How works Sound Spectrum Analyzer ?

[LeCaravage]

Hi,

Can someone give me the principle of the Spectrum Analyzer we could see in Protracker or some demos ?
I'm zero at sound so a simple explanation would be nice

[no9]

Bars are triggered by the notes replayed in pattern or by keyboard. Their heights are based on notes volumes which are added to the current bar heigh's value. It is not a real spectrum analyzer. Quadra Composer has more interesting one.

[LeCaravage]

Thanks for infos, no9

[Total Eclipse]

Originally Posted by no9

Bars are triggered by the notes replayed in pattern or by keyboard. Their heights are based on notes volumes which are added to the current bar heigh's value. It is not a real spectrum analyzer. Quadra Composer has more interesting one.

I presume the OP is talking about the spectrum analyser on the top right, rather than the bars above each channel? Doesn't that display actually respond to the frequencies being played? I'd guess that it doesn't actually analyse the sample data being played, rather it takes its cue from the pitch that's triggered as part of the pattern, though I could be very wrong here.

[no9]

Yes, the bars represent notes from C1 on the far left to B3 on the far right of the Spectrum Analyzer. Which also implies their frequencies (sample rates) at the same time.

[koobo]

Originally Posted by no9

Yes, the bars represent notes from C1 on the far left to B3 on the far right of the Spectrum Analyzer. Which also implies their frequencies (sample rates) at the same time.

To clarify, each note from C1 to B3 has a specific playback frequency in Hertz.
The actual audible frequencies are created based on this playback frequency and the form of the wave being played. From this point of view the Spectrum Analyzer in the picture does not actually display the frequency spectrum

[bloodline]

Ahhhh!!! I always wondered how Protractor faked the spectrum analyser. Very clever...

I have tried to write a Fixed Point FFT algorithm, but could never get it fast enough to run on a 68000 (especially after the windowing passes), and now even cheap microcontrollers have FPUs and run at hundreds of MHz, there isn't much point

[no9]

Look there at the Quadra Composer's Spectrum Analyser.
[ Show youtube player ]

[Estrayk]

I have never seen much sense about the spectrum analyser in protracker, because in order to visually understand what you see in the spectrum all the notes should be sampled at the same frequency, which is not very common in amiga mods.

It looks nice but it is not very effective.

Just an opinion

[koobo]

Originally Posted by bloodline

I have tried to write a Fixed Point FFT algorithm, but could never get it fast enough to run on a 68000 (especially after the windowing passes), and now even cheap microcontrollers have FPUs and run at hundreds of MHz, there isn't much point

Making an impossible thing work on 68000 is exactly the point!

In HippoPlayer (v2.52) there is a spectrum analyzer scope that runs about 5-10 frames per second on A500. Admittedly it is not super smooth but it's quite ok I think.

It is a port of an old C-algorithm. Technical details: Mix and resample four channels into input data, apply Hanning window, calculate 128-point fixed point FFT, calculate decibel representation, plot data.

[KONEY]

I was looking for a way retrieve actual sample address being played and landed here. From the HW manual it seems like audio registers are write only so it shouldn't be possible but maybe there is some trick?

[a/b]

You have to access player structures, it typically maintains a copy of sample ptrs, periods, volumes, etc. for all the (active) channels. So you take sample ptrs and then you calculate where exactly within a sample you are. This can get tricky due to sample looping, replaying the same sample with a different note/period, changing volume, etc. You also have to maintain frame counters for each channel you so know for how long it's been played.
I did that with the noisetracker player, in a music disc we released back in the 90's. So if you are using protracker it could be done in a similar way *or* if you switch to P61 player that has the oscilloscope already built-in you could just customize it (haven't used it myself, I only needed a pure player at some point).
IIRC you are primarily using octamed, so in that case you'll have to look into its source code but the principles are the same.

[Predseda]

It is indeed cool, but what does it tell me?

[8bitbubsy]

Originally Posted by Predseda

It is indeed cool, but what does it tell me?

ProTracker is not a professional music production tool. The basic (and improper) visualizers are there mostly to entertain you and make the tracker more interesting to use.

Both the spectrum analyzer and the VU-meters are fake.