Thursday, December 23, 2010

Getting the Smoothest Teleprompter Scrolling

This article will tell you how to  use TeleScript AVtm in a dual VGA monitor configuration to be sure your client sees the smoothest scroll on the teleprompter monitor. This discussion is a fairly advanced, technically – but if you’re a teleprompting professional, these are things you really need to know!

Artifacts – what are they?

Video artifacts are, basically, anything in the display that is not intended. Artifacts may include hum bars, ghosting, and other periodic glitches.

HUM BARS are periodic “stripes” that move from top to bottom or vice versa. They are caused by “hum” induced into the video by interference from the power supply causing a “beat” with the vertical sweep in the display monitor. Common sources of hum bars are poor grounding, inadequate or broken shielding on coax cables, and running power cables in parallel and too close to signal cables. Hum bars can be hard to track down, but are generally the easiest artifact to diagnose and understand.

GHOSTING is a shadow of the primary image appearing usually to the right or left (or both). Ghosting is usually caused by poor cables, impedance mismatch, or cable runs that are too long. The problem can usually be solved by replacing cables, or reducing their length. Often times, a line driver or distribution amp can solve ghosting problems.

The Artifact unique to teleprompting – Frame Sync Compression Bars

Even with the demise of CRT monitors, one powerful commonality remains: the manner in which each frame is synchronized with the display. A compression bar is an artifact that is caused by lack of synchronization between the computer and the display monitor is, while not completely unique to teleprompting, is still most troublesome and clearly viewable with a clearly delineated, steadily moving text display.

The computer maintains a “frame buffer” which is generally an area of physical memory (RAM) provided in the user’s video card.  The frame buffer is where the visual display is created by the system. The application programmer uses whatever tools are available to create the desired visual image in the memory provided. The video card then converts the frame buffer to signals that are meaningful to the display device where the digital signal become an image -- and our eyes do the rest.

The images created in the frame buffer are released at a steady rate – generally 60 fps (frames per second) or 70 fps. Today’s monitors actually “converse” with the computer system and can operate at a variety of frame rates. For teleprompting it has been common to adhere to the established NTSC frame rate of 60 fps to maintain compatibility, though this is NOT required in VGA (non-composite) systems.

Frame sync compression bars occur when partially formed frame buffer are displayed – we see the top part of one frame, and the bottom part of another. When this process is repeated, the “break” point generally moves toward the top or bottom. The visual effect to our eyes is similar to a hum bar – a line which moves up or down the screen. Frame sync compression bars are possibly the hardest artifact to understand, and certainly the hardest to tame in the world of teleprompting.

In the paragraphs that follow, I’ll explain how the computer using vertical and horizontal scan to refresh pixels in the display. I’ll also describe how the computer’s VBI (vertical blanking interrupt) is used by the video card to prevent frame sync issues. Finally, I’ll tell how to set up TeleScript AV in a way that will totally avoid frame sync compression bars.

How the computer establish sync with the display – the basics

Just as we were saddled with the low resolution of NTSC TV for many years because of compatibility issues, we’ve also inherited some baggage from early CRT (cathode ray tube) computer displays. CRTs create a display with a single moving electron beam emission which moves in a very systematic path across the face of the picture tube. There, the electrons strike a “phosphor” coating which glows from the electron bombardment.

Composite Video as described by the National Television System Committee in 1953, consists of a single modulated signal which combines not only the value for the intensity of the moving electron beam, but also several signals to synchronize the end users display to the originating device. The display is divided into 30 Frames Per Second – each frame consisting of 2 interlaced fields of 262.5 each. In actual practice, the frame rate is very slightly below 60 fps – about 59.9 fps. (This explain why 60Hz power signals cause predictable moving hum bars in NTSC CRTs.)

Interlacing fields make small text harder to read. Thus, computer systems flatten the interlaced pattern and generally display every vertical line on every field which makes them, effectively, 60 FPS.

The CRT electronics contain sweep generation circuitry which moves the electron field smoothly from the left to the right, and from top to bottom. Each time the beam has completed on horizontal line, in order to prevent an unsightly retrace line, the signal is blanked. A signal is sent in the composite source which tells the display to shut off the electron beam and to move the electron gun back to screen left. Horizontal Sync tells the display that it’s time to start the beam moving screen right again. (See the diagram below.)

Likewise, when the electron beam has reached the bottom right corner of it’s travel, the signal is blanked and moved back to the upper left of the screen. This is known as vertical retrace. When it’s time for the beam to start its travel again, a Vertical Sync signal is sent. The period time during which the electron beam is blanked while moving back to the top of the screen is called the Vertical Blanking Interval or VBI.

