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2.Getting Your Sound
On Tape
IN THE OLD DAYS... "Analog" was the only kind of recording
available to the average musician. The first sign of the digital revolution
reaching the streets in the late 1980's were DAT recorders, Alesis ADAT 8-tracks, Tascam
DA-88 8-tracks and later, hard disc recorders like the Emu Darwin, Akai and Vestax
machines came along and the genie was out of the bottle. Of course the early
digital samplers were also in the mix. Now you can run a digital multi-track
recording session from your iPad, for crying out loud!
Still, the process is the same
even though there are different technical considerations and format specific
issues to address.
a. The Analog Recording Process
Analog recording devices use a plastic tape coated with magnetic particles
moving across a magnetic recording head at a constant speed to record and
playback. There is always an "erase head", first in the tape path, to erase
and re-align the tape particles before they hit the "record head". In the
"two-head" machine there is one head for both recording and playback. The
"three-head" design features one head dedicated to recording, the "sync head",
and another for playback, "the repro head". Professional machines have three
heads.
There is a limit to the intensity of the signal that the tape particles
can actually absorb and reproduce. The two parameters that interact to maximize
the tape's ability to correctly record and playback are "tape speed" and "bias".
At a faster speed, there is more tape area for a given signal, i.e. more tape
particles to record. Most professional analog multitrack recorders run at
30 ips (inches per second). "Bias" is a process that was discovered by accident.
It was found that when a high frequency signal, 100 Khz or so, much higher
than human hearing, was recorded along with the normal signal, the magnetic
particles did a better job of recreating the higher frequencies.
It is a complicated process and there are lots of things to go wrong! The
tape machine must be mechanically and electronically aligned to very fine
specifications. First, to ensure that it physically handles the tape gently
during shuttling, rewind and fast forward. Although tape formulations have
improved greatly over the years, mechanical problems can damage the tape by
stretching or wrinkling it. There is no error correction for this! Treat your
tapes with care and respect. Other problems include loss of particles off
the tape, called shedding, speed fluctuations which produce "wow and flutter"
and improper tape to head contact.
Furthermore, the electronics have to record the input signal and play it
back faithfully. This is where tones on your master tapes becomes so important.
They are required to properly align the electronics in the tape machine so
when you work at different studios, your tape sounds like you remembered.
When all these parameters are aligned correctly, you stand a good chance of
hearing back a reasonable facsimile of what you recorded previously.
b. The Digital Recording Process
The digital recording process is far simpler mechanically, but much more
involved electronically. The input signal is sampled 1000's of times per second
and each acoustic slice is given it's own digital number, consisting of 0's
and 1's. Theoretically, the "analog-to-digital converter" (ADC) receives the
analog input and converts it into a stream of numbers and conversely, the
"digital-to-analog converter" (DAC) reverses the process.
The "sampling rate", or how many times per second the sound is sliced is
the main factor in how well the sound will survive its digitization. CD's
are sampled at 44.1 K or 44,100 times per second, and that has become an industry
standard. Some formats offer 48 K sampling as well. DAC's and ADC's aren't
created equally however and there are differences in how these machines sound,
despite the theoretical consistency of 0's and 1's!
Digital tape machines use mechanical transports and plastic tape as a storage
medium for the digital information. The Alesis ADAT and Tascam DA-88 are examples
of the inexpensive digital tape multi-tracks. Another approach gaining acceptance
are hard disc recorders. Some have computers with software as front-end controllers,
like the Digi-Design and Soundscape machines, while others are dedicated boxes
you plug hard discs into for storage, like the EMu Darwin, Vestax and Akai
recorders.
With these random access digital recorders, the size of the hard discs
limits the amount of recording time. Locating is a snap, as is editing. When
this approach is combined with a computer as the interface, you have a powerful
word processor for music. Anyone who has used a Mac or Windows on an IBM knows
how to drag and click with a mouse and that's basically how you manipulate
the sound files.
c. Theory of Multi-track Recording
Multi-track recorders are simply tape machines that allow you to record
tracks and then overdub additional tracks in any order. For instance, you
might record a drummer on four tracks, then go back and record a guitar part,
etc. To do this, the tape machine must be able to record one track while its
playing back the others. In an analog machine, it must do this from the same
recording head. This is the job of the "sync head".
Digital machines don't rely on sync heads and repro heads, they're just
reorganizing 0's and 1's. Depending on the device, sometimes the tape based
digital machines are not as flexible as the random access machines.
It is possible to "lock up" more than one multi-track tape machine to get
more tracks. This is usually done with two identical machines and SMPTE. SMPTE
is an acronym for a timecode that was originally developed for the motion
picture industry. It sounds like a high pitched squeel but to devices that
can "read" it, it looks like a running clock. For lock up, we would "stripe"
two multi-track tapes; for one song we might need five minutes, so we set
the SMPTE "writer" to write from 0:00:00:00 to 5:00:00:00 minutes.
SMPTE is displayed as "hours:minutes:seconds:frames:sub frames", although
not all devices read subframes (there are 80 subframes). There are four types
of SMPTE. They are 30 frame per second (fps) drop frame, 30 fps non-drop frame,
25 fps, and 24 fps. In the United States, audio professionals generally use
30 fps non-drop frame and in England and Europe, they use 25 fps. The 30 fps
drop frame, sometimes called "29.97", is used for video and film applications
in the United States.
As has become the industry practice, we record this SMPTE on the highest
edge track on each tape. For example, track 8 on an 8-track, track 24 on a
24-track, etc.. So, now we have identical "striped" tapes on their respective
multi-tracks. The next step is to use a synchronization device designed to
read the SMPTE off each machine and control the motors of both to keep them
locked together.
One machine becomes the "Master" and the other, or "Slave",
chases the master machine. Two of the most popular professional systems that
do this are the Lynx and Adam Smith synchronizers. This is the basic concept
and it is possible, with the right interfaces and connections, to lock up
different types of multi-tracks, VCR's, timecode equiped DAT machines, digital
editors, etc.
The concept is simple, the execution can be complicated. The most important
thing to remember is what kind of code you've got. Keep good notes!
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