Tom Barry wrote: > It sounds like "IM3 products" (whatever they are) and > good RF frontends are supposed to be known technology. IM3 is third order intermodulation distortion products. Created in receivers when multiple incoming carriers and the local oscillator beat together. Ideally, any such IM products would fall way outside the IF passband and be attenuated nicely. But with single conversion tuners, they are not all outside the passband. It appears that the low cost of single conversion tuners is enough of an incentive that manufacturers seem to jump at the chance of going that route. As we were informed a couple of weeks ago, even COFDM did this. As Al Limbert and others explained, if your RF amp up front is tuned to the channel you want and can reduce its gain in the presence of strong signals, the IM distortion can be greatly reduced, so single conversion tuners can work okay. With co-located transmitters, the problem might not be so acute. Because you will never see unwanted signals that are louder than the desired signal. But if the unwanted signal is louder than the wanted signal, then this can be a problem. The numbers we saw showed that a 3 dB increased level to the receiver, from unwanted signals, can cause up to 9 dB increase in this in-band IM3 distortion (IIRC). So, big deal. And a 3 dB increase into a receiver is not so hard to imagine if there are loud transmitters close by, even if your RF amp is tuned. The shoulders of the RF amp's passband won't be vertical. Dual conversion, i.e. where the receiver goes through two IF conversions, takes care of this nicely. > But if that's the case then maybe we (collectively, > not me) also know what it would cost to add them to a > box with an LG5 chip in it. > > How much extra are we talking about here? Well, since everyone was marveling at the low cost of the first gen Freeview boxes, and they were dual conversion designs, the cost cannot be prohibitive. However, it won't follow Moore's law, so the cost will not erode over time quite so fast. Bert ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.