Displaying Pictures on Mechanical TV Sets
We use three techniques to display pictures on our mechanical sets. First, we have a mechanical camera that generates 60 line pictures for display on our RCA set. Visitors to the museum can see their friends on this setup.
The second method uses the Aurora universal scan conveter developed by Darryl Hock. We use it for our Western Visionette (15 fps, 45 lines) and our Western Empire State (18 fps, 45 lines)
Finally, we use a PC to generate images for our Baird Televisor (30 lines):
Using a PC to Generate 30 and 60 Line Pictures
This is a technical description of how low definition video is generated to allow the mechanical sets in our collection to work.
First, a video is produced using a standard camcorder. Care has to be taken with the subject matter, because only closeups of faces, etc. will show up on the low definition sets.
Then, the video is played back into a video capture card in a PC. Snagit, a capture program, then converts the video to an .avi file. For the 30 line Baird standard, 12.5 frames are captured per second. For the 60 line standard, 20 frames are captured per second.
The .avi file is then converted to individual .bmp files, one per frame, by a program called AVItoBMP.
A Visual Basic program is then used to read each .bmp file and convert it to 30 or 60 line standard. The 30 line standard has vertical scanning lines, while the 60 line standard has horizontal lines. The video is recorded on the right channel. For synchronization, sync pulses are recorded on the left channel (more on this later).
This program then creates a .wav file which contains the data from each frame sequentially. This .wav file is played by the PC using the Media Player.
The right channel output of the sound card (video) in the PC is connected to one channel of a 40 watt stereo amplifier which it turn feeds the neon tube in the receiver. An interface circuit is necessary to provide the DC bias for the neon and to increase the voltage swing of the video to the neon.
Synchronization is different for the 30 line and 60 line systems. For the 30 line system, a sine wave at the line rate (375 Hz) is placed on the left channel. This wave is amplified by the other channel of the stereo amplifier, which feeds the phonic coils on the receiver.
The 60 line receiver relies on a synchronous motor for synchronization. Since it is difficult (if not impossible) to synchronize the PC sound card output to the power line frequency used by the synchronous motor, a 60 Hz sine wave (3 cycles per frame of the video) is placed on the left channel to drive the motor.
The sine wave is then amplified by the second channel of the stereo amplifier. The output of the amplifier is then fed to the motor through an impedance matching transformer.