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

[Tim]

On 4/5/19 3:16 PM, Tim wrote:
> 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
>   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.

Here two or more low-latency-triggered precision oscillators
  at non-harmonic related frequencies provide a signal to an
  input and the software measures the group phase delay in
  relation to the trigger time:

[PC] -->--[ex. Par. port]-->--[Triggers]-->--[Sin oscs]
      ---                                         |
        |                                         |
        --<--[Audio input]---------------<---------

It's similar to how round-trip latency might be measured,
  except there the sines are generated digitally by audio output.