Since LTC is a digital signal, the volume parameter is very important for it. The volume in analog format sets the digital amplitude between the logical zero and one, therefore we should be monitoring the SMPTE output signal from the sound card very carefully.
The operational level of LTC is from 0dB to +2dB, if the signal level is turned down significantly lower, Timecode will either not be detected at all by the receiving devices, or it will not be stable.
Often due to neglect, the working level of the sound card in the system is turned down on the working computer, causing not only the signal of the audio channels but also LTC drops. Add to this generation from a built-in sound card, an audio splitter or an audio console that divides the LTC signal and you will get a pretty “battered” LTC signal that can give you a number of sync problems.
The SMPTE graph is not sinusoidal, like an audio signal, it is square, because the signal is encoded in a binary system. At short distances, the type of signal is not critical. With large lengths of audio cable, factors such as cable capacity and its induction appear. What is bad? By the fact that the square waveform becomes sinusoidal and the phase of the signal ascent is shifted. This threatens with the fact that a strongly modified SMPTE code will simply not be readable by the receiving device or will not be stable and some frames will be lost, and as a result of this the synchronization will not be correct.
As we know, LTC is a digital signal that transmitted over an audio interface that connects by using balanced audio cables.
The principle is exactly the same as with DialUp modems. We all remember the electronic sound when the modem dialed to the server. Since then the home internet has changed but LTC has remained the same. Transmitting a digital LTC signal over an analog audio channel entails a number of features.
The same thing happens if the LTC signal is passed through an audio console or an audio splitter. Many “experts” do not fully understand the fundamental difference between the audio signal and LTC. It is naive to believe that you can work with the SMPTE digital signal in the same way as with a standard analog signal, which is a gross mistake, since LTC is a pulse signal with completely different characteristics.
For example, if we send an analog signal and LTC with the same level, SMPTE will “sound” two times louder. And a normal LTC level will be received as overloaded on a sound deck, which is normal, because it is a digital signal. In such cases, the “specialists” turn down the level of the generated LTC signal, thereby lowering the quality of the line noise immunity.
Since the LTC signal can be played as an audio track (initially this signal was originally recorded and played back from a magnetic tape), this LTC signal also dependents on the playback speed. If the playback speed is higher or lower than the original LTC, then the data flow rate will also change. Which effects the quality of synchronization. Some devices can compensate for minor changes in playback speed, but if the LTC speed of the timecode goes beyond the permissible limits, the device begins to lose frames, or determine the wrong time, which in any case leads to a loss of synchronization.
Now a couple of clause. Above, I mentioned the detrimental effect of using analog audio splitters on an LTC signal.
However there is an exception that exists which is the audio spitters. They do not really harm the LTC signal we can identify that splitters are only experimental by using the timecode generator and oscilloscope.
But again, this is more rare than the rule, since no audio equipment manufacturer calculates its characteristics for transmitting a square LTC signal.
The following clause, for work with LTC it is also possible to use some digital audio consoles. Of course, there may be some confusion, since this again contradicts the above information about sound consoles.
Now in the new digital era, more and more sound analog systems are being replaced by digital. Despite the fact that both analog sound and digital, perform the same task, they work fundamentally in different ways. In digital audio consoles there is almost no analog audio path, which introduces distortion in the digital LTC signal. And also, the working dynamic range of the audio channels of digital consoles is much higher than in fully analog models, which is also important for LTC signal.
To find out what digital consoles are suitable for work possible only practically and again with an oscilloscope, which can show the states of the digital LTC signal. Since it is a well-known fact that there are a number of medium-budget models of digital consoles, which like to enhance sound in their own way, which is again not acceptable for LTC.
When I work on projects, even if I know that the console that the sound department uses is suitable for working with LTC, I still try to avoid using it. Because, unfortunately, I cannot insure against the “clumsy hands" of the sound engineer, who may accidentally apply FX processor on my LTC channel or switch to the preset, where my LTC channel is closed.
In this case, the question of responsibility arises. The task of the sound engineer, that the sound in the venue should sound correct, this is his main priority and if anything goes wrong with it, he will think about the LTC signal at the last moment.
Therefore, I always insist that only the equipment that I and my team know is involved in the synchronization system. In this case, the responsibility for the reliability of the system will only be on me.
To summarize and once again define the factors that determine the quality of the LTC signal
* Low LTC signal. The digital LTC signal dependents on the playback or generation volume. The normal LTC signal range is from 0dBu to + 2dBu.
* Use for amplifying and dividing the LTC signal, sound equipment. Especially sound splitters and sound mixing consoles. Which distort the square waveform into a sinusoidal sound form.
* Changed LTC audio track playback speed, which is different from the original speed at which the timecode was recorded.
After that, the question arises, what then should we use to divide and amplify the LTC signal? At first glance it may seem that violations of the rules for correct work with LTC are caused by the lack of appropriate technical solutions on the market, but this is not so. Since LTC is one of the oldest protocols in the direction of synchronization, manufacturers also worked on creating special equipment for working with this interface.
We will examine this part of working with LTC in the next chapter.