The sound industry also uses a balanced 1/4 Jack connector, which also has three pins for transmitting a balanced signal. Some manufacturers also use this connector for transmitting, receiving LTC, which is quite acceptable.
Standard sound cards with mini-Jack outputs are not suitable for working with Longitudinal Timecode, due to the fact that the sound outputs of these type of cards are not balanced. To understand why this is important, let's examine the basic principles and differences of balanced and unbalanced signal type.
The unbalanced signal is transmitted over two wires: one wire is a signal, the other is ground. This method of signal transmission is extremely easy to implement, but is not able to resist interference at the physical level.
Despite the fact that this synchronization protocol is one of the oldest, it raises the most questions and lack of understanding among people. Below I will describe the principles of generation and what requirements for the equipment and connection makes LTC.
To generate the LTC signal, you need a sound card with balanced audio outputs or an LTC timecode generator. In this article, we will analyze the first type of equipment for generation. Later, I will describe the essence of working with professional LTC equipment.
The most common connector that is used for LTC is the XLR audio connector. This connector has three pins, which is necessary for transmitting a balanced signal, and the connector has a latch lock, which makes it more reliable.
In comparison with unbalanced connection, a balanced one has two main advantages, due to its technical features.
The first advantage is the ability to transmit analog and digital signals without significant distortion over a considerable distance.
The second feature, which is closely related to the first one, is the best noise immunity in comparison with the unbalanced connection.
Not balanced connection allows you to get a stable signal at distances no longer than 15-20 meters, then with a balanced connection, the actual line length can reach 200 meters and more. This does not mean that we can transmit LTC for the same distance as the audio signal. There is a number of conditions that will not allow us to do this.
If we do not take into account these conditions, then we can transmit via the balanced line, without the LTC signal loss at a distance of no more than 30-50 meters.
Now let's take a closer look at the conditions that can or can’t allow us to transmit LTC over a balanced line at a distance of more than 50 meters.
To understand the main idea, let's look at the transmission of an ordinary audio signal to a distance of 200m. The operating level of the audio signal is usually 0dBu (averaged value), respectively, when transmitting a signal via cable, we expect to receive the same signal level at the output of the line. When transmitting a signal to 200 meters, we get a significant loss of signal level.
To restore the audio signal at the end of the line, it is necessary to raise the volume of the sent signal to such a level that it can compensate for the signal loss at a distance of 200 meters. For this you need to use equipment that has good properties of the dynamic range of audio outputs.
If the equipment that sends the signal does not have the necessary power, then if you try to raise the signal level above the equipment's capabilities, the audio path of such a device will simply be overloaded and as a result the signal will be distorted.
another option, if it is not possible to raise the signal level at the beginning of the line, then at the end of the line use a device with a high sensitivity of the inputs. Such devices can easily recognize the signal with large losses and amplify it to the required level.
To understand what the parameters of the dynamic range and sensitivity I mean, I will give the characteristics of the sound card MOTU 8A, which is in the professional segment of such cards and whose parameters are quite impressive.
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Now in different equipment that works with timecode, a special algorithms have already been introduced to identify the most distorted timecode. Previously, this was not. And it was not necessary to transmit LTC over long distances.
If you want to be sure of the quality and reliability of the generated timecode, then always use external professional sound cards with balanced outputs.
As I promised, in the next article we will touch on the topic of specialized equipment which allows not only to generate timecode, also to properly split it and, if necessary, restore it.
Now let's imagine the same situation, but with the LTC signal.
For example, we have a synchronisation server that provides LTC and let’s say a device that must receive an LTC signal via a balance line.
Not one light console, not one end-client synchronisation card has the necessary sensitivity parameters of its LTC ports to receive LTC through a line of 200 meters or more! And besides, we will get on top of the attenuated LTC square-shaped distortion signal since the cable of such length has impressive size of its own capacitance and indicators .
Under these conditions, we can not transfer LTC to 200 meters and even more so!
We have two ways to remedy this situation.
The first way. As in the case of sound, we need to raise the level of the generated signal equivalent to the line loss. To do this, you must either use professional sound cards with good properties of the output dynamic range, or use timecode generators / controllers that allow you to adjust the levels of signals on LTC ports separately.
The second way is to install at the end of the line, a sound card with good sensitivity properties of the input ports to enhance the LTC signal to a working level, or to install an analyser / reshaper at the end of the line that can accept a low LTC level, correct all its distortions, raise it to the working level and give back to customers. Later in the next article we will analyze the functionality of one of these devices.
In general, I advise you to use both of these paths simultaneously, then the reliability of the system will be much higher.
Now we discover that it’s absolutely not easy to transmit LTC via 200 meters of the balanced line. LTC is not sound and it requires more attention.
Most external professional sound cards have balanced outputs that allow you to work better with the transfer of Longitudinal Timecode, also these cards allow you to fully work with two or more audio channels and still work with LTC on individual channels, which is not possible with integrated two-channel audio cards
The balanced signal uses three wires for transmission: HOT and COLD signals (direct polarity and polarity reversed) and the third is ground wire. The electrical resistance of the signal wires in relation to the ground is balanced (i.e. equal), which is reflected in the name. Signals on the two wires of the balanced line are transmitted in antiphase with equal amplitude relative to the ground.
The audio operational amplifier at the balanced input of the device turn back the inverse signal and then adds it to the direct signal. As a result, interference, equally induced on the two phases of the line, is subtracted, and the useful signal is doubled in amplitude
The LTC synchronization interface was originally designed for use at TV studios, where the most professional equipment is still used today.
The Longitudinal Time Code was not designed to transmit through a low-quality audio path and certainly not through a home audio card and mini-Jack connector. It was designed to work with professional balance inputs and outputs, which all professional audio and video systems were equipped with.