Virtual Televisor Interface Construction

Rather than attempt to describle all aspects of the construction of the Virtual Televisor interface on this page, the link above will let you download a complete manual that contains parts lists, circuit board diagrams, construction information, and alignment instructions. The manual is in PDF format and, if you do not have a copy of the Adobe PDF Reader software, you can get a free copy from the Adobe Systems website. The manual will be updated regularly based on reader feedback. The current version is dated 03 Feb 04.

Virtual Televisor PC Board

For those with access to commercial plotting and board production services, a complete set of  Gerber Files  in ZIP can be downloaded here. One of the unique aspects of this circuit board is that wiring to be board can be done in one of three ways:

• Use of a standard edge connector

• Use of header terminal strips and socket

• Direct connection of the mainframe wiring to the board.

Housing the Interface

Here the circuit board for the interface is housed in a relatively large cabinet. I intend to use the extra space to add an internal speaker and a few other features as time goes by. In this case, a header socket has been used to interface the mainframe wiring to the board. The rear apron, facing the camera, contains (from L to R) the video input, audio input, and external speaker jacks, the coaxial power connector (9-12V DC), and the DB25F parallel port connecrot. The front panel has the AUDIO GAIN and VIDEO CONTRAST controls, the POWER toggle switch, and a POWER LED indicator.

Parts Availability

Virtually all of the parts for this project can be ordered through DigiKey or major suppliers elsewhere in the world. The construction manual has a complete parts list, but the following notes may be useful with respect to the more specialized components:

• TTL Logic. U, U2, and U5 are identified at LS-family TTL devices. Virtually any TTL family (HCT, etc.) would do as well (provided the specific devices are available). However, you should try to keep the devices within the same family - in other words, don't mix families!

• A/D Converter. The MAX153CPP A/D converter (U4) has no replacement. If your distributor does not stock the device or it is back-ordered, you can often get it directly through the MAXIM  web site.

• Video Op-Amp. U3 is a moderately-fast op-amp that features industry-standard pin-outs for 14-pin quad stage devices. If you cannot get the MAX494CPA, you can try other devices, such as the LM324N or others. Note that garden-variety devices will have bandwidth limitations that may degrade video quality. However, there is nothing to stop you from using a marginal device at first, upgrading to a better chip once everything is working.The biggest limitation of using most op-amps is that they may not work well when operated at 5V. In such a case, the op-amps can be powered from the 9-12V bus. The input protection network for the A/D should prevent any damage from over-driving the device if you do run the op-amps on a higher voltage. If you do use a higher voltage, the bias resistor network on pin 6 of U3B will require modification to hold the inverting input at ~2.7-2.8V.

• Audio Amplifier. U6 is an extremely common low-power/low-voltage audio amplifier IC that is universally available.