LED AM Video link
(for high-power LEDs)

About this project:

After messing with several different photodiode detector circuits, I decided to see if I could pass video through an optical-only ("lightbeam") video link.

The answer was:  Yes - but this is not the way to do it over longer distances - I'll get to this later.


Video-Speed optical detector:
Figure 1:
  Schematic of video-speed optical receiver.
Bottom:  As-built prototype of this receiver.
Click on either image for a larger version.
Small image of schematic of video-speed optical
                prototype of the video-speed RX

One of the most vexing things about using solid-state optical detectors (such as a PIN photodiode) is that you can either make such a detector very sensitive, or you can make it fast - but not both at the same time.  The main reason is capacitance:  It is the very nature of capacitance to resist a change in voltage - and high-speed operation clearly implies very fast changes in voltages over time!

For photodiodes, there are several approaches that one can take to improve the speed:
The works of P.C.D. Hobbs go into some detail about extracting higher frequency response from a relatively high-capacitance photodiode.  In his paper "Photodiode Front Ends - The Real Story" he details a circuit that takes several approaches to do this:

An adaptation of the circuit is shown at the top of Figure 1 and the circuit works approximately as follows:

The upshot of all of this is that the output voltage of U1 will be generally proportional to the amount of light falling on D1, the photodiode.  U2 and its associated circuity simply provides an "artificial ground" to avoid the requirement of a bipolar (dual-voltage) power supply.

Important note concerning the circuit in Figure 1:

The circuit shown in Figure 1 was designed only to be fast and quick to build and no attempt was made to optimize sensitivity and I have little doubt that it can be improved in terms of both bandwidth and sensitivity!

Figure 2:
  Schematic of video-speed modulator
  As-built (and operating) video modulator.
Click on either image for a larger version.
Small version of schematic of video TX
                version of the as-built video modulator with LED
Unlike self-amplifying photodetectors (such as photomultipliers or avalanche photodiodes) the ultimate sensitivity of PIN photodiodes is limited, in large part, by the noise contribution of the amplifier connected to the photodetector.  In this circuit, there are a number of noise-contributing components - namely the transistors Q1 and Q2, but note that the resistors (especially the 4.7k resistor in Q1's emitter circuit) can also contribute significant noise - so it is recommended that one use metal film resistors in the Q1/Q2/U1 circuits instead of the more-typical carbon-film resistors.  The resistors associated with U2 are not noise contributors in this circuit.

Finally, this circuit does not have sufficient gain to provide the 1 volt peak-to-peak video signal necessary to drive a typical video monitor under most conditions and additional amplification is usually required.  While this could have been done using a number of different circuits (another LM7171 or two, a UA733-based amplifier, etc.) I used what was available:  The "Channel 1 Vertical Output" from my old Tektronix 465B oscilloscope.  I simply connected the video monitor to the appropriate jack on the back of my oscilloscope and used the 'scope itself as a variable-gain video amplifier!


Video-Speed optical modulator:

Initial tests were done using an LED strongly driven by a function generator and with this setup I could verify that the circuit in Figure 1 did, in fact, have MHz-range frequency response, albeit with some rolloff.  With these encouraging results, I quickly (in 10 minutes or so) threw together about the simplest modulator that I could, the result being shown in Figure 2.

While this modulator works, it does not work very well as there are several things wrong with it:
As I mentioned, I threw this circuit together in just a few minutes just to provide a usable source of video-modulated light and while there are any number of ways in which this circuit's performance could have been increased, I won't mention them here.  Note that the driving method described is not capable of fully-driving the LED at the highest video component frequencies, hence some of the noted rolloff:  A better system would involve a "stiffer" driver as well as appropriate pre-emphasis designed into the transmit and receive system to maximize bandwidth at the available modulation depth - not to mention various methods to achieve reasonable linearity.

Performance of the entire system:

Despite its many faults I did succeed in sending video across my basement (a distance of about 20 feet, or 6 meters) using an LED and photodiode receiver with plenty of margin to spare.  For transmitting I placed the LED at the focus of a 250x310mm Fresnel lens to concentrate the beam (but losing at least half of the optical flux by not using a secondary lens) with the LED simply sitting atop of CD-ROM Jewel case as shown in Figure 2 (the Fresnel lens is visible in the bottom picture of Figure 3) and using a camcorder playing a pre-taped video segment as a video source.

The receive circuit (see the bottom picture in Figure 1 and the top picture in Figure 3) was attached to some scrap cardboard using double-sided tape and a simple lens holder was fashioned (also from cardboard) to hold a large Plano-Convex (PCX) lens such that the photodiode in the receiver was approximately at its focus.  Despite about 10dB of chroma-frequency rolloff, the pictures received were fairly good, perhaps on par with a VHS home videotape recorder.  In the top picture of Figure 3 one may see, in the background, the video monitor with the actual transmitted signal:  The white horizontal line on the monitor is from the pre-flash of the digital camera that I used to photograph the scene.
Figure 3:  The video gear in action.
Top:  The video receiver is shown being bathed in red light from the transmitter with the actual video being received being visible on the monitor in the background.
Bottom:  The video transmitter.  The video source is from a video camera in the foreground while the Fresnel lens (being illuminated by the LED) is in the background, being held up by some lead-acid batteries.
Click on either image for a larger version.

                video-speed receive in operation
                Video transmitter in operation

As previously implied it takes a bit of careful adjustment to get this system to work:  The setting of Rbias on the transmitter is critical for best bandwidth and linearity, as is the gain setting of the video amplifier (my oscilloscope in this case.)  I also noted that at this distance it was very easy to saturate the photodiode, so the transmitter was purposely mis-aimed to reduce the signal enough to allow proper operation of the receiver - but this implies that there was a bit of signal margin in the receiver itself.

