Wibbly Wobbly Waveforms


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The very first analytical electronic instrument, developed in the late 1890s, was the oscilloscope. This used a cathode ray tube (CRT) to paint a graph of voltage on the Y axis versus time on the X axis. Once television became a practical reality in the 1930s, the same instrument was applied to the video output from the camera and became the very useful waveform monitor.

Early television cameras were delicate and temperamental beasts so a waveform monitor was an essential tool for examining picture levels at every part for the programme production and transmission chain. PAL or NTSC video signals are sensitive to a variety of transmission or recording nonlinearities and problems such as un-terminated cables. Every analog studio feed has to be precisely timed to ensure it stays in sync. Constant checks have to be made using the waveform monitors to ensure all the timing adjustments were perfect. Colour television gave the waveform monitor many new roles as even more adjustments were needed to make analogue transmission work properly.

Digital picture information is relatively robust and does not usually get corrupted at each regeneration or link. Precise signal timing is no longer crucial; every equipment input is normally capable of re-timing itself correctly or even has an integral synchroniser. CRTs have vanished from waveform monitors and are increasingly rare in the home but modern digital test instrument using LCDs or rasterizers look much as they did in the early days.

So why would you want to look at luma (picture brightness information) portrayed on the same analogue style instrument?

Modern cameras are essentially analogue, even after the big switch to digital transmission, and they still incorporate an automatic iris. Add in the human operator variables and there is always every chance for the picture to be less than optimal, particularly when matching cameras. There could also be differences in lenses, optics and sensors. Incorrect gamma settings can give rise to crushed blacks for instance. All can be easily spotted on a waveform monitor. There are many additional special monitoring modes to help with colour balance, such as the RGB parade. This simply shows all signals side by side or overlaid on a common timebase for easy comparison.
The most common timebase for monitoring is 1H: one horizontal scan line period. Usually all the frame lines are displayed overlaid in this mode so only horizontal features stand out. Most instruments can also alternatively show a frame-based timebase but traditionally still portrayed on the same horizontal time axis. Some unconventional instruments have a waveform frame view display turned though 90 degrees. This view makes for a better correspondence with the actual picture analysed.

Transmission problems that could give rise to blocking and quantisation errors also exist in the digital world. Transcoding and remixes can also affect the dynamic picture range. Waveform monitor displays are therefore still very much in use but no longer in a separate box like the old analogue instruments. Today they are always combined at least with a vectorscope. These features can be integrated into the monitor itself. The displays are in most cases only an indication of the levels with a crude graticule.

Fully featured test instruments with calibrated displays can provide much more detail and the ability to switch graticules and scaling as well as homing in on a region of interest in the image. These can be still found in a stand-alone box with or without a built-in screen. More often the waveform can be brought up on the computer monitor as a tool or plug-in to an editing or grading package. These too can have limitations. Sometimes their speed is well below real-time or the is size quite small due to the limitations of running within the editing software.

For real-time conventional 2D images, the Cel-Soft Reel-Check Solo uses PC software image processing that is fast enough even for 4K as well as 2K/HD monitor windows in multiple views, audio or other displays. And there is no special hardware. This type of software product can replace the rasterizer or stand-alone box.

For 3D TV production or post, the Cel-Scope3D software creates high-precision waveform monitors which can be combined so that left and right eye image luma or RGB are superimposed for comparison and matching.

A waveform monitor is very similar in concept to the oscilloscope in many ways. Digital processing has now completely done away with vacuum-tube technology but still provides the familiar waveform styles that are easy to understand and can provide a comforting consistency in a studio suite or on-set.



Robin Palmer is Managing Director of Cel-Soft and is habitually involved with solutions for 3D & TV technology.

Tags: iss070 | cel-soft | cel-scope3d | waveform | monitoring | vectorscope | N/A
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