Subject: Re: [linux-audio-dev] peakfiles and EDL's
From: Paul Davis (pbd_AT_Op.Net)
Date: Mon Feb 26 2001 - 20:03:59 EET
In message <01022618013502.07471_AT_pc-robert>you write:
>On Mon, 26 Feb 2001, you wrote:
>> >From: Robert Schrem <Robert.Schrem_AT_WiredMinds.de>
>> >
>> >I think the values in a peak file should correspond to the level
>> >meters of a compact casette recorders display. It should correspond
>>
>> It is often the peaks we are interested in to see in the waveform.
>
>If you would take only one peak value for 2048 samples you won't
>see much of a waveform anyway. Only singnals BELOW a frequency of
>44100/2048 ~ 20 Hz would show a recongizable waveform. And this
I'm not totally sure, but I *think* that this is completely wrong.
The waveform that you see when you display the amplitudes stored in a
PCM data stream representing complex sounds does not display any
particular frequency - it displays volume (albeit not necessarily in
keeping with our physiologic perception of it). You almost note this
yourself, but don't take the next step.
Its only when you display the amplitudes of simple wave forms that you
are seeing the frequency of the waveform. In a complex waveform, the
PCM values don't represent the amplitude of a sound at any particular
frequency in any way. Remember, PCM is in the time-domain, you can
only see the frequencies by doing an FFT and displaying the results.
>only if you would apply antialias filtering. If you use peak
>files you care more about the big picture: If you whould like
>to see 1 Minute of music at once. If your screen resolution
At 1 pixel = 1 sample, a typical modern monitor can display only about
0.03 seconds worth of 48kHz audio.
At the other end of the scale, you might be viewing 20 minutes of
audio on the screen.
So there is clearly no single peaks-per-sample figure that satisfies
all levels of viewing by itself. But I believe that about 2048 is a
reasonable value: below that number and its almost possible to display
the raw sample data for 16 tracks by reading it as you go. Go way
above it, and you simply have to compute the peak-of-peaks, so to speak.
--p
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