Quick, back-of-the envelope calculations indicate that this system, using for the receiver the same-sized Fresnel as used on the transmitter (but optimized) and using a similar-sized Fresnel at the receiver would be capable of spanning a distance of at least 1 mile (1.6km) with usable results:  Probably much more with more circuit optimizing - assuming that going through that much air wasn't going to cause its own problems!

Why this method is not good for longer-distance through-the-air video transmitting:

Having said all of this, I would NOT recommend these circuits for video communications at a distance of more than several hundred feet under any circumstances.  If you ignore the fact that they are finicky (requiring careful adjustments of both the transmitter and receiver to work) and that their frequency response is rather poor (something that could have been remedied with a little more work) and that the LED could have been driven to more than four times the output available from this circuit (theoretically doubling its range) there is the matter of the effect of atmospheric scintillation on any amplitude-modulated signal.

Because the video signal was directly amplitude-modulated onto the LED, critical signals such as horizontal and vertical synchronization are present as amplitude variations.  Also, in order for a video signal to work properly with any typical monitor it must be held to fairly close tolerances plus or minus 1dB or (much!) better - for acceptable viewing:  Commercial and even normal consumer video levels are held to much tighter standards than that!

Even with medium-sized lenses on both ends it doesn't take propagation through much of an atmospheric path to experience scintillation that greatly exceeds 1dB and it is likely that typical atmospheric conditions along a near-to-ground path length of a mile or so (1.6km) would probably be pushing the limit.  Why?  With the fluctuating light levels, the recovered AM signal will also be fluctuating wildly:  These changing video levels would play havoc with the TV synchronization signals - not to mention affecting apparent brightness of different parts of the video picture.

While it is theoretically possible to implement various schemes to normalize signal levels at the receiver (such as sync-keyed AGC circuits, etc.) it would still require at least 25dB of signal-noise ratio of the video signal in order to maintain anything closely resembling a noise-free picture and it is likely that such circuits would be problematic in the presence of higher scintillation levels, anyway.

One way around this problem would simply be to modulate the LED with a video signal carried on an FM subcarrier:  A variation of this method is used in consumer-grade videotape recorders for the same reason that we would need to use it:  The amplitude variation of the signal recovered from a videotape would cause unacceptable artifacts in the playback video.  With FM, the absolute amplitude of the signal is irrelevant - as long as it is above the noise threshold of the detector.  Another advantage that FM has is that with a signal-noise ratio of better than 10-12dB (depending on system parameters) the recovered signal is nearly noise-free.  The difficulty with using FM is that it would require higher-frequency energy to be modulated, and as we have already seen just getting video bandwidth modulated - and then demodulated - is a challenge with such simple circuitry!

One example of this is the system used by the German Laser ATV experimenter group. This web page is in German:  Click HERE for a Google translation to English.  In this circuit video was modulated onto a 20 MHz carrier using an NE564 PLL.  Likewise, demodulation is accomplished also using an NE564:  Those familiar with FM-ATV demodulator circuits (as well as those used in the early days of analog satellite TV) will immediately recognize these circuits.

Another example, already mentioned, is the method (often called the "color under" system) used to record video onto tape as used in consumer-grade videotape recorders.  In these systems, the luminance (black and white) portion of the signal is modulated onto an FM carrier in the 2-8 MHz range (the frequency and amount of deviation depending on the recording system) while the chroma (color) portion is heterodyne downconverted to something in the 500-900 kHz range (the precise frequency also depending on the recording system) and both signals are put onto the tape.  On playback, the chroma upconverted and re-united with the demodualated luminance signal, mostly recreating the original video signal.  It is interesting to note that even though the chroma signal is subject to significant level variations upon being read from the videotape (although some techniques are used to stabilize this level) the human eye is generally insensitive to such variations so comparatively little effort is needed to correct this effect.

It is likely that the "color under" system would eventually be limited in its range by the amount of scintillatory amplitude variations that the video monitor could tolerate, although this effect could be somewhat mitigated by keying an AGC circuit to the colorburst level of the received signal.  The main advantage of the "color under" system is that it would not require as high a frequency response as, say, the 20 MHz German system in which the color is sent along with the rest of the video:  Using photodiodes, reduced frequency response translates directly to better achievable sensitivity.  If photomultiplier tubes are used, however, the frequency response limitation is not as much a problem but this comes at the cost of the added complexity, fragility, and expense of the use of the tube.

Other related links:

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Keywords:  Lightbeam communications, light beam, lightbeam, laser beam, modulated light, optical communications, through-the-air optical communications, FSO communications, Free-Space Optical communications, LED communications, laser communications, LED, laser, light-emitting diode, lens, fresnel, fresnel lens, photodiode, photomultiplier, PMT, phototransistor, laser tube, laser diode, high power LED, luxeon, cree, phlatlight, lumileds, modulator, detector

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