RCA Model TM-10A
Color Video Monitor
By: A. H. Lind,
Mngr. Audio Projectors and Systems Engineering Group
Broadcast News #77, Jan/Feb 1954
One of the units of Color Television equipment which holds high interest for engineers and laymen alike is the Color Television Monitor. The RCA TM-10A Color Television Monitor is designed around the RCA 15-inch Tricolor Kinescope. It is capable of reproducing the color picture from a composite Color Television signal.
FIG-1. Cabinet mounted RCA TM-10A Color Television Monitor. The monitor may also be mounted in a standard 19" cabinet racks or another housing of the customer's choice.
The monitor is basically a chassis and front panel unit which can be housed in a cabinet, as illustrated in Fig. 1, mounted in a standard 19-inch cabinet rack by use of rack mounting hardware which is also available, or mounted in a housing of the customer's choice. To facilitate grouping when housed in cabinets, the cabinets have flat tops and are unobstructed on the top and sides. This permits placing monitors immediately adjacent to each other and/or stacking vertically as conditions require. Forced ventilation is accomplished by means of an exhaust fan near the top on the back of the cabinet and an intake vent at the bottom of the cabinet back. This vent also serves to provide ready access to the signal and power input connectors as well as several set-up controls. On the front panel there are eight operating controls positioned between the kinescope and a hinged cover panel which is located near the bottom. With the cover panel open the set-up controls located on the front of the chassis are accessible. Thus direct viewing of the kinescope is possible while making these set-up adjustments.
FIG-2. Unretouched picture make directly from the face of an RCA Color Monitor. In makeing the plates for this illustration every care was exercised to obtain a reproduction which would give a true impression of the monitor picture. Model is Marie McNamara "Miss Color TV" as seen by RCA Color Studio Cameras in NBC's Colonial Theater Studio.
The tricolor kinescope is a directly-viewed, glass envelope, three gun, shadow mask type shown in Fig. 3. This tricolor tube incorporates important differences from conventional monochrome kinescopes. First, instead of a uniformly coated phosphor screen, the color tube screen is composed of a regular pattern of small, closely spaced phosphor dots. These dots are arranged in triangular groups deposited very precisely on a glass plate support (see Fig. 4). Each group (or trio) consists of a red-emitting dot, a green-emitting dot and a blue-emitting dot. The phosphor screen contains 195,000 dot trios or 585,000 dots. The viewing screen provides a picture size of 11-1/2 inches by 8-5/8 inches high with rounded ends. Next, in the color tube there is a shadow mask. It is positioned parallel to and immediately behind (gun side) the phosphor screen as shown in Fig. 4. The mask contains the same number of small apertures as there are color dot trios on the phosphor plate. They are about the size of one phosphor dot and are located on the axis of their corresponding trio. By arranging the tube's geometry properly, an electron beam approaching the shadow mask at a slight angle from the line to the center of deflection, will land only on a single color in any one of three rotational positions 120 degrees apart. In order to obtain precise alignment between the apertures in the mask and the phosphor dots, the mask and phosphor dot plate are mounted together in an assembly. The mask functions to provide color separation by shadowing two of the three arrays of phosphor dots from each of the electron beams, while exposing the proper array to excitation by each beam.
Third, the color tube contains three electron guns. These parallel, closely spaced guns provide independent electron beams for excitation of each of the three phosphor dot arrays. The guns are controlled by the appropriate red, green and blue video signals supplied by the output stages of the color video channel.
FIG-3. RCA 15" Tricolor Kinescope around which the monitor is designed.
In operation the three beams are deflected by a common scanning field and must be made to converge at the aperture corresponding to the dot trio being scanned at the moment. This convergence of the beams is accomplished by an electrostatic lens system that causes the beams to bend away from their individual gun axis toward a common axis of the gun array. To aid in the uniformity of convergence, the Pure Yoke (Fig. 4), when properly energized, positions the three beams symmetrically with respect to the common kinescope axis as the beams leave their respective guns. A given combination of potentials will cause the beams to converge at the mask apertures in the center of the phosphor plate ares. However, because the shadow mask and the phosphor plate are flat, the distance from the center of deflection to the mask apertures varies as a function of deflection which makes it necessary that the focal length of the converging lens be made to vary as a function of the deflecting angle. The potentials required to maintain proper convergence vary a! function of the deflecting angle. I dynamic converging is accomplished applying voltage derived from the vertical and horizontal deflection circuits to result in a varying potential applied to the converging electrode.
FIG-4. Cross sectionaldiagram of the tricolor Kinescope showing an enlarged section of the shadow mask and phosphor dot screen.
The three beams are electrostatically focused. As in the case of converging the beams, the beam path length from the center of deflection to the shadow mask is a function of the area being scanned. Thus the focus potential must also be varied as a function of scanning. A voltage similar to that obtained for dynamic convergence is applied to the focus electrode to accomplish the dynamic focusing.
When properly focused and converged the beam from one gun sees only dots of one color no matter which part of the phosphor plate is being scanned. Thus, three primary color signals controlling the three beams produce independent pictures in the primary colors. To the eve these pictures blend because of the close spacing of the dots, and as a result the eve sees a full-color picture.
A simplified block diagram is shown in Fig. 5. To aid in understanding the circuits the blocks are grouped into five sections: Kinescope, Video Section, Color Sync Section, Deflection and High Voltage Section and Low Voltage Power Supply Section. Since the tricolor kinescope has been discussed, no further details on this section of the block diagram will be mentioned.
