[RTTY] RTTY Filters: 250hz vs. 500hz
Kai
k.siwiak at ieee.org
Fri Aug 23 14:59:53 EDT 2013
Yes this is good practical advise. Remember also that whether you employ a 300,
500 Hz or even 2.8 kHz filter
in the radio, the FINAL filtering is software-implemented in your RTTY
demodulator software - and in some
like MMTTY there is a dizzying array of software filter parameter choices. So,
try to keep things linear till you get to that decoder.
Play with RF-amp on or not, RF attenuation and AGC settings, that is, operate
your radio, not just your automated logging program.
73
Kai, KE4PT
On 8/23/2013 2:48 PM, RLVZ at aol.com wrote:
> Hi Guys,
>
> After participating in dozens of RTTY contests and using different radios,
> my layman opinion is that "not all radios and crystal filters are created
> equal". Further, some hams are comfortable using real tight filters and
> are not real concerned about decoding every RTTY signal possible, whereas
> other hams are more concerned about trying to copy every signal. A couple of
> examples:
>
> Difference in Radios/Filters: It seems like the 250hz filters on my old
> FT-1000-MP were broader than the 250hz filter in my K3.
>
> Difference in Operators: Some great RTTY contesters have told me that
> during RTTY contests, they always run their K3 with 250hz roofing filter and a
> DSP setting of 350hz. I've tried those exact filter settings on dozens of
> occassions on two different K3's whenever QRM gets bad. And I always go
> back to a wider setting as soon as possible because I'm unable to decode many
> of the weaker signals whenever using the 250& 350hz filter settings.
>
> Therefore, for the best possible reception, I believe it's best to have
> both a 250hz and a 400/500hz filter option and to be able to switch between
> them instantly, if possible.
>
> Hope to work you in the SCC RTTY test this weekend!
>
> 73,
> Dick- K9OM
>
>
>
>
> In a message dated 8/23/2013 11:45:02 A.M. Central Daylight Time,
> rtty-request at contesting.com writes:
>
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> Today's Topics:
>
> 1. Re: 300hz or 500hz IF filter? (Jay WS7I)
> 2. S5xxEB stations in SCC RTTY contest (Robert Bajuk)
> 3. Re: 300hz or 500hz IF filter? (Kai)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 23 Aug 2013 08:19:50 -0700
> From: Jay WS7I<ws7ik7tj at gmail.com>
> To: rtty at contesting.com
> Subject: Re: [RTTY] 300hz or 500hz IF filter?
> Message-ID:<52177D96.6030502 at gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Joe-
>
> You are just simply wrong. And in any case the question was about
> filters not the signal. Been using stacked 250 Hz filters for over 30
> years on Icom, Kenwood, Yaesu radio's. They simply work in RTTY
> contesting always have and always will. Theory, math, which I trust
> Chen on far more than I believe you may indicate something else but
> experience tells me narrow is better for contesting, perhaps wider for
> weak signals but if they are that weak most won't hear them at all
> during a contest which is why DXing if different from Contesting.
>
>
> On 8/23/2013 8:05 AM, Joe Subich, W4TV wrote:
>> And most FSK signals are 370 Hz wide or more
>
>
> ------------------------------
>
> Message: 2
> Date: Fri, 23 Aug 2013 17:47:40 +0200
> From: Robert Bajuk<rbajuk at gmail.com>
> To: rtty at contesting.com
> Subject: [RTTY] S5xxEB stations in SCC RTTY contest
> Message-ID:
> <CACzNusnqoVRG21Aqow4BGJee_Z3Y0NT-zPvKT2AGx+kH8R+ZTg at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Special callsigns S5xxEB have been issued in order to promote the
> greatest sporting
> event in Slovenia this year - EuroBasket 2013 (4. - 22. September 2013).
>
> See more about the event on the official webpage:
> http://www.eurobasket2013.org
>
> S5xxEB stations active in SCC RTTY contest will sign RST + *2013* for this
> purpose.
>
> 73 Robert, S57AW
> SCC Contest Manager
>
>
> ------------------------------
>
> Message: 3
> Date: Fri, 23 Aug 2013 11:58:54 -0400
> From: Kai<k.siwiak at ieee.org>
> To: rtty at contesting.com
> Subject: Re: [RTTY] 300hz or 500hz IF filter?
> Message-ID:<521786BE.2080503 at ieee.org>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> The RTTY elements are either 22 ms (bits and start bit) or between 22 and
> 44 ms
> (1 to 2 stop bit lengths), usually 33 ms (1.5 bit lengths)..
