Video emitter uses battery power

Edited by Bill Travis and Anne Watson Swager

JM Terrade, Clermont-Ferrand, France -- EDN, 8/30/01

The block diagram in Figure 1 shows how to make a cable-free, direct-video system. The system allows users to walk from booth to booth at an exhibition to interview people and to display the interviews in real time on three screens at key locations. You can use the small and simple system each time you need to capture image and sound on the run. Figure 2 (pg 96) shows a detailed schematic diagram of the video system. The system provides no stereo audio but rather mixes together right and left sources. IC_2A acts as an inverter/adder. At Point C, the ac signal represents the sum of the left and right channels: V_C=-R₁(V_A/R₂+V_B/R₃). With the same value for the three resistors, V_C=(V_A+ V_B). C₈ and C₉ block any dc voltage at points A and B. IC_2A works from a single 12V supply but needs a continuous bias voltage to provide a positive and negative swing around 6V. R₄ and R₅ create a 6V bias source for both IC_2A and IC_2B. IC_2B acts as an inverting voltage amplifier with a gain of P₁/R₈. With the values shown, you can adjust the gain as high as 4.7. You can adjust P₁ for audio gain as high as 13 dB. C₁₀ blocks the 6V dc at Point D, so only the ac audio voltage is present at the audio input of IC₄. The LM358N works well from a single supply, but when the output voltage is close to 0V, it needs some help to avoid signal distortion. Pulldown resistors R₆ and R₇ minimize the distortion.

IC₄ is a video-emitter IC from Aurel (www.aurel.it) that works in the UHF band at 479.5 MHz (UHF Channel 22). Its output power is 2 mW to a 75Ω antenna, A₂. Typical supply current is 90 mA. The signal from the videocamera connects directly to IC₄'s input. If you need more power, you can add IC₃, also from Aurel. This IC works in the same frequency band as IC₄ and boosts power to 19 dBm in the 75Ω antenna. Both IC₃ and IC₄ are available in small, single-inline packages. A Switching Level signal from Pin 8 of the SCART video connector is present when the videocamera is on. The current consumption of IC₃ and IC₄ is 90 (5V) and 100 mA (12V), respectively. To reduce power consumption, a dual-contact relay, K₁, connects IC₃ and IC₄ to the supplies only when the camera is on. When the videocamera is off or disconnected, the supply current decreases to only a few milliamperes.

IC₅ and C₄ through C₇ provide a 5V supply to IC₄. IC₁ and C₁ through C₃ provide a stable 12V supply to IC₃ and the LM358. You can connect a 12V battery directly to J₁. IC₃'s data sheet specifies a supply level of 11.4 to 12.6V, but tests show that the IC works properly if the supply is higher than 11V. The total 200-mA current consumption yields approximately three-hour battery life with 12 AA cells. If you use a switching power supply, you would obtain longer battery life, but you need to take filtering measures to avoid interference with the video path. If an ac outlet is available, you could use an 18V, 300-mA wall adapter to replace the battery.

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