Re: Dolby A example (was Re: Has anyone ever used `echos` or `chorus`?)

On 12/02/25 20:56, Doug Lee wrote:
> Off-list communication on this one welcome so we don't stray too far. :-)

Me, I tend to write off-list messages and then sent to the list by mistake.

Fortunately I seldom say anything too outrageous :-)

However, as long as it's relevant to anyone interested in precise low-level

audio processing...


> On Wed, Feb 12, 2025 at 08:21:34PM +0100, Martin Guy wrote:
>> Do the flow diagrams work for you
> On paper yes those can work if brailled right; but on a computer, at least I personally don't usually follow those.

Thanks, I'll keep that in mind.

> I understand compand and mcompand fairly well but am not an expert at crafting effects for specific

 > purposes with them.

For that I had to dig out Ray Dolby's original paper plus a few other 
descriptions

and measure his hand-drawn graphs with a ruler to figure out the curves' 
coordinates

but was happy when one followed by the other produced something that 
sounded the same.

The --plot output was very similar to the original diagrams so it went 
fairly well.

Dolby B and C instead have a single sliding frequency band, not 
something I think SoX can do,

but they are worse cheap versions for consumer boxes.


>>   A recent example where I did not succeed was a recording someone sent me where the
>> volume cut way down at a certain point.

The new "softvol" effect may help here. It's a simple volume multiplier that

immediately reduces the volume when a sample would have clipped and

optionally increases the volume continuously so that it doubles every N 
seconds.

I use it all the time to blast the quartiere with nonstop music as it 
makes the

audibility of the result independent of the original recording volume

and stuff in the quiet passages as audible as in the loud ones.

Since I start at volume *= 400 (!) the very start of each track is 
always interesting.

There's an extreme example of it at work under http://martinwguy.net/test -

the long filename full of sox effect names.


However, I'm not sure you *can* restore something like what you describe

to its original dynamics because information has been lost.

> I did, though, just write a small Python utility for scanning files via sound very fast

> using a two-stage SoX pipe and 1-10ms tones on a 256-frequency range to represent bytes

Do you mean a single tone that blips at one of 256 frequencies according 
to its value and

recognizing certain characteristic sequences? Interesting.

One of my passions is for log-frequency-axis spectrograms, which 
translate sound from

the audio domain to the visual one - not a lot of use to you but maybe 
for deaf people to

let them see speech and music - however there are inverse techniques to 
turn a spectrogram

back into the original sound, or a best effort at it. Peter Zinovieff's 
tried this in the 1960s

and in one of his notebooks describes scanning a picture of a flower and 
converting it into

sound as a particularly fascinating experiment. My own efforts in this 
direction are documented

at https://wikidelia.net/wiki/Spectrograms#Inverse_spectrograms

That suggests adding JPG and PNG as input and output formats to SoX

as synonyms for spectrograms without axes, presumably with the lower and

upper frequencies and the pixel-columns-per-second and dynamic range

in image comments or as format parameters.

One thing at a time though...

M