After passing through a wideband video amplifier the signal is fed into two channels. One is the luminance or brightness (monochrome or M) channel and the other is the chrominance (color) channel. The circuit functions accomplished in the chrominance channel result in two signals I and Q being recovered. The 1, Q and M signals are combined in a resistance mixing matrix circuit to produce R, G and B signals. (See Page 19, J. W. Wentworth's "RCA Color Television System.") These signals are then further amplified in the video output amplifiers and supplied to the appropriate tricolor kinescope control grids.
The monochrome channel includes a low pass filter, a delay line, an aperture compensator, and video amplifier stages to provide the required signal gain. The low pass M filter is carefully designed to provide the desired cut off at 3.58 megacycles, the subcarrier frequency, and maintain good phase response up to the cut off region. This is desirable to suppress moire patterns and color contamination in the reproduced picture. The delay line is required to make the signal delay through the M channel the same as that by way of the Q channel so as to affect a proper time match in all signals at the matrix inputs. The aperture compensator, which is of the constant delay type, provides adjustable high frequency peaking that is very effective in enhancing the fine detail structure in the picture.
The chrominance channel includes a bandpass filter, low pass filters for the I and Q paths, I and Q signal demodulators, a delay line in the I signal path, plus video amplifier stages to provide the necessary signal gain, and signal phase splatters to provide two I signals 180 degrees out of phase and two Q signals 180 degrees out of phase. The band pass filter serves to restrict the signal to the subcarrier and its sidebands, which contain the chrominance information. This band limited signal then is fed to the I and Q demodulators. The demodulators are two synchronous detectors operating in quadrature. The subcarrier signal is processed in the color sync section to provide the two signals phased at 90 degrees with respect to each other for one of the signal inputs to each of the demodulators while the other input in each case is the chrominance signal itself. The I signal out of the demodulator passes through a low pass filter (1.5 mc.) to insure that no direct signal transmission through the I demodulator occurs, and then through a delay circuit. The delay is required to make the signal delay through the I channel the same as that by way of the Q channel so as to affect a proper time match in all signals at the matrix inputs. Finally the I signal is split by means of a phase splitting stage into two signals 180 degrees out of phase which are utilized in the matrix section. Similarly, the Q signal out of its demodulator passes through a low pass filter (500 kc) to sup . press extraneous demodulation products. Because of this narrow band pass the total signal delay through the Q channel is inherentlv greater than through either the I or M unequalized channels. Thus it is used as the reference and delays are inserted in the I and M channels to insure time matching of the signal components at the matrix section. Finally, the Q signal is split bv means of a phase splitting stage into two signals 180 degrees out of phase which are utilized in the matrix section.
The matrix section is a passive network of fixed resistors that establish the ratios by which signals from nine input terminals are added to subsequently appear at three output terminals. The input signals are M, -1, I, -Q and Q signals. The circuit functions to transform the M, I and Q signal components into red, green and blue (R, G and B) components which describe the identical color signal in terms of the color primaries of the system. The R, G and B signals are then amplified to a level suitable to operate the respective electron guns in the tricolor kinescope. D.C. restoration is accomplished at the grids of the kinescope.
Color Sync Section
The color synchronizing circuits serve to supply the continuous quadrature phased subcarrier signals to the color signal demodulators from the reference color sync burst present on the back porch of an FCC Standard signal. The signal, after amplification which emphasizes the sync and burst portion, is gated to permit only the burst information to pass. The gating action is controlled by a pulse obtained from a winding on the horizontal output transformer. The burst of subcarrier signal is then used to excite a 3.58 mc. crystal which continues to oscillate or "ring" during the remainder of a line, thus providing a continuous 3.58 mc. sine wave. This wave is amplified, clipped and then split into quadrature phased signals in a special transformer.
Deflection and High Voltage Section
Scanning synchronization information is separated from the composite input signal after amplification and used in conventional fashion to synchronize the vertical and horizontal oscillators. External sync may be used if desired by switching to the external sync mode of operation and supplying the external sync signal to the proper input terminal at the rear of the chassis. The circuits for generating the scanning deflection fields are conventional, thus a functional description is not included here. However, the high voltage supply and convergence circuits are new and merit further consideration. The high voltage supply is a "kick-back" type that shares the horizontal output transformer with the horizontal deflection yoke. One rectifier circuit supplies the final accelerating voltage for the kinescope and a second voltage from a voltage divider that provides the d.c. component of the beam convergence signal. The kinescope high voltage is regulated by means of a shunt regulator tube. The action of this regulator tube is arranged to complement the load of the kinescope and thus maintain a constant load on the power supply. A second rectifier circuit supplies an adjustable output for the kinescope focus electrode.
The signal to accomplish dynamic convergence is generated by shaping the pulse trains taken from the vertical and horizontal output circuits, mixing them and then adding the mixed signal to the d.c. convergence voltage mentioned above. A similar voltage is added to the d.c. focus voltage to provide dynamic focus of the electron beams.
Low Voltage Supply
The plate and bias voltages are supplied by a conventional unregulated selenium rectifier power supply. If the 115 volt power source is not well behaved a line voltage stabilizer may be beneficial in maintaining freedom from line surge effects in the picture.
FIG-5. Simplified block diagram of the RCA Color Television Monitor.
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