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Appendix E, EC-1 AES/EBU Interface
EC-1 MANUAL 1.00 E-7
SELECTING AES/EBU (I/O CARD) AS THE DIGITAL SOURCE
When using digital inputs, the Digital Source function selects one of three digital input
options. The I/O CARD option will appear in the display only if an I/O card (such as the
Alesis EC-1 card) is installed in the M20. With the EC-1 card installed, pressing the Digital
Source button (next to the meter display) toggles through the available sources, such as
ADAT OPTICAL, TRACK COPY, and I/O CARD. To select AES/EBU as the digital
source, press the Digital Source button until I/O CARD appears in the DIG SOURCE group
of the meter display. Now, when Input Select mode is set to digital input for a given
channel pair (and dependent on how input routing is configured), an AES/EBU input will
be the source for the selected channels.
SELECTING AES/EBU (I/O CARD) AS THE CLOCK SOURCE
If a word clock, video, or optical clock source that is synchronous to AES/EBU digital
audio input is not available, sample clock for the M20 can be derived from any of the four
AES/EBU input channel pairs.
TO SELECT AES/EBU AS THE CLOCK SOURCE:
1. Press the Clock Source button (under the main display) until I/O CARD appears in the
CLOCK SOURCE group of the main display.
2. When I/O Card is selected, the alpha-numeric in the main display will indicate, “AES
Clk Source: Chan 1/2”. This implies that the M20 will look for clock input at the
channel pair 1 and 2 XLR input. If AES/EBU clock input is not detected, the I/O
CARD indicator will flash and the M20’s internal clock will be used until valid
AES/EBU input is detected.
3. To select the AES clock source, use the Up/Down arrows on the keypad. The
selections are Chan 1/2, Chan 3/4, Chan 5/6, or Chan 7/8. As noted earlier, if recording
multiple AES/EBU pairs, their clocks must be synchronous to avoid “ticks” and “pops” or
audio distortion. If you have multiple synchronous inputs (for example, from a single digital
mixer), it does not matter which channel pair is chosen as the clock source.
Example: Suppose you set the digital source to I/O CARD, and set the input selection
for channels 3 and 4 to digital (shown by the “D” illuminated under those channel
meters on the M20). Input 3/4 on the EC-1 card is the only jack connected to an
AES/EBU digital audio source (like a DAT recorder). If no other AES/EBU-
synchronous external clock source is available, I/O CARD must be selected as the
clock source. But when you do this, the clock source I/O CARD indicator flashes,
and audio monitored via the M20 is distorted. Why?
Because there still is not a valid clock. The M20’s default AES/EBU clock source is
channel pair 1/2. But in this example, channel pair 3/4 is the only AES/EBU input
present, so the M20 tries to use its internal clock. Since there is no connection
between the M20’s internal clock and the internal clock of the DAT feeding it in this
case, the numbers that make up the digital transfer are arriving in the wrong
positions, causing the distortion. The cure? Simply set the AES clock source to
“Chan 3/4" (or whatever channels you’re using), or plug the AES/EBU source into
channel pair 1/2.