Some Thoughts About Standardization
Page 6 of 7
Audio channel assignments
Audio channel assignments are another troublesome point.
These days, most people can cope with concept of stereo. Two channels,
left and right. Many planetarium soundtracks are mixed in stereo; show
packages deliver their soundtracks in a 2-channel format, such as CD or
cassette.
The problems comes in when the soundtrack contains more than stereo audio
-- such as the SMPTE time code for the automation. A multi-track tape is
often used; 4- and 8-channel tape decks are the most common.
In theory, prudent planetarians should use their show package tape as
their source material: make copies for show use and then safely store the
original in their archives. In practice, and almost unbelievably, many
planetaria do not have two of the same multi-track decks, so they can not
make backup copies of their show tapes. So the original package tape gets
used directly, and planetarians continually ask us for their soundtrack
tapes to be made with the channel assignments matching their theater
configurations. Of course, these are all different.
Which two channels hold the stereo audio? Which channel contains the
SMPTE time code? We've had requests for practically every possible
combination.
| Studio 8-track |
| Track | Contents |
| 1 | Left Audio |
| 2 | Right Audio |
| 3 |
|
| 4 |
|
| 5 |
|
| 6 |
|
| 7 |
|
| 8 | SMPTE |
| 9 | SMPTE |
|
In audio and video studios, the usual practice is followed that SMPTE
time code is recorded on the highest-number track: 8 for an 8-track deck, 24
for a 24-track. But some 8-track decks allow for SMPTE to be recorded on an
additional "ninth" channel, without having to "waste" an
audio channel.
Laser shows are often presented in planetaria, and the International
Laser Display Association adopted their own "standard" for storing
audio and data signals on an 8-track ADAT tape. Unfortunately, they put the
audio on the highest numbered tracks, 7/8 -- and if the ADAT doesn't have
the 9th channel SMPTE capability, the time code gets recorded on track
6!
|
| ILDA 8-track |
| Track | Contents |
| 1 | Red |
| 2 | Green |
| 3 | Blue |
| 4 | X |
| 5 | Y |
| 6 | (SMPTE) |
| 7 | Left Audio |
| 8 | Right Audio |
| 9 | SMPTE |
|
Things really fall apart when we move beyond stereo, to multi-channel
sound. Here we have 4-channel systems, 5-channel systems, 5.1 surround
systems, and discrete 6-channel surround systems. Of course, they all have
incompatible channel assignments for the audio tracks.
5.1 Surround
(DVD/home theater) |
| Track | Contents |
| 1 | Left Front |
| 2 | Right Front |
| 3 | Center |
| 4 | Low Freq./Effects |
| 5 | Left Rear |
| 6 | Right Rear |
| 7 |
|
| 8 | SMPTE |
|
5 channel
(Sky-Skan) |
| Track | Contents |
| 1 | Center |
| 2 | Left Front |
| 3 | Right Front |
| 4 | Left Rear |
| 5 | Right Rear |
| 6 |
|
| 7 |
|
| 8 | SMPTE |
|
6 channel
(IMAX) |
| Track | Contents |
| 1 | Left Rear |
| 2 | Left Front |
| 3 | Center |
| 4 | Right Front |
| 5 | Right Rear |
| 6 | Top |
| 7 |
|
| 8 | SMPTE |
|
We like the 5.1 format, because it keeps left/right pairs on odd/even
assignments. Audio mixing consoles all put their track assign controls such
that the left side of the pan pot points to tracks 1, 3, 5, 7, etc. -- and
the right side 2, 4, 6, 8, etc. So with the 5.1 assignment, to send a
signal to the left side (front or rear), you point the pan pot to the left.
You can't do that with the other formats; mixing becomes much more
difficult.
IMAX is a registered trademark of IMAX Corporation
|
