I knew someone was going to ask this! The above post was actually an oversimplification, in actuality the impedance can vary with frequency. Its similar to saying a speaker is 8 ohms, very rarely is it 8 ohms at all frequencies in the audio range. Some reviews will publish the full graph of speaker impedance VS frequency, but how does a consumer use this? How can a consumer figure out whether this speaker is going to sound better than another using a graph for each?
The same sort of thing applies to S/PDIF systems. IF you want full information you need a impedance VS frequency chart (covering at least 1MHz to 200MHz). So you have one of those for the source and one for the connecting cable and one for the receiver. What do you DO with this information? A human can probably do some gross interpretaions with these, particularly if the spread for each one is not too large, but if each one has a large spread, how do you make meaningfull interpretations of how the system as whole is going to behave? There ARE ways to take this information into a computer and simulate what the total SYSTEM behavior is going to be, but then you have to know how the particular DAC product is sensitive to this, different ones are sensitive in different ways. Once a group of people have discovered the sensitivities of a particular DAC then this information might be useful. (note that very few manufacturers even think about any of this stuff, so information on this is NOT going to come from a manufacturer) There certainly are test equipment that can measure this, the telecom industry uses this stuff all the time, but they are not cheap, $20,000 to $100,000. It should be possible to design a piece of fairly inexpensive test equipment that has a USB interface to a computer so the computer can do all the heavy lifting and have it just do this one function over just the frequency range we are interested in. Unfortunately the market will be small, so even if technically it is not too difficult, it will still be fairly expensive. Somebody could probably market such a device for $4k-$5k. If a bunch of DIY types get together to do the circuit design and programming it could be a DIY project for a lot less. Note that DIY, open source, cooperative projects for test equipment are quite rare! It would be interesting to see what would happen in the industry if reviwers started publishing this information. My guess is that LONG term it might be usefull, manufacturers might actually start measuring their products and put some effort into getting things closer to spec. But short term it might be a major problem because most consumers would have no clue what it all meant, the market (audiophiles and reviewers) would wind up fixating on some aspect to make comparisons on, which may or may not have any relationship to something important. The important thing would be to get the market to focus on what is primarily important, low spread, and averaging around 75 ohms. This is something that is very obvious by looking at the impedance VS frequency charts, as long as all the charts are done the same way, have the same units etc. For a particular peice of equipment built using surface mount parts on PC boards the impedance is going to be quite uniform from box to box as long as the board design is not changed and the components stay the same. Changing a chip from one manufacturer toanother CAN have a significant change impedance, since that is usually not a parameter that is "tied" to the part number. Yes it IS possible in many cases to build a device that you can insert between boxes to improve the match, but they would have to be engineered for a specific combination, there is no such thing as one box that you can use that does everything. I suppose it's possible to build a device that measures the system and configures a network to optimize it, but such a device would be exceedingly expensive. It would be way cheaper to just build the sources and recivers in such a way that they don't care. John S. ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=95614 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
