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VE2HAR > MT63 13.03.05 20:04l 122 Lines 4261 Bytes #-7304 (0) @ WW
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Subj: Re: [MT63] Re: Crest Factors
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Date: Sun, 13 Mar 2005 13:35:23 -0500
Subject: Re: [MT63] Re: Crest Factors
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Tomi Manninen wrote:
> On Sun, 2005-03-13 at 17:35, Mark Miller wrote:
>
>>Envelope crest factor
>>To determine the envelope crest factor, first determine the power of a
>>sinusoidal signal that has the same peak voltage as the modulated signal's
>>peak voltage. This value is then divided by the average power of the
>>modulated signal. The result is defined as the translated signal's envelope
>>crest factor.
>
> So envelope crest factor is peak crest factor minus 3 dB, right?
>
> Anyway, a while back I did some calculations with the numerical
> sample data right out of the MT63 modem. The (peak) crest factor
> number I got was around 13 dB, slightly dependent on the data.
>
Interesting -- I played Linux geek for a while --
Create a file of random characters
$ dd if=/dev/urandom bs=1 count=1024 of=random.dat
Use the command-line MT63 to create a raw data file of the audio
$ mt63tx -srandom.mt63 -trandom.dat
Use 'sox' to get the statistics on the file:
$ sox -t raw -sw -fs -r8000 random.mt63 -t wav /dev/null stat
Maximum amplitude: 0.795074
Minimum amplitude: -0.766235
Midline amplitude: 0.014420
Mean norm: 0.132586
Mean amplitude: 0.000000
RMS amplitude: 0.166712
Peak amplitude / RMS amplitude =
((0.795074+0.766235)/2) / 0.166712 = 4.68
Converting to dB: 20 log(4.68) = 13.4
and dropping 3 dB because we're measuring the instantaneous
peak value rather than the RMS value of the highest cycle
of audio, we'd get 10.4 dB
The questions remaining is what portion of time does the
waveform exceed a particular value? -- if these peaks occur
less than 1% of the time, for example, they'll probably get
lost when monitoring on a scope. They'll also probably get
clipped by the output stage of the transmitter and no one
will notice. Is this important? Given the amount of
redundance built into the signal, probably not from the
perspective of data loss -- but how about from the standpoint
of spectral purity?
It wouldn't be too difficult to write a program that
reads in the computed values to be transmitted and get
statistics on the percentage of time the waveform is
more than 3dB, 6dB, 9dB, etc. over the RMS value.
Given what the numbers say, I'd guess that MOST stations
running MT63 are driving their transmitters into clipping.
A typical 200W+ PEP output rig would have to be running at
a level of about 20W PEP output to stay completely clean.
The good part of having such a high peak/RMS ratio is that
those finals will stay nice and cool during long key-down
periods.
73,
Paul / K9PS
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