> I've never heard of a half bit length tone (11 ms) sent in isolation.
>
> That means the spectrum will be dominated by the 1000/22 = 45.45 Hz
> component,
> and will have a fine underlying structure of 1000/33= 30.3 Hz component.
> 99% of
> the energy is contained withing 250 Hz.
>
> I agree with the K3 comment - that is one cool radio.
>
> -Kai KE4PT
>
> On 8/23/2013 11:05 AM, Joe Subich, W4TV wrote:
>> No, the half bit makes the baud rate effectively 90.9 (the shortest
>> element is now 11 ms) thus the calculation is:
>> (2 * 90.9) + (1.2 * 170) = 385.5 Hz.
>> although the actual occupied bandwidth will be dependent on the
>> information content (how often/how regularly transitions occur will
>> effect the value of "K" in the previous formula).
>>
>>> Alternatively, observe RTTY signals on-air.
>> And most FSK signals are 370 Hz wide or more depending on the care
>> with which the manufacturer has designed the FSK circuits. The only
>> exception are later versions of the K3 firmware which generate very
>> clean FSK using DSP.
>>
>> 73,
>>
>> ... Joe, W4TV
>>
>>
>> On 8/23/2013 8:15 AM, Kai wrote:
>>> Absolutely incorrect. Consult ITU-R SM.1138: BW = 2M + 2DK; D=shift/2;
>>> M = Baud/2 K = 1.2 (typically)
>>> BWrtty=2M+2DK = Baud + shift*1.2 =249.5 Hz
>>>
>>> If you consider the effect of the 33 ms (1.5 bit) stop bit, that effect
>>> has a narrower spectrum which is contained entirely within the 249.5 Hz
>>> BW of the 22 ms start and Baudot bits. The shortest element is still 22
> ms.
>>> Alternatively, observe RTTY signals on-air.
>>>
>>> Kai, KE4PT
>>>
>>>
>>>
>>>
>>> On 8/22/2013 10:34 PM, Joe Subich, W4TV wrote:
>>>> On 8/22/2013 9:42 PM, Kai wrote:
>>>>> The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just
>>>>> under 250 Hz.
>>>> Absolutely incorrect as 250 Hz does not account for the necessary
>>>> modulation sidebands or for the discontinuity (additional bandwidth)
>>>> generated by the 1.5 bit stop. Due of the half bit, the necessary
>>>> bandwidth for 170 Hz shift RTTY approaches 170 + (2 * 90.9 * 1.2) or
>>>> slightly over 370 Hz as the shortest element is now 11 ms.
>>>>
>>>> 73,
>>>>
>>>> ... Joe, W4TV
>>>>
>>>>
>>>> On 8/22/2013 9:42 PM, Kai wrote:
>>>>> The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just
> under
>>>>> 250 Hz.
>>>>> 73
>>>>> Kai, KE4PT
>>>>>
>>>>> On 8/22/2013 6:54 PM, Joe Subich, W4TV wrote:
>>>>>> The -6 dB bandwidth of the INRAD "300 Hz" filter is shown as 340 Hz
>>>>>> which is slightly less than the theoretical 370 Hz required for 170
> Hz
>>>>>> shift 45.45 baud RTTY.
>>>>>>
>>>>>> That said, performance will be a trade off between improved
>>>>>> selectivity and interference rejection - up to a point. If the
>>>>>> receiver can withstand AGC effects of close in interference, a 400
>>>>>> to 500 Hz filter will generally provide better copy than a 300 Hz
>>>>>> filter. Note: no amount of selectivity is useful when signals
>>>>>> overlap or the interfering signal includes distortion (spurious)
>>>>>> products that overlap the desired signal.
>>>>>>
>>>>>> 73,
>>>>>>
>>>>>> ... Joe, W4TV
>>>>>>
>>>>>>
>>>>>> On 8/22/2013 5:38 PM, David VE3VID wrote:
>>>>>>> Hello everyoneI would like to outfit my FT-857D portable rig with
> an
>>>>>>> IF filter on its 455khz stage. INRAD sells a suitable 500hz unit.
>>>>>>> They also have a 300hz unit. I am leery about the 300hz filter
>>>>>>> being too narrow.
>>>>>>> Any opinions?
>>>>>>> 73Davidhttp://www.ve3vid.webs.com/
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>
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