Audio Codecs

Pengu

Pengu

Digital Music Pimp
Heyas..

Im just wondering a bit about codecs/audio quality.

First off. I understand kilobits/sec, ie bitrate. But i don't understand the sample rate.. how exactly does the sample rate (44.1 khz, 48 khz, etc) affect the audio quality?

Next, does anyone know of a tool to compare an original track, and a compressed file to calculate the audio quality (by checking the waveform perhaps?) of the compressed file (as a percentage/grade of the original)

Lastly, does anyone know of a lossless codec that works with iTunes (remembering that itunes 4 should work with anything that has a quicktime plugin/component)??

Cheers,

Pengu
 
Uh... I'm definitely not a complete expert at codecs, but I DO know that if you go to the "Import" preferences in iTunes, you can choose to use "AIFF," which is a lossless codec... and, coincidentally, I believe it's actually the basis of the Audio CD format.

EDIT: Lossless, assuming you set it for 44.1khz, 16bit, Stereo sound.
 
I thought CD-Audio was 48khz on a CD.. anyways.
AIFF doesn't compress the file at all. So an hour of music will be about 600mb as an aiff. I was looking for something like FLAC (Free Lossless Audio Codec) that will work with iTunes.

Anyone know how a 320kbps AAC goes against the orig. CD? I ripped a track to 256kb AAC, and then to 320kb at 44.1 and 48khz, and I can't tell the difference on these speakers (so very craptacular) but I can definitely hear the difference at higher volumes (on my car system)between a CD and a 192kbps MP3..
 
Its hard to explain the sample rate with out a little physics...

I presume you know what a sine (math funtion) looks like? Imagine that sine wave is actually the music you are encoding, say the sign wave has a frequency of 1 hz (one cycle per second). The sample rate is specifys how often the encoder looks at the sine wave. If the sample rate is really low say the same rate as the sine wave, every time it goes to take a sample the sine wave is at exactly the same position. It thinks this is a constaint singal not one that is going up and down. Therefore you have a useless encoder.

Imagine sitting at an intersection counting cars, you can only count cars when your eyes are open, the more you have your eyes open the higher the sample rate, you could probably count every car that went past. If you spend half the time with your eyes shut (low sample rate) you would miss most of the cars and your car counting results would not be that good!

On the radio for example you can hear the difference when the dj is having an interview with someone over the phone compared with having the person in the studio. The phone conversation has a much lower quality. I think the public phone service has a sample rate of about 10Khz - but dont quote me, i cant remember!

In general you need to have twice the maximum frequency of the music as the sample rate to get the "full" sound, so if the maximim frequency of your music is 20kHz (ear splitingly high) you should set the sample rate to be 40Khz.

The bitrate i take it you understand, but it is the detail of each sample that the encoder takes. The two go hand in hand. It is pointless having a high bit rate with a low sample rate or visa versa.

Apple proclaims that you can't tell the difference between 128Kbps AAC file and the original, im picking you could but only if you had top-o-the-line sterio with exceptionally good speakers.


I probably overkilled the explation, a little more detail than was required but i have a bit of spare time at university today!
 
Cool, thanks.. Didn't quite get it until you explained it like the traffic lights thing. I understand the bit-rate...

Any idea what quality mp3 compares to 128 aac, or how much difference there is between a 320kbps mp3 and 320kbps aac? i think im gonna test a 320kbps mp3 in my car (mp3-cd player) but i cant compare the aac's as it wont play them..
 
First time i've seen that analogy pretty damn good. So, does this work on dual processor chips. I have two people watching cars and their eyes are open all the time, they still see the same amount of cars. If they eyes are open closed and intersecting intervals they would see all the cars. If their eyes are closed at uniform intervals they would only see half the cars. And if their eyes weren't opened at all they wouldn't see any cars. So, if this were true I would want the exact opposite of me in a mate to trully see everything the world has to offer...or better yet all I need are eyes in the back of my head and I wouldn't have to remember any birthdays except my own.
 
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