Subject: Re: [linux-audio-dev] peakfiles and EDL's
From: Robert Schrem (Robert.Schrem_AT_WiredMinds.de)
Date: Tue Feb 27 2001 - 18:23:17 EET
On Tue, 27 Feb 2001, you wrote:
> >Well I'm pretty sure: if you take a sample each 2048 samples
> >of a sample stream that was recorded at 44,1 kHz you would have
> >a resulting sampling frequency for the peak data of about 20 Hz.
> >I think this is pretty right, isn't it?
>
> Well, yes, but notice that you said that the SF for the *peak data* is
> 20Hz. Thats not the same thing at all as saying the SF for the
> original audio signal was 20Hz.
OK, sorry for my misinterpretable description.
> >Here the sampling frequency is only the double of the
> >sampled singnal frequency, you already can't tell anymore
> >if it's a sin wave or a square wave:
> >
> >.. . . . . .
> >
> > . . . . . .
>
> But thats not what's proposed. You're talking about traditional
> sampling of an acoustic pressure wave or analog signal. In this case,
> we are doing computational determination of the peaks. At the
> resolutions we're talking about for peakfiles, we're not trying to see
> the waveform, we're trying to see the amplitude curve for the sound.
OK - someone else on the list stated, that he would expect to see
the WAVEFORM from the peak data and I just wanted to explain, that
this is will not be possible.
> >I don't understand why you take min AND max peak data - for usual
> >audio singnals sampled without an DC offset they would be almost
> >the same (except for the sign), wouldn't they?
>
> Because you're displaying the entire block of 2048 samples with a
> single x coordinate. No, there is reason for the max/min to be the
> same, and you want to display both.
To be honest: Can understand your point percisely :)
I think min and max value will have almost the same absolute
value if you compute the minimum and maximum of all samples
in block of 2048 signed sample data values with no DC offset.
> >I also think 2048 is resonable. And maybe whatever downsampling algorithm
> >you coose, you won't get representative peak data values anyway, because
> >each of those peak value would have to speak for 2048 wildly spread
> >data values. A hard job I think :)
>
> the min/max values are not representative of the 2048 samples,
> sure. but thats not the point. when you stick the successive max/min
> values together on a screen, you end up with something that is a good
> visual approximation to the perceived amplitude signal. and *thats*
> what's important here. Fidelity to the exact sample data matters only
> at high resolution, and there, we use the sample data anyway (though
> still peak-sampled, just at a ratio of peaks/sample that works with
> the specific values).
I think we agree on this completely :)
Maybe you would like to discuss this:
We could show the min/max values in one colour and
in another colour the 'loudness' you would hear with
your ears. Also the peak meters could be switchable
from 'digital' min/max metering to a 'loudness'
metering you have in traditional studio equipment
for the mixup process... I fear you will call this
sensless featureism... but...
But if your digital system works with floats
internally anyway, then you would only have to
watch for clipping while recording and just before
you send your final mix to a DA converter.
In all other places clipping would not be a problem
you have to be bothered with anyway, do you?
just my 0.02 euro :)
robby
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