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6 Display Adapters

There are three types of display adapters used in this project. First, there are two LCD shutter adapters, which carry or generate the control signal for the LCD shutter glasses. There is the RF modulator, which modulates the NTSC video onto a television channel, and finally the VGA converter, which converts the NTSC video format to VGA format for display on a VGA monitor.

6.1 Shutter Adapters

Two different adapters were developed for this project. These are devices that use vertical sync (or generate a signal like it) to create the alternating left/right signal to turn each LCD shutter panel on and off. Shutter glasses such as the ones used in this project have a small circuit that generates an AC signal that powers the LCD panels in the glasses. This circuit resides in a small adapter, which has three connectors: power jack, stereo audio jack, and a DB-25 parallel port connector. The stereo audio connector carries the AC signal to each panel. The DB-25 connector usually mates to the DB-25 female connector on the rear of a PC or laptop. The shutter adapters described here will present a signal to a DB-25 connector on pin 4, which is where the shutter circuit expects to find it. And because there is no PC available to adjust the time at which the shutter glasses change between left and right eye, something is needed to perform that function, which is where the shutter control circuit comes in.

6.1.1 TV adapter

This adapter consists of a DB-25 female connector, two short pieces of stranded wire, a DC power connector (same as the one used on AC-DC adapters - see the parts list below) and optional DB-25 backshell.

Since the signal needed to operate the LCD shutter glasses is readily available from the camera module, all that's needed to get the signal to the glasses is this adapter, which connects to the 3D-Spex adapter. The 3D-Spex adapter will expect to see the left/right signal on pin 4 of its own DB-25 connector.

A 22- to 24-gauge wire is soldered from the metal frame of the connector to the outside conductor of the power connector, and another wire is soldered between pin 4 of the D-sub and the center conductor of the power connector.

Figure 37 TV Adapter

Remember to use two different colors for the wires.

6.1.2 VGA Adapter Module

The VGA adapter generates the LCD shutter control signal directly from the video signal for those applications where the camera module is somewhere at the end of a radio link.

When the video is going to be displayed on a television set, the output of the radio receiver is connected to one of the RCA jacks on the VGA adapter. The video is sampled as it then travels to the RF modulator to be modulated onto channel 3 or 4, and used to generate vertical sync.

When the video is being displayed on a VGA monitor, the radio receiver output is first sent to a VGA converter. This device converts the composite video signal from the cameras into separate red, green and blue signals, and horizontal and vertical sync. The output of the VGA converter is sent to the VGA adapter by one of the 15-pin connectors, where the vertical sync is sampled as the VGA signals continue on to the monitor.

Once the vertical sync is generated from the received composite video, or taken from the incoming VGA video signal group, the shutter control signal is generated using virtually the same circuit as on the sync board.

Figure 38 VGA Adapter module

6.1.3 VGA Adapter Circuit Board

The VGA adapter circuit board provides three functions: vertical sync extraction from composite video, vertical sync source selection, and shutter control circuit.

The two RCA jacks make the normal connection between the radio receiver and the RF modulator, as well as make the video signal available so the vertical sync can be extracted. Likewise, the two DB-15 connectors provide a place to tap into the VGA signal path so the vertical sync can be available for the shutter control. Without these pairs of connectors, it would be necessary to fabricate special breakout cables with tiny surface mount printed circuit boards mounted inside the connector housings. I tried it, and it was messy. This is better.

Figure 39 VGA Adapter Block Diagram

A National LM1881 receives the composite video signal and generates the vertical sync (among other signals). The LM1881 is the sync separator box in figure 39. The vertical sync is tapped off the VGA signal group that's coming in one DB-15 from the VGA converter and out the other to the monitor.

The vertical sync signals from the LM1881 and the VGA signal group meet at J7.

J7 is a 3-pin header, with the middle pin (J7 pin B) connected to the input to the shutter control circuit. When a shorting plug is connected across pins A and B of J7 the vertical sync output of the LM1881 is used to generate the shutter control signal. When pins B and C are connected together, the vertical sync from the VGA converter is being used.

The shutter control circuit is virtually identical to the shutter control circuit on the Sync board.

The output of the shutter control circuit is sent to a DB-25 connector that mates to the one on the McNaughton shutter adapter (which generates the AC signal needed to operate the shutter glasses).

Connectors J1 and J2, as well as J3 and J4, are bi-directional. It is not required to use one connector as input and the other as output.

There is one or two circuits available on the Internet that illustrate the operation of the Nuvision adapter. In fact, it should be possible to use the circuit at http://www-wjp.cs.uni-sb.de/~jofis/shutter.html to combine the Nuvision adapter and the VGA adapter. The world could use one less adapter. However, that circuit would not able to adjust the timing of the shutter signal. It is usually assumed the LCD shutters are controlled by application software or graphics card driver adjusts the shutter timing.

Figure 40 VGA Adapter attached to VGA Cable

The ExpressPCB circuit board is 2.5" by 3.8", which makes it a "mini-board" special, qualifying for the $62 price for three boards. (The price was $59 when the project started.) As usual, for this price there is no silkscreen or solder mask.

