[net.audio] pre-echo and print through

rzdz@fluke.UUCP (Rick Chinn) (09/11/84)

re: pre-echo on vinyl discs:

You're all more or less on the right track (no pun intended). Tapes are
stored tails out in studios that care about the archival value of their
client's product. The reason is two-fold:

    first is that if you have to re-wind the tape first to play it, then
    after you play it, it is wound on the take-up reel under more or less
    constant tension, certainly less than if you had the tape machine do it
    at fast-wind. With today's crop of servo-controlled machines, this is
    less of a problem, since most of these machines have a "spool" mode,
    that allows the machine to "sorta-fast-wind" the tape onto the reels
    under controlled tension, so you don't have to wait as long (usually ~60
    ips).

    second is that any print through produced during storage will occur
    after the start of the material, rather than before. I think that it's
    unlikely that print through is the cause of the pre-echo because most
    master tapes are leadered up to within a [pick a suitably short
    interval] of a whatever. Since the leader tape isn't magnetic, it can't
    be the recipient of any print-through. Now, some producers or engineers
    like to not use leader, so you don't hear the sudden onset of tape hiss
    (especially since its currently in vogue to not use any sort of noise
    reduction) between selections. These guys use erased tape as leader. In
    this case, you may very well be hearing print-through.

Regarding groove echo, as trsvax!mikey said, if its about 1 revolution from
the beginning of the music, it's probably groove echo. 

Mikey also mentioned sampling the content of the music before recording.
This has been common practice for some time now. I tried to nail this down
to a date, but the best I could do is find an article dated 1955 which
described the practice (and made it sound like old hat). What actually
happens is that the material is sampled about one second before it's
actually recorded using a tape machine with two playback heads, with the
first one driving the disc recording lathe's preview channel and the second
one ending up at the cutterhead. At any rate, the preview channel tells the
lathe's leadscrew what is coming up, before the cutterhead gets it, and in
its own small way, cheats on father time.

Newer systems (like the Neumann VMS 80) use 1/2 revolution for their preview
delay. Then they further delay one channel by 1 revolution. This makes the
preview system aware of what happened one revolution previous to current
time, as well as what's coming up. They use both amplitude and phase
information between the two stereo channels to tell the leadscrew to back up
or go forward, and how fast. This way, they "snuggle" the grooves together
when its practical, and push them farther apart when not. Smart, these
Germans, no?

Of course, any concept of electronic preview goes out the window in a big
hurry when you talk about direct-to-disc. There, the mastering engineer has
to decide whether he/she wants to vary the pitch manually, while following
the score, or to compromise, setting the pitch as wide as he/she dares,
gambling that the music still fits on the disc.

As a parting shot, I refer any and all who are interested in this and other
aspects of analog disc recording to the following anthology:

    Disc Recording Volume 1
    Groove Geometry and the Recording Process
    An Anthology from the pages of the Journal 
    of the Audio Engineering Society

Available from:

    Audio Engineering Society
    60 East 42nd St.
    New York, NY 10165

At last count, it was about $40. Volume 2 may even be out by now.

There are a couple of articles about groove echo in lacquer masters as well
as a wealth of information about a subject that most recording engineers
regard as a black art. If someone wants the exact AES reference for any of
these articles, email me and I will respond.

Rick Chinn
John Fluke Mfg. Co MS 232E
PO Box C9090 Everett WA 98206

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