[Elecraft] APF and audio limiting

[Mike]

Yes!

And to avoid some potential ear damage as well. Normally I wear phones, and I think 
that blast may have caused bleeding if I'd had them on then.... >:o

Mike NF4L

On 11/6/2010 11:51 PM, Joe Subich, W4TV wrote:
>>> Since very weak signals (which is what APF is all about) are well below
>>> the level that AGC action starts, when trying to dig out such a weak
>>> station, it may be wise to turn off AGC altogether (provided you have
>>> set the AF limiter beforehand).
> It might be wise to ask Wayne and Lyle to consider activating the AF
> Limiter when APF is turned on just to prevent a problem it a strong
> signal jumps on top of the weak one we're trying to copy.  It is not
> much of a stretch to think that a local might call that weak UA0 on
> 160 or 5R on 80 while we're running "all knobs to the right" trying
> to dig it out of the noise.
>
> Looking at W8OZA's AGC test data, it appears that the audio output
> can vary more than 40 dB from a -140 dBm (MDS) signal to a -43 dBm
> signal (S9 +30 dB) depending on the user's selection of AGC slope
> and threshold.  If one adds another 10 dB of APF gain, a sudden 40
> to 50 dB audio "shot" can be quite dangerous depending on the initial
> listening level.
>
> In addition, given reports of damage to the speaker amplifier with
> sudden noise bursts with AGC off, enabling AF LIM with APF *might*
> prevent some potential amplifier damage.
>
> 73,
>
>      ... Joe, W4TV
>
>
> On 11/6/2010 9:20 PM, Don Wilhelm wrote:
>>     Mike,
>>
>> The AGC works at the IF level, not the audio level.
>> What that means is if you crank the AF gain way up to listen to a very
>> weak signal, AGC action is probably not even started because the signal
>> level is at or near the level of the band noise - In other words, the
>> receiver is operation at "full gain, all stops removed".
>>
>> Under those conditions, if a very strong signal suddenly comes into your
>> passband, yes, it will be loud simply because the AF Gain is advanced.
>> Yes, the AGC will respond to that new signal - just how much depends on
>> how you have the AGC menu parameters set.  If it does cause an AGC
>> response, it will reduce the gain of the receiver, and you will no
>> longer be able to copy the signal at or near the band noise floor.
>>
>> So perhaps what I am saying is that everyone might run their AGC
>> parameters so that all stations, no matter what their relative strengths
>> will have the same audio level - that would cure the condition you have
>> stated - but I do not really believe that is a good solution - for one,
>> it will make band noise the same audio level as the signals - result is
>> "noisy K3", and we have been there before.
>>
>> Myself, I prefer to be able to determine the relative strength of
>> various signals "by ear" rather than "by S-meter" - that means the
>> strong signals will have greater audio levels than weaker stations -
>> that is just the way it is.
>>
>> Since very weak signals (which is what APF is all about) are well below
>> the level that AGC action starts, when trying to dig out such a weak
>> station, it may be wise to turn off AGC altogether (provided you have
>> set the AF limiter beforehand).  With AGC off, the Audio limiter is
>> active to protect you from ear-shattering events such as a strong signal
>> suddenly coming on frequency.
>>
>> An additional benefit to copying very weak signals with AGC off is that
>> a strong signal coming on within the passband will not de-sense the
>> receiver (as it would with AGC on), so you may be able to use the
>> "filter between the ears" to continue copying the weak signal.
>>
>> 73,
>> Don W3FPR
>>
>> On 11/6/2010 7:46 PM, Mike wrote:
>>> Thanks Tom.
>>>
>>> Well, I kinda thought it shouldn't blast my ears that hard. Audio *WASN'T*  up too
>>> loud for the signal I was after. Maybe I don't understand what AGC does. That's why I
>>> asked the questions I did.
>>>
>>> On 11/6/2010 6:21 PM, Amateur Radio Operator N5GE wrote:
>>>> On Sat, 06 Nov 2010 18:04:18 -0400, Mike<nf4l at nf4l.com>      wrote:
>>>>
>>>> Sounds normal to me.
>>>>
>>>> Audio up too loud and APF on, then a strong signal transmits on the frequency
>>>> you are listening to....
>>>>
>>>> What do you think it SHOULD have done?  You didn't say.
>>>>
>>>> Tom
>>>> Radio Amateur N5GE
>>>>
>>>>> On 11/4/2010 9:48 PM, Mike wrote:
>>>>>> I'm playing with the new APF, and trying to get a REALLY weak QSO to pop. I have the
>>>>>> volume cranked up pretty good, and a REALLY strong station throws in their call. I
>>>>>> thought my speakers had blown. Glad I wasn't wearing cans.
>>>>>>
>>>>>> My CONFIG:AF is 18, AGC = F.
>>>>>>
>>>>>> Was I doing something wrong? Anybody else experience this? Do we need/can we have
>>>>>> better audio limiting?
>>>>>>
>>>>>> 73, Mike NF4L
>>>