So, what does this have to do with modern displays and teleprompting?

Who knows what system computer engineers might have come up with if not for the constraints of CRTs and NTSC video. But when LCDs displays became commonplace, it was still required to maintain compatibility existing CRT displays. Modern video cards still use the same system… horizontal and vertical retrace, vertical blanking and all.

Particularly of interest is the Vertical Blanking Interval. The designers of video editing equipment learned that all edits would have to be performed during VBI. Otherwise, you’d see part of one scene on the top of the frame, and part of another scene on the bottom of the frame. The resulting glitch is very distracting and is not acceptable to video producers. Vertical Interval Switching is used on all modern video editing and switching equipment.

In teleprompter displays, the scrolling text is the equivalent of scene changes. The illusion of moving text is created by displaying successive frames which are displaced one scan line up or down from the previous frame. This glitch is exacerbated by the fact that it will be repeated with a frequency determined by the scrolling rate and the teleprompter program’s drawing rate. To get the smoothest scrolling, the display of successive frames must be synchronized with the VBI.

So what’s the problem – TeleScript AV is VBI synchronized, isn’t it?

True… all TeleScript software is VBI synchronized. At least, it is if the computer reliably reports the Vertical Retrace. Here’s where problems sneak in.

There’s a scan converter between the computer and the teleprompter display monitor.

In this case, the VGA signal may be resampled before it’s converted to composite form. If the VGA signal is 1024x768, for example, some horizontal lines must be dropped and interpolated. If the VGA is runnin gat 60 Hz, then an occasional frame must be dropped to maintain the required 59.94 NTSC vertical frequency. But must specifically, the VBI of the display device is no longer the same as the VBI reported by the computer’s video card. This is why it is impossible to guarantee that there will be no frame sync compression bars when using a scan converter! Removal of artifacts is entirely in the realm of the scan converter – not the application program!

So… staying away from scan converters in general, what’s source of the problems in VGA mode?

Dual video cards

I recall my early conversations with ATI, nVidia, and other designers such as Paradise who have long since departed from the competitive arena. I asked for laptops that could simultaneously display on an external monitor and the internal screen. The problems involved in this design task are not, by any means, trivial, and the solutions were a long time in coming. However, virtually all notebook, netbook and laptop computers now feature simultaneous display.

End of problems - end of story --  right?

No… not at all. Beginning of new problems.

How portable computers create simultaneous display

Most – I’d say ALL, but that leaves room for error - modern computers that feature simultaneous display have a video with TWO video controllers. These controllers can either share a frame buffer (clone mode) or they can divide video RAM and have their own frame buffer (extended desktop.) Each of the two video controllers is truly independent and the respective VBI of each controller is not related or synchronized. This is GOOD because it allows you to use different manufacturer’s displays, different screen resolutions, etc. If the external video were just an electronically buffered “Y” connection, this would not be the case and no one would be happy with the result.

Clone Mode is problematic, as far as Vertical Interval Sync to the external display. Clone mode is established by a means that’s dependent on the manufacturer and the creator of the driver software. In some schemes, the video card itself uses the two controllers and frame buffers independently. It “clones” the display by “blitting” (quickly copying) the contents of primary display’s frame buffer into the frame buffer of the secondary display. In other schemes, the video controllers are allowed to share the frame buffer, scanning at an independent rate. The “blitting” scheme actually is capable of fewer artifacts – because a wise hardware/driver designer can be specify that the blitting is performed and displayed with Vertical Interval Sync… from both controllers. However, this is not frequently the case. Though individual manufacturer’s schemes are seldom public information, it would appear the virtually all either use the shared memory concept, or use a non-synchronized blitting scheme.

In clone mode, frame sync compression bars appear on the external monitor because the video is READ by the secondary video controller. However, it’s WRITTEN and synchronized to the VBI of the primary controller. Consequently, the secondary controller displays the frame buffer as it is being refreshed, resulting in a frame sync compression bar on the display.

Finally, the best bet for avoiding frame sync compression bars: use the Extended Desktop mode with TeleScript AV.  The application senses which video controller is servicing the external display and synchronizes with that VBI. This inquiry is done each time you move the window, so it’s generally transparent to the user. In extended desktop mode, using a VGA-DA to feed the prompter, and with a confidence monitor, the display will be synced with the external video controller’s VBI. Use the primary display for Runlist, auxiliary on-screen dialog controller, timer, Find-Replace dialog – whatever will help you to be the best operator.

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