tones -- for 170 Hz shift, you can
think of this as an optimal "dual peak" filter that is 216 Hz wide, and a
hole in the middle. With overall -6 dB width of 216 Hz, and a very sharp
skirt, falling to below -80 dB by around 260 Hz.
Don't try this with any dual peak filter/I.F. filter combination that is 216
Hz wide, however. The shape of a filter (plus the group delay) can cause
much intersymbol interference for an arbitrary 216 Hz filter. To achieve
216 Hz (and not suffer from intersymbol interference) the filters need to be
a pair of perfect Raised Cosine shapes around the mark and space tones. So,
it looks like a dual peak filter, but is not any arbitrary dual peak filter,
and you need to change the width of the Raised Cosine when you change baud
rate.
As long as the QRM is not clipping the sound card, the raised cosine filter
will completely reject any QRM (and noise) passband that rejects the QRM
completely outside the 260 Hz passband. If you receiving chain and sound
card combination has enough dynamic range, just let the modem reject the QRM
.
When there is no QRM, a Matched filter outperforms the Raised Cosine by just
a tad, but it is also very wide. For 45.45 baud, the -30 dB points is more
than 500 Hz on each side of the Mark and Space tones. At some loss in
sensitivity relative to the Raised Cosine filter, you can "roof" the Matched
Filter with an IF receive filter when QRM is present, but open the I.F.
filter up where there is no QRM (weak DX working split, for example).
Both Raised Cosine and Matched Filters are very difficult/expensive to
implement in hardware. That is why a third order Butterworth filter is used
in the past to achieve optimal RTTY copy in the better TUs. In software,
both Matched Filters and Raised Cosine Filters are a matter of properly
designing the coefficients for an FIR filter.
73
Chen, W7AY
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