From b61ea15ed764171aad28939718f36146f8490f0c Mon Sep 17 00:00:00 2001 From: Carl Hetherington Date: Wed, 15 Jul 2015 00:12:54 +0100 Subject: [PATCH] More vague resampling notes. --- doc/design/resampling.tex | 27 ++++++++++++++++++++++++++- 1 file changed, 26 insertions(+), 1 deletion(-) diff --git a/doc/design/resampling.tex b/doc/design/resampling.tex index cb7388eb8..c1a1d3463 100644 --- a/doc/design/resampling.tex +++ b/doc/design/resampling.tex @@ -2,7 +2,7 @@ \usepackage{amsmath} \begin{document} -Here is what resampling we need to do. Content video is at $C_V$ fps, audio at $C_A$. +Here is what resampling we need to do. Content video is at $C_V$ fps, audio at $C_A$. \section{Easy case 1} @@ -60,4 +60,29 @@ per second, so they last $R_A / F_A$ seconds. Hence there is a scaling between some content time and some DCP time of $R_A / F_A$ i.e. $C_V / F_V$. + +\section{Another explanation} + +Say we have some content at a video rate $C_V$ and we want to +run it at DCP video rate $F_V$. It's always the video rates that +decide what to do, since we don't have an equivalent to audio +resampling in the video domain. + +We can just mark the video as $F_V$ and it will run $F_V / C_V$ faster +than it was. Let's call the factor $S = F_V / C_V$. + +An equivalent for audio would be to take the content audio at a rate +$C_A$ and mark it as $C_A S$. Then the same audio frames will be run +more quickly, just as the same video frames are being. The audio would be +in sync with the video since it has been sped up by the same amount. + +In practice we can't do this, in general, as the only allowed DCP +audio rates are 48kHz and 96kHz. Instead, we'll resample to some new +rate $P$ and mark it as $Q$ where $Q / P = S$. Resampling does not +change the sound, just how many samples are being used to describe it, +so this is equivalent to marking the original, unsampled audio as $C_A S$. + +Then we set $Q = 48$kHz so that $P = 48000 / S$, or $P = C_V F_A +/ F_V$. + \end{document} -- 2.30.2