Technical notes on MDs / ATRAC

[Karen Nakamura]

Hi Folks -

Responding to the recent discussion of MD recorders. If I wasn't
interviewing deaf people for whom a MD recorder is rather useless (see
previous e-mail on DV camcorders, which I love), I'd be using MDs. My
folks, who are both Indonesia scholars (ok, enough with the jokes on kids
of anthropologists) use MDs extensively and can't stop praising them.

The benefit of MD recorders are:

   * Very compact size
   * Very high quality recording
   * Long recording span (140 minutes in mono)
   * Automatic date/time stamp on all recordings
   * Ability to label recordings ("Interview with Bill Gates")

The disadvantages are:

   * Currently no easy way to go from digital->digital into your computer,
you have
      to go out to the analog realm and redigitize. Hopefully as IEEE1394
(FireWire/iLink)
      catches on as a general A/V link, there's be more choices

         - Some pro decks have digital out (usually TOSLINK optical), but
I've only seen
            a few TOSLINK boards for computers, the Creative LiveBoard and
Optical/Digital
            IO board attachment for SoundBlaster cards ($59):
	http://www.americas.creative.com/accessories/audio/

         - For a while Sony had a MD-DATA drive, which hooked to PCs. But
since the raw
            data storage of a MD player is only about 140 megabytes, it
wasn't a commercial
            success. I haven't seen MD-DATA sold anywhere.

         - You can get some newer MD decks in Japan that hook to your
computer, but it's unclear
           if they allow audio transfer, or just titling capabilities from
your PC.

   * The ATRAC compression method.

As mentioned above, MD disks have a raw data storage capacity of about 140
megabytes. This is approximately 1/5th the capacity of a CD which records
in PCM at 16 bits stereo / 44 mhz. So in order to get 74 minutes onto an
MD, you have to compress the audio signal.

ATRAC stands for Adaptive Transform Acoustic Coding. It uses psychoacoustic
principles to remove frequencies and portions of the sound track that are
inaudible to the human ear (either due to the physical mechanics of the
ear; or the properties of the auditory nervous system).

If you hook a spectrum analyzer to an audio signal before and after ATRAC
compression, you'll notice differences. However, they should be inaudible
to the human ear. The earliest ATRAC units (in the first MD recorders)
didn't have optimal algorithms and they had audio defects -- sort of the
rap that MP3 has right now. The latest ATRAC3.5 units in the newest MD
players are virtually indistinguishable from the originals.  In fact, on
the basis of technical specs alone (signal to noise ratio, etc.), ATRAC 3.5
is superior to the PCM recording method on CDs.

For more info on ATRAC: http://www.minidisc.org/aes_atrac.html
(this white paper should answer all of your questions and more)

What does this mean to linguists and linguistic anthropologists? Basically,
if you're only going to be listening to the recordings (transcription,
etc.) then ATRAC won't get in your way. But if you're going to be doing
spectrum analysis of the recordings, you may want to take a look at the
ATRAC white paper and see if the compression method is going to get in your
way.

BTW, someone mentioned that some MD recorders allow for ATRAC to be turned
off. I've never seen such a beast, and without ATRAC an MD would only hold
about 15 minutes of sound.

What would be nice is if future MD players came with USB or IEEE1394 ports
so that we could upload and download recordings to them. Unfortunately,
Sony is also a media conglomerate as well as a electronics manufacturer.
Since the MD system was devised before there was the fear of rampant MP3
piracy, there isn't that much protection built into the system (except for
SCMS, mentioned by another poster). So I don't think we'll see this in the
short term future.

Will MP3 make MD players obsolete? I don't think so, not yet. First, MD is
a huge commercial success in Japan (this year, more MD players were sold
than audio tape players (aka Walkmans)). MP3 player/recorders based on
flash memory are very expensive, you can't beat the price of a $2 MD disk
that records 74/148 minutes. But I think MP3 will start making inroads.
We're starting to see units that allow you to record to a removable
SmartMedia card, which you can then plug into your computer. But since the
RIAA (http://www.riaa.org/) seems intent on making life for MP3 recorders
very difficult, we'll have to see how they fare in the courts.

That's about the limit of my knowledge on MDs, but if folks have any
technical questions, I'd be happy to answer them.

Karen Nakamura
ABD, Yale University




ps. http://www.minidisc.org/ is a good site for general info on minidiscs,
they have links to all of the manufacturers of minidisk players/recorders