Posted by
k6dgw on
Jun 20, 2019; 9:36pm
URL: http://elecraft.85.s1.nabble.com/Measuring-SSB-RMS-Power-tp7653233p7653253.html
Brown noise is supposed to be sleep inducing. The security folks used
pink noise in the plenum above the SCIF's to mask conversations. I
can't remember exactly what the spectrum of A-weighting was, I think it
had a dip in the mid-range.
The TV station I was employed by in college would "rent" us out to
smaller AM and FM stations who didn't have a resident engineer. The
standard AM configuration was an aggressive peak limiter followed by a
"Sta-Level" with a very long time constant. Result was the modulation
was pegged at 99.5% all the time, and it sounded that way too ... zero
dynamic range. Years ago, I looked at KFBK's waveform when the then new
Rush Limbaugh was beginning his bloviation career. It too was nearly
constant since Rush could inhale very very quickly and talked very fast.
[:-))
Non-classical FM stations [we serviced 4] used peak limiters only since
100% FM modulation is an arbitrary number [75 KHz deviation then]. The
one classical music FM station we had used no limiting/compression. Our
TV station used no limiter, NTSC aural deviation then was 25 KHz,
however our two network feeds sounded fairly well compressed.
I seem to get best audio reports, and recordings a couple of folks have
sent me sound best to me when my K3 compression reliably goes to 10 dB
with occasional peaks to 15 dB on the bar graph. Much more than that
and there's clearly room noise in pauses and the audio begins to distort.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 6/20/2019 11:15 AM, Jim Brown wrote:
> Bob is correct to the extent that Pink Noise approximates the SPECTRUM
> of speech, but John's is on target that speech is not continuous, but
> highly variable in amplitude. For uncompressed speech, the average
> value is typically as much as 20 dB below the peak value; compression
> can reduce that to 10 dB or less. Highly sophisticated audio
> processing that has been used by radio broadcasters for at least 40
> years reduces that ratio to 6 dB or less. "If the modulation meter
> moves below 100%, it isn't loud enough!"
>
> 73, Jim K9YC
>
> On 6/20/2019 10:13 AM, John Oppenheimer wrote:
>> Hi Bob,
>>
>> Viewing pink noise with an oscilloscope, it is continuous, where as
>> speech has breaks.
>>
>> The web page is evaluating both expected power to a load and compression
>> gain. Using a little over six second recorded speech. Recording contains
>> fast spoken words, mostly numbers and a few "this is a test" etc, for
>> maximum power over time. Evaluating scope shoot, about 25 words during
>> six second recording. Not even close to pink noise.
>>
>> John KN5L
>>
>> On 6/20/19 11:27 AM, Bob McGraw K4TAX wrote:
>>> To evaluate speech use a pink noise source. That is the standard
>>> source signal for acoustic measurements.
>>>
>>> Bob, K4TAX
>>>
>>>
>>> Sent from my iPhone
>>>
>>>> On Jun 20, 2019, at 10:34 AM, John Oppenheimer <
[hidden email]> wrote:
>>>>
>>>> Hi Don,
>>>>
>>>>> On 6/20/19 9:21 AM, Don Wilhelm wrote:
>>>>> John,
>>>>>
>>>>> That connection is OK if you realize that your scope is reading
>>>>> the DC
>>>>> voltage developed by rectifying the RF from 1/2 of the load. You
>>>>> still
>>>>> have to use the power formula that is in the DL1 manual.
>>>> Well yes, power references are DCV^2 / 25
>>>>
>>>> DdB ratios computed using voltage ratios in the form of
>>>> 20 * Log(v1/v2). For example, 0 compression Pep/Pav dB =
>>>> 20 * log(15.3/5.24) = 9.3. I do need to flip the table title to
>>>> Pav/Pep
>>>> for proper negative dB values, Done.
>>>>
>>>>> I prefer to allow the 'scope to read the full peak RF voltage.
>>>>> Connect
>>>>> the 'scope probe to the side of one resistor which is connected to
>>>>> the
>>>>> BNC jack (one of the leads closest to the BNC), and the 'scope ground
>>>>> probe to one or the resistor leads furthermost away from from the
>>>>> BNC jack.
>>>> Using detected RF envelope for oscilloscope average RF measurement at
>>>> each digitized point during long, six second, trace. This will result
>>>> with increased accuracy when measuring modulated speech.
>>>>
>>>> John KN5L
>>>>
>>>>>> On 6/20/2019 9:38 AM, John Oppenheimer wrote:
>>>>>> Using DL1 with Oscilloscope to measure KX3 SSB RMS power:
>>>>>>
>>>>>>
https://www.kn5l.net/ssbpower/>>>>>>
>>>>>> Question, is this technique correct?
>> ______________________________________________________________
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