The board is mounted on a 3" by 6" by 1/8" piece of clear Lexan (cut from the same piece as the camera module), held there by 4-40 screws, nuts and washes.

Figure 41 Schematic, VGA Adapter

The Digikey part, 929400-01-36-ND is actually a 36-pin header; Digikey doesn't provide the header 3 pins wide.

6.2 RF modulator

The RF modulator is used to modulate the video stream onto a television channel, usually either channel 3 or 4.

The one used in this project (www.jameco.com, P/N 141639) uses an F-type RF connector for output to the television. There are many varieties of the RF modulator. There are some that provide connectors for both input and output video as well as DC power. This $4.95 version will require soldering of video input and DC power connectors.

Figure 42 RF modulator

A VGA converter is also called a line doubler, or an up-converter. It has two basic functions: convert NTSC video to VGA format, and increase frame rate from NTSC's 30 frames per second to 60 fps for VGA.

It is beyond the scope of this book (to say nothing of my ability) to explain how NTSC is converted into separate red, blue and green signals, and horizontal and vertical sync, but line doubling is straightforward.

First, an NTSC video frame is made up of two fields, one containing the odd- and the other containing the even-numbered lines of the complete frame. A line doubler will convert a line of analog video into digital at the 30 frames per second rate, and store it in a buffer. Once the line has been completely received and stored, the buffer contents are read out and converted back to analog at double the incoming rate, and sent to the monitor. Then it will send it out again. For each line of video received, two identical lines are read out.

VGA converters are used to interface video games, VCRs, DVD players and even television to personal computer monitors. But this convenience is going to cost you in video quality, and we haven't even mentioned stereoscopic 3D.

As just explained, the converter will turn each field into a frame. This means each frame will have half the video content of the original frame. One result of this will be a "flickering" of lines that are almost perfectly horizontal: in the original frame, a line that traverses between two fields is now partly there in one frame, and partly not-there in the next.

Now we add stereoscopic 3D. The sync adapter board in the camera module is going to throw away every other field from each camera's output, and combine the remaining fields into a single video stream of alternating left and right views. This is presented to the VGA converter (or to the RF modulator for viewing on a television screen). Viewing 60fps stereo video on either a VGA monitor or television screen with LCD shutter glasses means each eye will see only every other frame, or 30 frames per second. Not only will there be a lot of flicker, but it could cause headaches.

Two different VGA converters were tried during the project. One was an incredibly cheap bare-bones device that got the job done, but had problems just staying in one piece. The other not only had better quality construction, but it turned out to be not that much more expensive, and had other features as well.

The minimum I recommend is the CM-330 sold by Avtoolbox. It is also available on the Web from other sources. This unit handles the basic converter function and also provides controls for adjusting brightness, contrast, color and tint, as well as other special effects.

Figure 43 AVToolbox's CM-330 VGA Converter, front view

Figure 44 CM-330 VGA Converter, rear view

6.4 Shutter glasses

There are three basic types of shutter glasses, based on their interface to the computer. These interfaces are called interlace, page flipping and sync doubling. And since at the hardware level page flipping is basically the same as interlace, we will consider them the same.

Shutter glasses that use interlace expect to get a signal that alternates high and low for each displayed video frame. The electronics required by the shutter glasses is minimal - usually just an oscillator that generates the AC signal for the glasses, and a left/right control generated by the incoming interlace signal. These glasses will be the cheaper models; not that there's less quality, but they are very basic in operation. Their advantage for generating live 3D video is that no computer, graphics card or programming is needed. All that's necessary is a small electronic circuit that converts the vertical sync pulse into an alternating left/right signal that can be used to control the LCD glasses.

Shutter glasses that use sync doubling will get a pulse from the computer that will have timing similar to vertical sync. The glasses will have a small circuit board that doubles the frequency of the pulse and then sends it to the monitor to be used in place of vertical sync. The picture starts out as two pictures compressed together. When the doubled sync pulse is received at the monitor it effectively doubles the refresh rate of the monitor. One half of the original picture is shown at a time on the display, and the shutter circuit will use the doubled sync pulse to create its own left-right signal.

The advantage of sync doubling is the shutter glasses do most of the work and the computer doesn't need to generate two different scenes. The disadvantage is it's more expensive because more electronics has to be crammed into the frame of the glasses, or into a dangling umbilical. Besides, it can't be used for generating live 3D video when using the method described in this book to create the stereo video stream.

The shutter glasses used in this project are the 3D-Spex from McNaughton (formerly Nuvision). These are probably the simplest, most basic shutter glasses. They are reported by McNaughton to be good for 60Hz to 160Hz frame rates.

The 3D-Spex has a cable from the glasses that terminates in a 3-wire stereo audio jack. This plugs into an adapter, which contains the electronics to operate the glasses. The adapter also has a jack for a 9V AC-DC power supply. The other end of the adapter is a male 25-pin D-sub connector.

Figure 45 McNaughton 3D-Spex shutter glasses

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