Quick Tutorial

Mark Echeverri edited this page Jul 3, 2014 · 5 revisions
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After following the installation instructions contained in README.txt, run postslate on a Mac OS X, Linux or UNIX system; or run postslate.bat on Windows. You should be greeted with Postslate's main window:

Postslate's main window

On startup, Postslate checks for the presence of two third-party components: FFMPEG and MPV. If it fails to find them, you will be prompted to select their location or to install them. Doing so is relatively easy. If you have no other purpose for installing FFMPEG or MPV, you may wish to download statically-linked binaries. There are several options for obtaining and installing these packages, ranging from downloading binaries to compiling it yourself from source code.

Basic Workflow

Using Postslate to synchronize a collection of video clips with their corresponding audio recordings boils down to a three-part process:

  1. Scan the contents of a pair of directories, one containing video, the other audio.
  2. Synchronize matching clips and discard unmatched clips.
  3. Merge the matched pairs of video/audio clips.

Scanning Directories

If you're interested in Postslate, you presumably have a set of camera video clips that you want to be matched up and synchronized with a set of corresponding audio clips from an external recorder. Begin by selecting the video and audio directories, or folders. Select the clip locations, either by clicking on the ... buttons or by typing the paths. (The clips can be in different locations or under the same directory. For the latter, set the same path twice.)

Set the video/audio paths first.

Next, click the Scan button. The process may take some time as Postslate makes two passes, one through the video files and one through the audio:

Scanning progress

When scanning is finished, the Postslate window contains a list of your video and audio clips.

After scanning, Postslate shows a list of clips

Synchronizing Clips

For Postslate to work well, the order of your camera video clip names should approximately track that of the audio segments. Naturally, there are likely to be files that don't match--video segments where the camera was running accidentally, or when audio recording was solo--, but these are easily dealt with. If both the camera and the audio recorder name their files with some sort of ascending sequence pattern, you should be fine.

Initially, the first video clip, 00128.MTS, is paired with the first audio clip, STE-000.wav. This may or may not be what you want. Begin by clicking on the top row in the list at left:

Selecting a pair, with auto-preview

Soon after you click on the pair, a video preview starts, displaying a 5-second preview combining camera video with external audio. The preview is centered on the portion from each clip that Postslate believes is most likely to be the synchronized clap. If you both see and hear the slate during this preview, and the scene announced matches what's shown on the slate, then it worked correctly the first time. Usually, this is what happens.

There are several items worth pointing out here:

  • When you clicked on the pair of files, the preview began automatically. This behavior can be changed via the Auto checkbox to the right of the window.
  • The preview showed only the clap. Alternately, you can choose to preview the full merged file (camera video+external audio), the camera video by itself, or just the external audio. Set this by changing the radio buttons on the right.
  • The automatic preview played once. To repeat, click the Play button.

After the preview ends, Postslate displays graphs of the audio waveforms:

Waveform graphs displayed

In each graph, a full green line marks the point where Postslate thinks the clap is. (These points are always shown aligned.) Shorter green marks represent other possibilities for the clap location. Blue lines mark the start and end points of the synchronized pair of clips (based on the selected clap positions), trimmed to be the same length. To the right of each graph, a list box shows a set of numbers. The number highlighted in the list box represents the selected clap position in seconds after the beginning of the clip.

Moving on, we now select the second pair at left:

The second pair

The preview for this pair is very brief and contains a clap sound, but there is no slate clap shown in the video. Also, the graphs look mismatched: The video clip is much shorter than the audio. A reasonable conclusion is that video clip 00129.MTS is junk and needs to be discarded. To do this, hold down the Ctrl key and click the video clip's name:

Stag video

This action marks 00129.MTS as "stag", and it now appears grayed-out in the list. Meanwhile, the audio clip STE-001.wav now appears alongside 00130.MTS. Click on that pair to see if it is a good match:

The third pair

The preview includes the clap for Scene 9N, Take 1, in both video and audio. On to the next.

