[LAD] Latency measurements: External round-trip vs. absolute in/out values

With a discussion about latency in the Jack lists, I thought I ask a question I've been wondering: By connecting a cable from a HW output to a HW input, one can measure the round-trip latency. But if the purpose is to try to determine the absolute output latencies of the output port and the input port, by say, dividing the round-trip value by 2 and assuming this value applies to both ports, then won't this concept fail if the output port has a different latency than the input port? Say, by using two different devices for the input and output ports. I find myself asking what exactly transport position is and how it relates to the data in the outside physical world. I wonder if it would make sense to build an external hardware transport position indicator so that the latency test signals could be compared with it. For example to measure an output port's latency: ------>--[HW transport position indicator]-->--- [PC]------- | --- | | | -->-[Audio output port]-------->---[Comparator] | | -----<--[Delay measurement]-------<----------- The remaining unknown - the input port latency would be given by subtracting this measured value from the total round-trip latency. Sound crazy? Thanks. Tim.