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The general goal is to align transport-sample to be the audible frame and use that as "anchor" for all processing. transport_sample cannot become negative (00:00:00:00 is the first audible frame). Internally transport pre-rolls (read-ahead) before the transport starts to move. This allows inputs and disk to prefill the pipeline. When starting to roll, the session counts down a global "remaning preroll" counter, which is the worst-latency from in-to-out. Each route in turn will start processing at its own output-latency. Route::process_output_buffers() - which does the actual processing incl disk i/o - begins by offsetting the "current sample" by the route's process-latency and decrements the offset for each latent processor. At the end of the function the output will be aligned and match transport-sample - downstream-playback-latency (if any). PS. This commit is a first step only: transport looping & vari-speed have not yet been implemented/updated.
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