
The Ghost in the MP3 - fugyk
http://theghostinthemp3.com/
======
daturkel
This was posted almost a year ago and enjoyed a decent conversation in the
comments [0]

I'll repeat my comment from the time:

"For what it's worth, the presence of seemingly significant signal in the
difference between the original and compressed tracks does not necessarily
mean that significant sonic/perceptual loss has occurred. Operating correctly,
the encoder is designed to cut not just sounds that the human ear cannot hear
in general (e.g. sounds above 22kHZ) but also sounds which may not be
perceptible in context (e.g. the quietest signals in a loud section). So if
you find something beautiful about the ghost tracks (and I think there is
something beautiful to find), don't immediately jump to concluding that mp3 is
awful for cutting these sounds—they might be hardly perceptible when added to
the mix.

Of course, at high-compression rates mp3 does begin to significantly degrade
fidelity.

Edit: all of this is not to put down the project—I still think it's pretty
cool as art and as a demonstration of the encoder, I just didn't want people
to think that this was some sort of massive failing of mp3."

[0]:
[https://news.ycombinator.com/item?id=7955917](https://news.ycombinator.com/item?id=7955917)

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dankoss
The information present here isn't really "lost" as much as it can't be heard
in the context of the other sounds in the original recording. These forms of
audio compression take advantage of auditory masking[1] which means those
sounds likely wouldn't be heard in the original.

[1]
[http://en.wikipedia.org/wiki/Auditory_masking](http://en.wikipedia.org/wiki/Auditory_masking)

~~~
sejje
The information is lost in the sense that it's in the original, and not in the
lossy version.

Whether or not you can hear it, the information is gone.

~~~
mdisraeli
If you can't hear it, was it ever information?

~~~
daeken
We can't see in infrared, but there's clearly information there. Same with
infra/ultrasound and other sounds that are buried in our hearing.

It may not be pertinent information in the case of music, but it's definitely
information.

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joe_bleau
A quick scan didn't reveal to me whether he's time aligning the signals during
the subtraction. I've played with listening to the wav-mp3 signal before, and
I seem to recall that the mp3 encoder would introduce a little delay.

I added a transient pulse in front of the music so that I could (visually)
time align the signals before subtracting them.

~~~
to3m
I wondered what my favourite test tracks would sound like, so I made a
(somewhat stupid, and a bit slow...) program to produce the difference between
an MP3 and a WAV: [https://github.com/tom-
seddon/bin/blob/master/find_mp3_resid...](https://github.com/tom-
seddon/bin/blob/master/find_mp3_residue.py)

(Dependencies: python 2.7, lame, GNU make, mpg123, and (if you use FLAC files
as input) flac. Tested on my Linux PC with LAME 64bits version 3.99.5 and
mpg123 1.14.4 from the debian stable package repository. Run with -h to get
some "help".)

It uses lame to compress and mpg123 to decompress, and I don't know if there's
something special going on but the output WAVs always seem to have the same
number of samples as the original. And they seem to be aligned - if you use
this program you'll find that the difference between WAV and 128kbps MP3 is
somewhat noisy, but WAV vs 320kbps MP3 is pretty much silent.

(Or maybe you'll find something totally different! Who knows. I only tested
this on my system.)

~~~
mdisraeli
Neat, thanks for running the experiment to see how differing MP3 encoder
settings affect the lost portion. This explains why 320kbps is generally
accepted amongst DJs, as any loss is significantly less than that caused by
the club sound system :P

~~~
to3m
I did a blind test when I was in my 20s, and while on a couple of tracks I
could actually tell the difference between 320kbps and the original, I did
have to concentrate. And I couldn't really have said that one was necessarily
better than the other; the effect was as if one type of noise-y sound was
being replaced with a subtly different type of sound with the same noise-y
quality. Different, but overall the same.

Listening to the diff of one of those tracks today was interesting! All I can
hear is the drums... and where the sound I'm thinking of plays, it sounds like
rather quiet interference! But the drums as I recall sounded absolutely
identical. Interesting that the ears can detect one thing but not the other.

(I didn't bother to re-run the full comparison, as I'm no longer in my 20s.
One good (?) thing about getting old is that your hearing deteriorates, and
issues such as this become moot. You can also afford the disk space to just
compress everything at 320kbps. Then you don't have to worry about it, and it
fits OK on your phone too.)

------
im3w1l
The file we are watching is lossily compressed. So we are watching the lossy
compression of a delta between original and lossy compression.

How good is the lossy compression at capturing that delta?

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Buge
It gives an error when I try to play the video in Firefox or Chrome.

~~~
tveita
It played for me 30 minutes ago, but it doesn't anymore.

I thought it was a embedded Youtube video at first, but it's actually a .mov
file hosted in Google docs. First time I've noticed that way of hosting, maybe
they have a bandwidth limit?

~~~
eitland
Played fine on my phone (Android) just now. Seems like they are using Vimeo
now.

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chanux
Suzanne Vega - Tom's Dinner
[https://www.youtube.com/watch?v=FLP6QluMlrg](https://www.youtube.com/watch?v=FLP6QluMlrg)

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intopieces
If this kind of thing interests you, I highly recommend the book "MP3: The
Meaning of a Format" by Jonathan Sterne [0]

[0][https://www.dukeupress.edu/MP3/](https://www.dukeupress.edu/MP3/)

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MrJagil
How does he get the information lost in compression? Does he put the
compressed and uncompressed version on two different tracks with one phase-
flipped?

------
magwhyr
on vimeo: [https://vimeo.com/107845118](https://vimeo.com/107845118)

