Increasing cable length in precision video applications is a problem for you? This article will help you!
The solution described in this article proposes as the physical environment the use of a simple twisted pair cable; in addition to its lower price (compared to the shielded coaxial cable) it also features a much smaller weight, which sometimes can be a critical design criteria, especially for instance, in automotive environments, where cable weight account for the most percentage of the mass of a car.
Usually the regular configuration of a video transmitting system consists of an output amplifier (which either amplifies or acts as a simple repeater) mounted in the part of the system which generates video signal, connected via a shielded coax cable to the receiver part of the system. At the receiver, the signal is filtered, then amplified again, and fed to the display or perhaps to a video processing unit (i.e. DSP).
Figure 1 – Usual configuration for transmitting video on coax cable
As stated above, using this configuration has a significant impact on both the price and the weight of the system. A much more advantageous solution, from these points of view, may be achieved through the use of a differential amplifier at the transmitting side, which would thus be able to confer a good degree of immunity to the video signal with respect to outside influences.
The Video Driver is shown in Figure 2. It converts a single-ended input signal from a camera or DVD player into a differential signal that drives the twisted-pair line. The input receives an NTSC composite video signal with 1VPP amplitude, and the output drives the twisted-pair with a 2VPP differential signal. A 50Ω source resistor is in series with the outputs of both op-amps, matching the Video Driver output resistance to the twisted-pair characteristic impedance.
Figure 2 – Two polarity supply differential transmitter
In the receiver circuit of Figure 3, R16 is adjusted so that the overall gain of the system is unity (gain of the last op-amp is greater than one to compensate for signal loss). C1 and R14 provide a zero-pole function that compensates for attenuation of higher frequency signals in the twisted pair cables.
Figure 3 – Two polarity supply differential receiver
This is a fine and good solution, however, its main disadvantage is that it needs a negative powers supply. Most digital systems do not include such a supply, as generally there is no need for it, and introducing it only for the purpose of transmitting video cheaper might prove to be working against this particular goal, through the additional pricing required by a negative supply both in the video receiver and in the video
These disadvantages may be eliminated through the use of a single-supply configuration. We will detail this architecture more, as it is a realistic one, and involves only common components (discrete components and op-amps). Basically, what needs to be done, is to generate two “differential” signals. They are differential with respect to some specific positive voltage, which generally may be half of the supply voltage of the operational amplifier, in order to allow full use of the linear characteristic most op-amps have at VDD/2.
As such, the video driver in Figure 4 performs both a summation and a subtraction operation. The upper op-amp, adds the video signal to a constant voltage, called VBIAS, and the lower op-amp subtracts the video signal from the same constant voltage, yielding two differential signals, that will be used by the receiver to rebuild the original video signal.
Figure 4 – Single supply differential transmitter
The two 50 ohm resistors are used for impedance matching with the video receiver. All other resistors are used to implement the two arithmetic operations with the amplifiers. Generally, for good results, these should be 1% tolerance or even lower (0.1% is preferred).
The common waveform of the NTSC video signal is shown below (roughly corresponding to a color bar, but only with the luminance information).
Figure 5 – Luminance NTSC color bar
As stated, the purpose of the above circuit is to transform this signal in two signals which are symmetrical with respect to the VBIAS voltage, which in this particular case is 1.65Vor exactly half of the supply voltage of the op-amps. The VBIAS voltage can be easily generated with a simple voltage divider from the 3.3V supply voltage followed by a simple repeater op-amp. The repeater op-amp may be skipped if small value resistors are used to generate the biasing voltage, but this will adversely affect the current consumption of the overall circuit.
Figure 6 – Symmetrical signals transmitted over the twisted pair cables
The green trace illustrates the output of the upper op-amp, the red trace illustrates the output of the lower op-amp, and as we can see, the 1.65V (blue trace) is exactly between them. The “green” and “red” signals are the ones fed to the video receiver, using twisted pair cables. Their symmetrical configuration renders them too immune to external voltage noise, as the differential amplifier (Figure 7) which is to be found in the video transmitter makes the simple difference by subtracting the red signal from the green signal. Thus, as in any differential signal transmission case, the external voltage influences will be void.
Figure 7 – Single supply differential receiver
Besides making the difference, the receiving differential amplifier also needs to perform some kind of amplification, so that the total gain will be (again) unity. This is simply achieved with a few resistors around the op-amp, resistors which again, need to be at least 1% tolerance. Figure 8 illustrates the reconstructed video signal against the initial single ended video signal which was fed to the differential structure in Figure 4.
Figure 8 – Initial video signal (above) and reconstructed video signal (below)
The circuits in Figure 4 and Figure 7 transmit and receive NTSC video signals over twisted pair wire. They were designed and tested for transmitting video on inexpensive CAT-3 twisted pair wire. Even when transmitting on 300m of wire, good quality color video was displayed on a monitor with an NTSC input. Both transmit and receive circuits use the LMH6643 dual op-amp which has the proper bandwidth and slew rate for this application.