Two pairs down, when previewing the pair 00132.MTS / STE-003.wav, we encounter something different:

No clap in preview

No slate appears in the preview, and no clap can be heard.

The first thing to check is the waveform graphs:

Graphs for no clap

The video and audio clips are long and approximately the same length, suggesting they could be matchable. But, at the least, the clap location needs to be adjusted. (The clap position near the end of the clips isn't necessarily a problem--the scene could have been tail-slated.)

There are several things we could try:

  • We could try playing the camera video by itself, in its entirety (select Video, click Play) to verify that it contains a valid take.
  • We could listen to the audio solo (select Audio, click Play).
  • We could try adjusting the clap points.

Let's try the latter. Looking at the list box next to the camera waveform graph, we can see other candidates for the clap position. These possible clap points are ranked in the order of likelihood. When scanning the video and audio clips, Postslate looks through each audio stream for percussive sounds; the rank order reflects the percussiveness" of the audio. Depending on the scene being shot, there may be factors to throw off this guess--e.g., a person playing the drums. In any case, we see here that while Postslate put the first-guess clap point at 78.816 seconds into the video, its second-guess clap point occurred at 17.007 seconds. This likely corresponds to the spike (circled).

A likely candidate

Changing the clap point to this position, we see:

Changing a clap point

This is encouraging: The slate now appears in the video part of the preview. Since no clap is heard, let's try changing the external audio's clap point to the second-guess position:

Changing another clap point

And this does the trick.

You can change clap points by clicking either on a time in the list of possible clap points, or by clicking on a green tick mark in a graph.

If you mouse-over a spot on a graph, you will see/hear a brief preview of the media at that point. This feature is familiar to users of some commercial editing programs; try it out. It adds another tool to make finding a clap easy.

From the discussion so far, you now have a basic understanding of the process. Lather, rinse, and repeat for each clip.

Merging the Results

After working through the whole list, you will have a set of matched, synchronized pairs of video/audio clips. At this point, you can "merge" the clips by combining the camera video with the external audio. Merging, by default, simply means repackaging the synchronized, trimmed portions of each clip and placing them into the same container. It does NOT involve transcoding, and thus you should incur no loss of quality. In the screenshots shown here, the source camera clips are encoded using H.264 and the files are in MPEG Transport Stream format, while the audio clips are WAV files. After merging, we end up with the same H.264-encoded video stream, plus AC3 audio inside a MOV container:


There are several options to control the merge:

  • If Camera audio is checked, the camera audio is included in the merged output as a secondary audio stream. You may find it useful for reference during editing, though doubtless most of the time you'll want to mute that channel. It provides a helpful comparison of the camera audio vs. externally-recorded sound. You may find it useful to merge your clips with this option enabled, do some editing, and then merge again with this option disabled.
  • If Extra video is checked, any secondary video streams from the camera clips are included in the merge output. By default, only the primary video is included.
  • If Data streams is checked, any data streams from the camera clips are included in the merge output.
  • Finally, if Separate A/V outputs is enabled, the trimmed and synchronized pairs of clips are written to the merge folder as separate files. In the above example, the output names would look something like 00132.mov / 00132.wav.

After setting merge options, click the Merge All button. The process should be fairly quick since it usually doesn't involve transcoding.

Keystroke Shortcuts

The most commonly-used actions can be performed either by mouse clicks or by keystrokes:

Keys Action Alt+UpArrow Previous clip/pair Alt+DownArrow Next clip/pair Alt+RightArrow Next clap candidate from camera audio Alt+LeftArrow Previous clap candidate from camera audio Shift+Alt+RightArrow Next clap candidate from external audio Shift+Alt+LeftArrow Previous clap candidate from external audio Alt+c Select play Clap Alt+f Select play Full Alt+v Select play video Alt+a Select play audio Alt+Space Play preview Alt+q Quit