HD24/96 Technical Reference

101

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.

 

Gain-Adjusted

SMPTE In

SMPTE Out

 

 

 

Regenerated /

Distorted

 

Reshaped

SMPTE In

 

SMPTE Out

 

 

 

 

 

 

 

Time Code Reader /

 

 

Generator

Small Mixer or Line

Distribution Amplifier