MACKIE HDR24/96 - Manual (Page 200)

HDR 24/96
transport synchronizer that is genlocked to video. In this scenario, the synchronizer reads time code
from the multitrack and regulates the speed of the tape transport so that the time code coming off the
tape is resolved to the video signal. The synchronizer works just like the cruise control mechanism
on your car: it monitors the tape’s playback speed and regulates the capstan motor to keep the tape
moving at a constant speed. A typical setup using a synchronizer looks like this:
It is important to note that MTC should not be used when resolving sample clock to time code; LTC
is the preferred choice. This is because the LTC signal has much greater timing resolution than MTC.
LTC is a continuous signal that has a timing resolution of 80 subframes per frame, effectively
producing 2400 “ticks” per second for 30 fps time code (20 samples per subframe at 48 kHz). On the
other hand, MTC is a non-continuous computer data signal that is sent out in “packets” 4 times per
frame. The timing of these packets is often jittery and irregular due to the nature of the hardware that
generates MTC. Resolving to MTC produces a jittery sample clock that degrades the performance of
any A/D and D/A converters that are slaved to it.
Another important point to note is that because MTC is sent in ¼-frame packets, it is not possible to
achieve positional synchronization that is as tight as LTC. Whereas some devices may not get much
better than ¼-frame accuracy using MTC, some devices like the HDR24/96 are able to achieve
sample-accurate positional lock using LTC. This is because LTC acts very much like a clock signal
and LTC reader hardware can accurately latch on to the edge of an LTC subframe. The bottom line is
that when faced with a choice to use LTC or MTC, always choose LTC.
SMPTE LTC is an analog signal that can be distributed through any path and recorded onto any
medium that you would normally use to distribute and record analog audio. The HDR24/96 supports
two standard SMPTE LTC levels: -10dBV and +4dBu. The standard level for recording LTC onto
analog tape is –10 dBV. This insures that the signal is hot enough to be decoded by the LTC reader,
but not so hot to cause excessive crosstalk on adjacent tracks.
Always record SMPTE onto an edge track and if you project allows, leave the adjacent track unused.
This “guard” track will guarantee that no SMPTE crosstalk will be heard on the closest audio track.
Be sure to defeat noise reduction on the SMPTE track, as this will distort the signal. The +4 dBu
level is typically used when distributing SMPTE between devices. The SMPTE output level on the
HDR24/96 can be set to either –10 or +4 operation from the Sync Setup window. The HDR24/96 can
lock to incoming time code at levels within the range of –25 dBV to + 15 dBu.
When time code is recorded onto analog tape, the signal quality is slightly degraded. Some devices
may not be able to read SMPTE from third or fourth generation copies, especially if care was not
taken during the transfer process. In practice this happens often. You can generally suspect either a
SMPTE level or quality problem when the slave device stops and starts frequently, or simply will not
lock to SMPTE at all. You can use a small mixer or line distribution amplifier to change the level of
the time code signal going to or coming from a device. Be sure to defeat the EQ when you do this.
You can use a SMPTE regenerator to correct problems with signal quality. These devices are
designed to read distorted or otherwise poorly reproduced time code and either clean up the quality of
the original (called reshaping) or generate an entirely new SMPTE signal (called regenerating). It is a
good idea when copying analog tapes (or even digital tapes through an analog signal path) to reshape
or regenerate time code going to the copy. The picture below illustrates the use of both types of
devices.
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