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(or, how to put 10 pounds of music into a 5 pound bag)

Let's straighten out a few mixconceptions (I just typed that by accident but I think I'm going to leave it...who knows what I will actually be remembered for? :-) about media and compression and so on.

OK. Slowly. There's some math here, but I'll do it all. Just follow the thinking.

Let's assume that we're recording digital audio at the all-time world standard of 44.1 KHz. I keep harping on that because there are folks floating around who think their music will sound better if they use 48 KHz or more, but eventually they will have to do a sample rate conversion back to 44.1 and they will wish they had just done it the same way as everyone else.

Anyway, one reason 44.1 was picked is because you can think of it as representing that many bit changes per second, so if you imagine a sine wave going up and down, you will have just half of that as frequency response, or just a bit over 22 Khz, which is 22,000 cycles per second, as we used to say before digital. That's slightly more than most humans can hear, and not so coincidentally allows a full symphony to be stored on a single CD.

Right. So if we have 44,100 bits per second, then to find out how many bits per minute, we'd multiply by 60. But wait, there's more! Aren't we using 16-bit resolution in our K00L sound cards? So 44,100 x 60 x 16 = 42,336,000. That's a lotta bits!

But we don't store things as bits on a computer, we use 8-bit bytes, so dividing by 8 gives 5,292,000. And that, kiddies and kiddos, is why Dragon always says that digital audio takes 5 megabytes per minute, per track (that's 10 MB/minute for stereo, no discount when you buy two). And these are the laws which G-d did not give unto Moses (notice, He never said DAT in the Bible, as anyone from Brooklyn could tell you), but which are as inviolate as if they were.

If you can't follow that, get some more coffee. I'm on a roll here...

The point to remember here is that uncompressed 44.1 KHz audio takes 5 MB/minute. Period. (that's the little black dot at the end of the...never mind). So if you know the size of your hard disk, you can do the math. 2.5 GB can be written as 2500 MB, which is roughly 500 minutes of audio (2500 / 5 = 500 for the atheists in the audience). So when those "other hard disk recorders" say they have 60 minutes' worth of 8-track recording, they're actually being a bit conservative, since 500 / 8 = 62.5 (and in case it's not obvious yet to the most casual observer, we divide by 8 because there are 8 tracks).

As a corollary of this (my high school math teacher would be proud of me...hi Mrs. Landsman!), note that if you don't use all 8 tracks in any given song, you should have more total time available to you, because you're only using the disk space you're actually using, if you get my drift. This is assuredly the case in computer-based hard disk recording; I can't vouch for all the self-contained units because some of them may have operating systems written by morons.

Now, class, if 2.5 GB gives you 500 minutes of pure CD-quality audio, but Roland's 2 GB gives you 800 minutes, does Roland compress the audio?

You bet your bippy.

Do not confuse this with the kind of compression people pay money for, as with an RNC or Alesis 3630. That is a, er, dynamics processor. The compression that Roland is doing to give you "more minutes for your money" is akin to JPEG compression of images...you're losing something -- permanently -- in the translation. And it's up to you to listen to the results to see if you like their compression algorithm.

If you want to be a bit cynical, the Roland (and any other hard disk recorder that uses compressed audio) is nothing but a non-removable Minidisc recorder.

Not to say that any hard disk recorder that uses compressed audio is no good. As a counter-example, Fostex never compresses their audio, but look at all the trouble they're causing by only letting you record only two freaking analog tracks at a time!!!


That's not to disparage Fostex's audio quality, just their limitations.

Now, tape is an entirely different animal. It's...linear, D00d :-). And the tape goes by whether you put something on it or not, so when they say 128 minutes, they mean a full 128 minutes of 8 tracks. Think of it as the equivalent of, oh, a bit over 5 GB (5120 MB for those who like extra credit).
-- Dragon

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