How Adat Optical Handles A 96 Or 88.2K Signal; Using The Hd24 With An External High Sample Rate-Capable A/D Or D/A Converter; The Optional 96K Analog I/O Boards - Alesis ADAT-HD24 Reference Manual

high sample rates • chapter 7
How ADAT Optical handles a 96 or
88.2k signal
The ADAT Optical "lightpipe" was designed to
send eight streams of 24-bit digital audio data at
standard sampling rates. When it is used for high-
resolution audio, it isn't "sped up" but instead uses
two channels to send the data for one track,
sending it at standard speed with some special bits
set in the code that tell the receiver to decode it as a
high-speed 4-channel interface instead of a
standard-speed 8-channel interface. Track 1 sends
odd-numbered samples down channel 1 of the
"lightpipe" and even-numbered samples down
channel 2. Track 2 uses channels 3 and 4, Track 3
uses channels 5 and 6, and Track 4 uses channels 7
and 8.
This allows compatibility with mixers and older
ADATs that can't handle the higher sample rate.
As far as they're concerned, they're seeing two
different 44.1 or 48 kHz signals; if you listen
through them, they'll sound basically identical. But
devices that can read the User bit* in the ADAT
Optical subcode, identifying it as a high-sample
rate signal will automatically decode it as four
high-sample rate signals.
* There are some ADAT Optical devices that
are 96/88.2k capable, but which do not set
or read this user bit. These can still work with
the HD24, but the HD24's sample rate
indicator will flash to show that a possible
inconsistency has been detected.
However, even though a recorder or mixer can
read these pairs of ADAT signals, you may not be
able to work with these files. For example, if you
record a stereo 96kHz recording to four tracks of
Digidesign's Pro Tools® , cut and paste the tracks
and play them back into the HD24, the tracks
should record back in fine. However, if you try to
add a crossfade, EQ these tracks, or process the
tracks in other ways, you won't be able to record
them back into the HD24 at 96kHz.
66
Using the HD24 with an
external High Sample Rate-
capable A/D or D/A converter
The HD24 can be used to record the output of a
A/D converter which converts at 88.1 or 96kHz.
To record a digital signal at
96kHz:
1. Press
NEW SONG
sampling rate.
See page 32 for more information on creating a
new song.
2. Plug the ADAT Optical output of your A/D
converter or digital mixer into the Digital
Input of the HD24.
3. Press the
INPUT SELECT
is set for Digital input.
If there is not a 96k Analog Input board installed,
the only choice will be Digital.
4. Press the
CLOCK SOURCE
display reads Optical.
5. Press the Record Enable buttons on the tracks
you wish to record on, then press
The optional 96k Analog I/O
boards
Alesis offers an optional set of converter circuit
boards which, when installed, allow you to
record and play back at 88.1kHz or 96kHz using
the 1/4" analog inputs and outputs. Without this
option, 88.1k and 96k recording is available only
through the digital inputs.
For more information on the 96k Analog Input
board, contact your Alesis dealer or visit the
Alesis website (www.alesis.com).
and select "96.0k" as your
button until the HD24
button until the
.
RECORD
ADAT HD24 Reference Manual