Help: Audio Compression (not MP3)

Subject: Audio Compression (not MP3)
From: Ed Pellow
Date: 25 Jul 00 - 05:26 PM

I've recently bought a bootleg CD Rom of the entire Beatles catalogue.

The files are .wav files, and beyond reducing the 44.1khz sampling rate by half to 22.05khz, there seems no other reduction in quality. They are still 16 bit recordings.

Can someone explain how so much material has been squeezed onto a 650mb disc?

Ed

- who already owns all the recordings in various other formats - in case anyone was going to moan...

Subject: RE: Help: Audio Compression (not MP3)
From: Alan of Australia
Date: 25 Jul 00 - 08:23 PM

Ed,
Give us an example of one of the songs: What is the file size & the length of the song in min, sec.

They are probably compressed files in mp3 format or similar with a .wav extension. If you open them in an audio editor to see their parameters you will just see the 'before compression' figures.

If there is an example less than around 1MB email it to me at [e-mail].

Cheers,
Alan

Subject: RE: Help: Audio Compression (not MP3)
From: John in Brisbane
Date: 26 Jul 00 - 02:40 AM

Ed, as I recall the maths is pretty easy, even though I may not have the exact numbers correct. Each mono channel at 16 bit requires (something like) I Meg per minute. If it's stereo you need twice the capacity and conversely if you reduce the sampling rate by half you only need half the capacity. If I'm exactlty right (which I doubt) you would get 325 minutes at 44 kHz Mono and 650 minutes at 22 KHZ Mono, which is probably of the order of 150 - 180 songs. Is that within the ballpark? Are the songs mono? Regards, John

Subject: RE: Help: Audio Compression (not MP3)
From: Alan of Australia
Date: 26 Jul 00 - 08:39 AM

G'day,
OK, here's the maths:

CD quality is 44100 samples per sec in each channel. Each sample is 2 bytes. Stereo: 2 channels.

44100 x 2 x 2 = 176400 bytes / sec.

176400 x 60 = 10,584,000 bytes / min

1 MB = 1024 x 1024 bytes, i.e. 10,584,000 bytes = 10.093688 MB. Close enough to 10MB / min.

At 22.05 kHz Stereo 5MB / min

650 MB on a CD ROM / 5 = 130 min

So that's about 50 Beatles songs or 25 folk songs

Double that if they're mono. Still a long way short of getting the entire collection on 1 CD ROM.

I reduced my complete collection of Beatles CDs down to 6 CDs containing 161 songs. That's about 3/4 of their total. If I had done that at 22kHz it still would have taken 3 CDs (except that's not a valid audio CD format). Also note that audio CDs have a capacity of 740 MB, not 650 MB. So I've used 6 x 740 MB. Divide by 2 for 22kHz and I'd need 3.4 CD ROMs, which of course means 4. More for the entire collection. Halve all that for mono.

Cheers,
Alan

Subject: RE: Help: Audio Compression (not MP3)
From: Ringer
Date: 26 Jul 00 - 09:15 AM

Are they then compressed in a similar way to zip files? Or the drivespace utility?

Subject: RE: Help: Audio Compression (not MP3)
From: Alan of Australia
Date: 26 Jul 00 - 11:17 AM

I presume you mean lossless compression. There are several lossless methods for wave files but they all achieve a 40 to 50% reduction at best. Still not enough to get the whole lot on 1 CD ROM. And the files can't be played without a special player (or Winamp plugin) if at all. You wouldn't be able to play the files without being aware of the file type.

Cheers,
Alan