Korgscrew
Group: Super Admins
Posts: 3511
Joined: Dec. 1999 |
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Posted: April 14 2005, 18:13 |
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| Quote (hiawatha @ April 14 2005, 18:04) | | I know someone who hates CD's because he claims he can hear the buzz of the tiny silences between the "bits" of the data. |
I actually wonder if it really is that he's hearing. The convertors in PCM digital machines (that includes CD players) have a steep filter stage which is there to cut off frequencies beyond the system's upper limits - if they got through, they'd end up being wrongly reproduced as frequencies lower down. Those filters can have resonances, though, which colour the sound. It's possible he's hearing those.
Whether you can hear the effects of MP3 compression or not does indeed depend on what you're listening to it on. The average computer speakers don't normally show it up, and the headphones which come with MP3 players often don't either.
It works by taking out parts of the sound that (according to the psychoacoustic model they use) we won't hear. One of the phenomena it relies on is that of masking - if you pkay a quiet sound after a loud sound, you're not going to hear it with the same detail as if what you're listening to is all quiet. It therefore tends to throw away certain things which the model suggests are being masked in this way by loud sounds, and does it mostly on frequencies which we're less likely to pay attention to. At 128kbps, that can definitely be heard at work - play something with a lot of percussion crashes in succession (like cymbals, with their high frequency content), and they'll sound slightly 'squelchy'. That's the data reduction algorhythm gating out certain frequencies which the model says it's ok to touch...just of course, if you can hear it, then it's obviously not ok to touch them that much! If you've ever used noise reduction tools (I'll bet you all do that sort of thing for fun, eh?  "Come on guys, let's not go out tonight, that's boring - I've got in some beers, and a selection of noise reduction tools to play with"), it's the same kind of sound you can get with those if they're used wrongly, and for the same reasons (gating out certain frequencies).
It does depend on the material - some things doesn't trip the algorhythm up nearly as much as others do.
Something you can try, if you want to hear the data reduction working, but can't notice it on anything in your MP3 collection (other than just making some at a very low bitrate):
Something common in a stereo mix is for the really important things to be in the centre. MP3 takes advantage of that, and will eat away at the stuff at the sides, whilst leaving the things at the centre with less signs of having been touched. What it's done to the more ambient things at the sides isn't really noticeable, because it's masked by what's at the centre...but...aha! We can foil its cunning plan, by taking away what's at the centre, and thereby uncovering all of its dirty deeds!
For this you will need a Groucho Marx (failing that, Karl Marx will do) disguise set, a set of ears (Mr Spock-style pointy ones are strictly optional), either something capable of reversing the polarity of signals or an ill-fitting jack plug, and a flask of weak lemon drink (you may not substitute Guinness for this).
After putting on the disguise (in case you get caught in the act - this way you'll not be recognised later), and checking that the ears are affixed, you can get to work.
In a stereo signal, things which are in the centre are, as you'll probably know already, common to both channels. You'll also probably know from school maths classes that -1 + 1 = 0. What we can then do is create a negative version of one channel, and all the things which are common to both channels will magically disappear when they're added together (and you will be hailed as audio's answer to David Copperfield...possibly). You can do that in software (split the stereo file into two separate channels, then look for an option to reverse the polarity - it may well be called phase - of one. You'll probably have to convert the file to wave/AIFF or something first), or in hardware (look for a button marked with an ø on mixer channels, or wire up a pair of leads with one where the wires are swapped around at one end) - either way, one needs to be reversed, then the two have to be mixed together. An alternative involves the ill-fitting (stereo) jack plug (preferably a minijack - I've never come across this phenomena with the 1/4" types). If they're not quite seated right, they can have the same effect of making the centre signal disappear - you'll have to fiddle around a bit to get that to work, though!
At this point, you may drink the weak lemon drink.
What will you then hear? Usually a rather nasty, squelchy-sounding signal which bears some kind of relation or other to the track as you know it. It's usually quite extremely mangled-sounding, even if the full stereo track isn't (the effect can be quite pronounced with Minidiscs, which don't tend to display many disturbing artefacts when the signal is played normally).
Perhaps that'll give at least some insight into the workings of the system - enjoy your weak lemon drink, everyone!
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