On a fine day, 02-02-2004, John James wrote: >As some one who is fairly new to computer based recording in >general, but slowly getting my head round it all I wonder if someone >could take the time out to explain to me what the actual (audible) >difference between these three settings are. As far as I have >gathered the higher the bit, the wider the frequency range? Is that >correct. Also, what does "float" mean? (as in 32 bit "float"). The bit depth describes the length of the (binary) number that's used to encode signal levels. An 8-bit sample uses only 2^8 (2 to the power 8) = 256 different levels. So if you record e.g. a sine wave at 8-bits and then look at it, you'll see a 'stepped' waveform consisting of only 256 different levels. Small variations in signal level will be lost, because they simply can't be distinguished with only 256 levels. With 16 bits you already have 2^16 = 65536 levels available. Needless to say that your 'stepped sine' will look (and sound) much smoother, and more "real sine-like" than the 8-bit version. 24 bits give 2^24 = 16,777,216 different levels and is thus again a massive improvement: even very small fluctuations in signal level can be faithfully recorded. The maximum attainable frequency is independent of this, and is determined by the sampling frequency (i.e. how many times per second is a sample recorded). As a rule of thumb: the max. reproducible frequency is half of the sampling frequency. So a CD-quality 44.1 kHz recording can contain frequencies up to 22.05 kHz, and a 96 kHz recording can go up to 48 kHz. Bit depth and sampling frequency are independent: in principle you can sample at 8 bits, 96 kHz (high top-frequency but low resolution), or 24 bits, 22 kHz (low top-frequency, but high resolution), or any other mix. So: bit depth determines the vertical resolution (level) and sampling frequency determines the horizontal resolution (time). Increasing bit depth has a greater effect on audible fidelity than increasing sampling frequency has. I.e. the difference between 16 and 24 bit is very well worth it. Increasing the sampling frequency from 44.1 to e.g. 48 kHz is not really worth it (esp. since you then have to "downsample" again to 44.1 at the end, because that's what CDs use, and downsampling is tricky and can produce artifacts that negate the 'gain' you hoped to get from using a higher frequency). >I hear that recording in "24 bit" and dithering down to "16 bit" for >cd is higher quality than just doing the whole thing in 16 bit. Correct. That's because all the effects-processing, summing of audio signals, etc, etc (in short: all the stuff that goes on in a digital audio system) necessarily introduces rounding errors and such. If these occur in a 16-million level system (24 bit) they'll be far less audible than when they occur in a 64-thousand level system (16 bit). The concept is rather straightforward: if you're off by one on a 256-point scale (8 bit) the error is 1/256 of the max signal level. If you make the same off-by-one mistake in a 24 bit system, the error is only 1/16-million-th of the full scale. The tricky part of course is how to get from your nice clean 24-bit recording to the standard 16 bits used by CDs. There's a whole bunch of mathematics involved in doing this "nicely", which is all implemented in Logic's 'dithering' algorithms (press the Bounce button on the Output object, and in the bounce dialog you'll notice a popup with 3 dithering options: that's the stuff that can turn 24 bits into 16). So you needn't worry about that. Just record at 24 bits (if your hardware supports that!), and when it comes to bouncing the final mix for CD, pick one of the dithering options. Warning: never dither more than once, and only do it at the very last stage of production. E.g. if your recording is going to a mastering engineer, just bounce at 24 bit and let him/her do the dithering. >What about doing it in "32 bit float" and dithering from there. >(these seem to be the three settings available to me in Logic Plat >6). The '32 bit float' is Logic's internal processing resolution. You can't record at 32 bit float. Logic simply uses an even higher resolution internally to calculate effects, audio-summing, and such. Completely transparent and not something to worry about. The positive spin-off of Logic's approach is that you have plenty of headroom in the audio mixer and will find it very hard to cause real digital clipping. The only place where this plays a role for the user is when setting up the "Freeze" options. Normally Logic will use 32-bit float for its freeze files. If you want to use the freeze files in other programs, or if you want to manually place (a copy of) a freeze file on an Arrange track, you need to tell Logic to create 24 bit (or 16 bit) freeze files, since 32-bit-float isn't a format any software can use. If you don't do any of this, but use 'freeze' the plain regular way, just leave this set to '32 bit float'. Since the freeze files then already are in the format Logic uses internally, you'll save some CPU cycles (a 16/24 bit file has to be converted to 32-bit float by Logic internally, before Logic can start doing calculations). >How do these different settings then affect your CPU and I/O loads? The higher the bit-depth, the bigger the file. A 24-bit file is 1.5 times a 16-bit file. So that means more disk activity (and more disk space). About cpu use: I think this shouldn't be affected -- both 16 and 24 bit are converted to 32 bit float anyway, so processing is always done in the same resolution, regardless of the input format. Finally, about the "float" in "32 bit float". "Normal" 16 and 24 bit files use whole numbers (integers) -- i.e. numbers like 256 and 345678 without a decimal dot in them. A "float" is a computer number with a so-called "floating point" -- like 56.8765. Generally speaking, there's plenty of reasons for preferring one over the other in various situations, and here in the LUG there have been heated debates over whether using floats is better than using integers for audio or not -- but that's the kind of stuff you probably don't want to get into at all (and neither do I :). To sum it up: record at the highest bit-depth your hardware supports (i.e. preferably 24 bit), use a standard 44.1 kHz sampling frequency, and dither down to 16 bits at the very last step. -- Hendrik Jan Veenstra h @ k n o w a r e . n l Omega Art: http://www.omega-art.com/