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Chapter 15, Tutorials and Applications
ALESIS M20 REFERENCE MANUAL 1.06 15-7
TUTORIAL 3: RECORDING DIGITAL AUDIO FROM
OTHER SOURCES
Recording digital audio onto the M20 from a source other than an ADAT-compatible device
requires a digital audio interface, like the Otari UFC-24 or Kurzweil DMTi, or inserting an
Alesis EC-1 AES/EBU interface into the M20’s Expansion Card slot. These convert other
digital audio formats to the Alesis Optical format. The ADAT-PCR card can also output
audio recorded into a hard disk editing program as ADAT Optical format-compatible
signals.
When recording digital audio into the M20 from another ADAT-family device in a multiple
ADAT system, the M20 recording the audio is already synchronized with the other machines
because all slave machines are automatically set to External Clock mode. They follow the
master precisely.
When recording digital audio from some other source, the M20 must synchronize its clock to
the incoming digital audio. How this is done depends on whether you are using a single M20
or a multiple ADAT system. We will assume an ADAT Optical interface cable is carrying the
incoming digital audio, perhaps from an ADAT-PCR card.
With a single M20, connect the incoming Optical Out (e.g., from the ADAT-PCR card) to the
M20’s Digital In using a single fiber optic connector. Press the M20’s Clock Source button,
and set its clock source to Optical. The M20 will synchronize to the clock information present
in the Optical Out.
In a multiple ADAT system, you have two choices: synchronize the digital audio source to
the ADAT system (as described above for a single ADAT), or sync the ADAT system to the
digital audio source.
If the source does not have a clock input (i.e., it cannot sync to an external clock), you need to
choose the latter option. Connect the Optical Out to the master ADAT’s Optical In, whose
Optical Out already connects to the next slave, and so on. Set the master’s Clock Source to
ADAT (or I/O card if using the AES/EBU interface). If no digital audio clock is detected, the
master M20 will continue using its own internal clock. When the master receives digital
audio (and therefore, a digital clock), the master M20 will sync to the incoming digital clock.
DIGITAL CLOCK CONSIDERATIONS
Switching between the M20’s two internal clock sample rates (48 kHz and 44.1 kHz), changes
the reference used for computing tape location. Therefore, the same tape location will appear
to have two different time addresses depending on the selected sample rate.
Since the M20 cannot detect an external clock’s sample rate, you need to choose the reference
base the M20 should use to calculate tape position while syncing to the digital clock. Select
either 48 kHz or 44.1 kHz with the Sample Rate button.
If you hear clicks in the audio after recording from a digital source, either the Clock
Source is set incorrectly, or there is some other timing problem. Most commonly,
clock problems are the result of trying to use SMPTE Chase and receive digital input
simultaneously, where the digital source itself is not chasing the same SMPTE or
clock reference as the M20, or the SMPTE isn’t referenced to digital clock. There
must be one, and only one clock source in a multi-machine system.