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Re: [ARSCLIST] Audio History In a Nutshell?
At 12:14 AM 2007-12-07, Mike Richter wrote:
Briefly, the digits represented in the .CDA point to the desired
blocks on the audio recording.
One area of confusion stems from the two different approaches to
"digital" that are being used -- on the same physical media type and
sometimes on the same instance of that media type.
Engineering's first forays into digital was to take the overall
analog topology and substitute digital coding for directly
transmitted analog signals.
This created bitstreams that come over wires and need to be played
out in order and need to arrive over the wire at the time they need
to be played out.
SPDIF and AES/EBU audio interconnects behave this way, as do CCIR601
and FireWire video interconnects.
CDs are built on this model. The bits stream off the CD. There is no
"file system" on an audio CD. There are a few reserved places for
some additional information, but audio CDs just stream bits. There
may be a little buffering (especially on portable devices to reduce
shock susceptibility) but in the early players, the bits came off the
disc and into the D-to-A converter and now continued to stream as
analog signals into your earphones.
There is one and only one option for streaming audio off a "Red Book"
CD-DA and that's at 44,100 samples per second, stereo, 16 bits per
channel. Things start and end at discrete points, and 75 of these
points are available in each second.
NOW, this little 5-inch disc looked like a useful thing, so the
CD-ROM spec imposed a file system on it that looks like (but isn't
quite the same) as a Windows or Mac file system. There is a directory
structure and a File Allocation Table. With this type of file system,
you can put word processing files, or database files or music WAV
files or music MP3 files or even highly compressed video files on this CD-ROM.
Clever people doing what they do, certain versions of these CD-ROM
files could then be played in certain newer audio CD players giving
you the option of all 9 Beethoven symphonies on one CD-ROM.
Now, of course, you no longer have linear coding at 44.1/16/stereo
when you make a CD-ROM file.
To understand the numbers for MP3 or Windows Media -- and how much is
being thrown away, the bit rate for a CD-quality WAV file is 44,100 x
16 x 2 bits per second or 1411.2 kb/s (kilo bits per second). The
default Windows Media bitrate is 128 kb/s. As I said, I prefer 256
kb/s MP3 files for quality and portability.
So how do MP3 and WMV codecs make the file smaller? They throw away
bits that you won't hear. It's called "perceptual coding". It's not
as bad as Steve's example of recording an LP onto a 78, but it's the
Most archivists on this list stay away from 44.1/16 as an archiving
standard but rather use 96,000 samples per second, 24 bits per sample
(per channel). This provides an effective audio bandwidth of
approximately 40 kHz which will capture just about everything on an
analog tape (I just measured a Studer A810 record/play as only down a
few dB at 28 kHz at 7.5 in/s). The 44,100 sample per second systems
cut off sharply just above 20 kHz.
MP3s and WMVs are convenience delivery formats (as are CDs). At a
given bitrate, it is said the WMV sounds slightly better than MP3.
Remember CDs are 1411.2 kb/s. Archival formats (for stereo at 96/24
are 4608 kb/s.3.3 times the CD bitrate.
Back to files. When we moved from streaming to files (in audio,
everything past DAT, CD, and that ilk are file-based not streaming
based) we now have entities that can be copied, moved, and verified
that work within the commonly used computer file systems, be that
Windows, Mac, or Linux/Unix.
Once we move from streaming to files, there is nothing conceptually
different between audio, video, word processing, or database files --
they can all be stored and transmitted in the same way.
Now, just to confuse you, the CD Extra can have a streaming RED BOOK
(CD-DA) component as well as an ORANGE BOOK CD-ROM component. I have
had one of those pressed. We put ten songs on in CD-DA which was the
"release" but since it was most likely the artists last CD, we also
put a few historical and "bonus" tracks as both MP3 (and two as WAV)
on the filesystem CD-ROM side. This also had a snapshot of the entire
website. It was an attempt across four CDs to archive a good portion
of the artist's life work and apply the LOCKSS principal Lots of
Copies Keep Stuff Safe. What better thing than having people pay for
the privilege of keeping the copies for us?
So, lots of tricks can be done with the little 5-inch disc and we
always have to be mindful of what mode we're using it in.
Of course, the next thing to consider is that there are now two
generations of development past the CD density in the same sized disc:
DVD at 4.7 or 9.6 GB/disc
BluRay and HDDVD
Does this help or confuse more?
Richard L. Hess email: richard@xxxxxxxxxxxxxxx
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.