Thanks to all for the ideas! Spread spectrum, ODFM, etc. would, as I understand it, pretty much be RF over IR, similar to what we're doing now (multiple RF carriers on IR). For the digital data, I'm currently running FSK with +/- 100 kHz shift at 1.8 MHz. I am running 10 kbps. As I tried increasing the bit rate, the coverage area dropped substantially. I'm using an SA639 for the receiver. It's designed for the Digital European Cordless Telephone system which runs at about 1.152 Mbps when using FSK. I use an op amp based preamp after the photodiodes with bias to the photodiodes applied through a slug tuned inductor which becomes a parallel resonant circuit when combined with the photodiode capacitance. The preamp drives the SA539 which upconverts the 1.8 MHz to 10.7 MHz. Ceramic IF filters and a ceramic resonator in the quadrature detector complete the receiver. The max2990 is interesting. It only supports 100 kbps in a 10 kHz to 490 kHz spectrum. The LEDs we're using can be modulated up to about 30 MHz, so more spectrum is available. The MAX2990 appears to require two way communications for ack, which would not be possible in our application. It would be nice if I could just on/off key the IR LEDs instead of having to amplitude modulate them, but that may not work out. For the analog FM (carrying audio) receiver, we have two designs. One is a TRF receiver using ceramic filters. A second design upconverts the RF over IR signal to the FM broadcast band where it is demodulated in a standard single chip FM receiver (DSP-based). Thanks for the comments! I'll do some more research. Harold > If I did my calculations right, you're at about 30 meters per half bit. > Not sure about your environment, but it would seem like 30m isn't far > enough especially considering possible strong ir reflective surfaces. M= y > guess is that you'd end up with at least some dead areas, possibly a lot > of > dead areas, where a simple on off bitstream couldn't be recovered. This > is > even more true if you are relying on reflections to get the ir signal > through. If you decide to try this, please report back with your finding= s > as I'm kinda curious. > > It would be interesting to investigate whether one of the rf digital > modulation methods (ofdm/qam, psk, etc) could be applied. Some of these > are multipath sensitive, some are not so much. > > One possible ic resource is the power line modems such as the max2990. > These are generally ofdm modems in various frequency bands. > > > On Dec 26, 2017 12:26 PM, "Harold Hallikainen" > > wrote: > > Several years ago I designed a system that transmits audio and data over > IR. The audio is transmitted as frequency modulated RF over IR. The data > is transmitted is frequency shift keyed RF over IR. I've been thinking > about direct digital modulation of the IR. The IR LEDs would be pulsed. > The bit rate to carry the audio and digital would need to be in the area > of 5 Mbps. Does anyone have experience, thoughts, or references on this? > The IR is transmitted in a large room (movie theater auditorium). I'm > concerned about reflections causing intersymbol interference. What do > people think would be a good method of encoding the data (async, biphase, > etc.)? Are such systems existing? Are there any chip sets? > > Thanks! > > Harold > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com > Not sent from an iPhone. > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 FCC Rules Updated Daily at http://www.hallikainen.com Not sent from an iPhone. --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .