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LTC-Disp

Longitudinal Time Code Display

German Version: LTC-Disp

(Still?) no finished device, but at least a finished development

Content

Introduction

Longitudinal Time Code - also called Linear Time Code - is a method to transmit time information by means of a pure audio signal. It is not about clock times, but about the time that has passed since the beginning of a recording or a broadcast, for example. It is used in studio technology.

There are devices which can generate and display different LTC signals very flexibly. But they also cost a lot. I designed the LTC-Disp as a simple, pure display of the time codes, based on a suggestion from the PA sector (more precisely: the lighting department, which also works with it). There is no finished device available, at least so far it is only a finished development!

Granted: This photo is a photomontage, there is no finished housing. A single complete set of front and back panels with display contrast filter screen, milled and engraved as shown here, is simply too expensive for me.

Technical background information

The LTC is related to the Vertical Interval Time Code (VITC), which has the same purpose for video signals. For this reason, fractions of seconds are not given in e.g. milliseconds, but in frames. A frame is a picture in film or a frame in television. Different standards define different numbers of frames per second. In the case of film, 24 frames per second, in the television systems commonly used in Europe 25, and in the USA and other countries almost 30 frames per second. This is of course not interesting for pure audio productions. What is set there as frames/s is more or less a matter of taste. In Europe, 25 frames per second are probably common.

LTC and VITC are standardized according to EBU Tech. 3097-E, which I have in the version of November 1985.

Basically the LTC uses the Bi-Phase Mark Code with 80 bits per frame for the data. At 25 F/s this results in 2000 bit/s, i.e. one edge every 0.5 or 0.25 ms. So the bandwidth of the fundamental frequency is 4 kHz, but the harmonics of the square wave signal should be preserved for a safe decoding.

The content of a data packet or frame, summarized in a simplified way: The time information is transmitted in BCD code for 8 digits (hh:mm:ss:ff). This requires less than half of the 80 bits. The other bits are used for synchronization and for a few control and monitoring bits. The majority of the rest can be used freely by the user. More detailed information can be found at https://en.wikipedia.org/wiki/Linear_timecode .

Features of the LTC-Disp

Display:

• 8-digit 7-segment display, white, 8.5 mm digit height
• Hours - minutes - seconds - frames
• With or without leading zeros (00 03 42 17 or     3 42 17) selectable
• With or without decimal points (00.03.42.17 or 00 03 42 17) selectable
• Brightness adjustable
• LED "SIG" Green: A signal is present
• LED "ERR" Red: The signal cannot be decoded correctly

Connections:

• Audio: XLR balanced, male and female, for looping through
  Signal level approx. -23 dBu to +23 dBu (50 mV_rms to 10 V_rms)
  Input impedance 25 kΩ
• Power supply: low voltage socket 5.5/2.1 mm
  8 - 15 V, 170 mA max.
  

Decoding:

• Longitudinal Time Code according to EBU Tech. 3097-E with approx. 1500 to 2500 Bit/s

Housing:

• 55 x 105 x 38 mm³ (L x W x H)

A view (or insight) into the inside of the LTC-Disp