Kurzweil DMT interface, User Manual, Page: 34 / 44

DMTi 34
This is caused by the way the
DMTi
handles certain data in the digital audio
stream. In the AES/EBU digital word is a set of bits known as the “channel
status field” which contains, among other information, a flag denoting the
sample rate of the signal being transmitted. This can be set to “44.1”, “48”, or
“unknown”. When the
DMTi
’s Clock Select switch is set to Int 44.1K , this flag
in the output signal is set to “44.1”. When Clock Select is set to Int 48K or to
KDS
, this flag is set to “48”. With any other settings—
AES1
,
AES2
,
MDM
, or
WDCLK —this flag is set to “unknown”. Some receiving devices may not
accept a digital input with an “unknown” flag, and will not record or lock to it.
Alternatively, some devices, when they receive a signal with an “unknown”
flag, assume that the sample rate is at a particular value (say 48 kHz) and if the
incoming signal is at the wrong value (say 44.1 kHz), it will not record it.
If this situation occurs, use one of the “known” settings for Clock Select :
Int 44.1K or Int 48K . If that is not possible, contact the manufacturer of the
unit in question for assistance.
Clicks and Pops
These occur when trying to combine two digital streams that are out of sync,
or when trying to convert a digital stream on the fly. The
DMTi
deals with out-
of-sync streams, but if you are sending data from the DMTi to a device that has
trouble with external synchronization (or doesn’t allow it at all), you can
experience problems of the latter kind. First of all, make sure that the receiving
device is set up to lock to incoming digital data, if possible. Then, check that
the
DMTi
’s output sample rate matches the input sample rate of the device in
question—often you will find that the device has no trouble working with an
external 44.1-kHz clock, but will have a very difficult time if it expects 44.1
kHz and the incoming signal is 48 kHz!
If you are still having trouble, you may have to lock the
DMTi
to the receiving
device. Many AES/EBU devices generate a valid signal at their outputs even
when there is no audio playing, and this signal can be used as the
DMTi
’s clock
master. For example, the output of a DAT deck that you are recording to can be
looped back to the
DMTi
through one of the AES inputs (as long as you’re not
using that input for anything else), and the Clock Select switch can be set to
follow it. Be sure not to route the audio signal from the device’s output back to
its own input, or you will get a feedback loop.