Hah, I found a PDF in German detailing a similar idea:
http://www.explicitlabs.de/wp-content/F ... verter.PDF
Full Size:
http://i.imgur.com/sEnZrNa.jpg
Video Converter
RGB / YPbPr Converter
By Christian Tavernier (F)
Since the appearance of high-definition television (HDTV), it has been almost certain that the analogue device connections will be replaced by the digital interface HDMI or another digital standard in the long term. But before this is done, the old analog technology will continue to exist for a while.
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On many SAT and cable receivers, which do their duty today, a digital HDMI output is not yet available. The picture and sound signals are routed to the TV set via the old trusted SCART cable. Modern HDTV flat-panel units and HDTV projectors often have a so-called component input as well as several HDMI inputs. The jacks labeled "YPbPr" are sometimes labeled "YUV", but this is not exactly correct.
The SCART outputs of most consumer devices have separate video signals of the primary colors red, green and blue (RGB). Although these signals are not HD signals, the quality is clearly superior to the S-video and, in particular, the composite signal. Unfortunately, the signals red, green and blue can not be connected directly to the inputs Y, Pb and Pr of a TV set or projector. For the adaptation, the trade offers signal converters, which cost about 100 € and more with good quality. The converter described here achieves at least the same quality, it is built up quickly and the costs are only a fraction.
Red, Green and Blue
On some TVs or beamers, the Y, Pb, and Pr inputs are falsely red, green, and blue. Nevertheless, these inputs must not be connected directly to the outputs R, G and B of a SCART output. The Y is a sum signal from the weighted signals R, G and B. The signals of the inputs Pb and Pr are so-called color difference signals, they are also formed from the signals red, green and blue.
The rules, according to which the signals R, G and B must be added or subtracted, are, of course, no secret. For this reason, the design of a signal converter from RGB to YPbPr is not difficult. For the addition or subtraction of analog signals, operational amplifiers (opamps) are suitable, although not every opamp type is equally suitable. With the well-known TL081 the success is only moderate, because its bandwidth is not enough. The frequency spectrum of analog video signals can easily reach the 100 MHz mark, especially when the video signal comes from an HD signal source. Opamps with low bandwidths would have a deleterious effect on the video signal, the contours would be blurred. An opamp family designed for this purpose has been marketed by Linear Technology under the LT1398 and LT1399 types. The bandwidth of these opamps is 300 MHz, the amplification is almost linear in the range 0 ... 150 MHz (deviations less than 0.1 dB). Linear Technology manufactures these opamps only in SMD design, but the SMDs are still so large that they can be soldered by hand with a fine soldering iron and point-shaped tip.
Converter Circuit
Our video signal converter is based on documents and circuit proposals by Linear Technology [1]. The converter operates with four opamps, which are integrated in an LT1398 (IC1) and an LT1399 (IC2). The LT1398 is a double opamp, one half remains unused. As the circuit diagram shows, Opamp IC1A adds the weighted signals R, G and B to the luminance signal Y, the calculation rule Y = 0.3R + 0.59G + 0.11B. The factors in this formula are realized by the resistors R4... R7. For the signal R the gain is equal to the resistance ratio R4 / R7, for G is the gain R5 / R7 and for B the gain is determined by R6 / R7. The resistors R1 ... R3 at the inputs cause the input impedance to be 75 Ω. Values of input impedance which deviate from the standard value may result in signal distortions.
From IC1A, the inverted luminance signal passes to IC2B where it is inverted again and amplified by a factor of 2 (= R13 / R8). This gain is necessary because R16 at the output of IC2B causes an attenuation by a factor of 2 together with the input impedance of the connected device.
Signal Pr must be formed according to the relation Pr = 0.71 (R-Y), and Pb = 0.56 (B-Y) for Pb. Since these equations are identically constructed, they are realized with opamp circuits according to the same pattern. For signal Pr, IC2A is switched as a subtracter while IC2C has the same function for signal Pb. The signals at the outputs of these opamps depend on R9 and R10 (or R14 and R15). The output signal of IC2C is equal to 2 · (B - Y), while the signal 2 · (R - Y) is at the output of IC2A. The factors 0.56 and 0.71 are formed by the voltage dividers R11 / R12 (R17 / R18) and the input impedances (75 Ω) of the connected device.
The opamps operate with a symmetrical operating voltage of ± 5 V. The center tap on the low-voltage side of the mains transformer is grounded so that a bridge rectifier can supply the positive and negative voltage. In both branches voltage regulators and capacitors ensure stability at the outputs. LED D1 indicates the operating condition.
Build Up
Although the resistance values given in the circuit are neither of the E12 nor the E24 series, the values are standardized. If they are not available at the component retailer on site, they can be ordered together with the LT1398CS and LT1399CS, for example, from RS Components or Farnell (privately via
http://www.hbe-shop.de). It is important for the order that the versions of the LT1398 and LT1399 with Suf x "GN" are not usable. The design belonging to this Suf x does not fit the board, because of the smaller design it is no longer possible to solder by hand!
The board [2] was designed in such a way that all cables are confined to the absolutely necessary lengths. The input and output sockets are not located on the circuit board in order for the printed circuit board to be accommodated in a housing of your choice. The SMD opamps (IC1 and IC2) have their place on the PCB bottom (copper side). A fine soldering tip with a point tip and a steady hand are indispensable for soldering. All other components are fitted as usual on the PCB top.
When mounting the resistors, the right value is at the right place. The resistors are characterized by more color coding than usual because of the high accuracy. In order to avoid confusion, it is recommended to measure with the Ohmmeter before soldering. The jacks, which are to be installed in the housing, should be of the "RCA" type, because in the case of consumer video systems, cinch sockets and RCA plugs have become the quasi-standard. This applies not only to the video signals, but also to the audio signals that belong to them.
Connect
After the circuit configuration, the converter is immediately ready for operation, settings do not have to be made. The converter inputs are connected to the RGB outputs of the controlling device, there is no more to be done.
Sometimes no signals are present at the RGB outputs of a SCART socket. As a result, no or faulty signals appear at the converter outputs Y, Pr and Pb. In this case, the controlling device must be set via its configuration menu in such a way that the RGB signals are connected to the SCART connector. SCART connectors are provided for RGB, S-video and composite video signals, but depending on the device configuration, only the composite signal is present. In many models with two SCART connections, in particular with satellite receivers, the signals R, G and B are only connected to one SCART terminal. Whether this is the case, a SCART connection change can show quickly.
This is a well-known unit
