It 'important to realize that even if the oscillators sample may vary from one another, they tend to be extremely accurate in time, we are referring to themselves. This means that if you know what the clock speed of your recorder audio''autres sample clock of the camera, so a single correction factor will both return path for the synchronization.
A simple procedure is described below, using freeware programs, for calculating the correction factor necessary for your particular record. Necessary programs (all free):
1) AoA audio Extractor --- (not necessary if you can extract the audio track from a video recording in your NLE)
2) Audacity , a free audio editor
3) Switch , a free audio file type converter
Step 1: Set up your camera and audio recorder next to each other, and start both running
Step 2: Make a good audio 'clapperboard' point by tapping a pencil firmly onto a hard surface, as close to both camera and recorder mics as possible. This will cause a fast 'edge' to become the reference point, which will be useful later.
Step3: Let both devices run for at least 10 minutes. 20 minutes are better, but not essential.
At the end of the period, (timing not critical), make a second similar 'clapperboard' reference point. Allow a few more seconds recording, and then stop both devices.
Step 4: Download the video from the camera, and the audio from the recorder to your computer.
Step 5: Open the video file in 'AoA audio Extractor', and extract the audio from the video. Convert this audio file (if necessary) to a 48KHz 16 bit LPCM (wav) file, using the 'Switch' program linked to above. This file will be your reference audio file.
Step 6: Open the WAV file saved in the previous step in 'Audacity' - a free audio editor.
It is now necessary to save only the part of the file between the two 'clapperboard' references, which hopefully stand out clearly (if you hit the pencil hard enough in steps 2 and 3!).
This must be done accurately, to within 100th of a second, or better.
Click and drag the file to highlight the beginning section, including the first 'clapperboard' waveform. Use the 'fit selection to window' tool (immediately to the right of the 'zoom' tools) to magnify. Delete as much as possible before the 'clapper' reference. Repeat the operation until you can clearly identify the first distinct waveform of the 'clapper' reference waveform. Click, hold and place the cursor directly over the first full (clipped) waveform, where it crosses the centre line. Delete every thing to the left of that point, by clicking and dragging to highlight, and then deleting with the scissors tool.
Step 7: Repeat the above procedure for the end part of the waveform, again using the start of the second 'clapper' waveform as your reference cutting point. In this case, cut everything to the right of your selected reference point (not the left, as in the first case.)
Step 8: It is necessary to record the exact length of this new cropped waveform. This information is listed at the bottom of the window, but in a rather inappropriate format. Locate the toolbar
at the top of the window, click 'Edit'-'select all' and then 'Effects'-'change tempo' . Note the length of the file in seconds in the right hand box (the left hand box is greyed out).
Record this figure. It is the reference file length
Step 9: It is necessary to convert the file recorded on the audio recorder into an identical format to the camera audio track, namely 48 KHz 16 bit PCM. If the track is not already in this format, open it in Switch, a free file transfer program, and select the output encoding as 48 KHz 16 bit, mono or stereo to fit your audio file. This program will then convert whatever format your input file is (MP3, WMA, 44.1 KHz PCM, etc) into the required 48 KHz, 16 bit format required.
Step 10: Repeat the procedures in steps 6 and 7 for this new file. Edit the 'clapper' points with as much accuracy as before, but notice that the 'clapper' waveform will appear different from the first, although hopefully with as clearly defined first ' zero crossing' point.
Step 11: Repeat the procedure in step 8, and you will notice that the overall file length is different.
Overwrite the original file length into the right hand box, and take careful note of the resulting figure in the percent change box. Make sure you record whether it is a plus or minus value - it could be either! This is the most important figure, because it represents the percentage variation between the two units.
Once you have obtained this value, simply apply it to all files recorded with the same recorder and this camera. Providing you can align the start of the file with a single 'clapperboard' mark, there is no need to repeat the rest. Simply change the length of the external audio file by the correct percentage value, to allow the two audio tracks to remain synchronised.
It may seem a long winded process, but it only needs to be done once, and could then save quite a lot of time, against the option of trying to align tracks manually in your NLE, in small sections.
You may of course wish to use an alternative audio editor, and the details of this precedure may then be slightly different, but the principle remains the same.
A simple procedure is described below, using freeware programs, for calculating the correction factor necessary for your particular record. Necessary programs (all free):
1) AoA audio Extractor --- (not necessary if you can extract the audio track from a video recording in your NLE)
2) Audacity , a free audio editor
3) Switch , a free audio file type converter
Step 1: Set up your camera and audio recorder next to each other, and start both running
Step 2: Make a good audio 'clapperboard' point by tapping a pencil firmly onto a hard surface, as close to both camera and recorder mics as possible. This will cause a fast 'edge' to become the reference point, which will be useful later.
Step3: Let both devices run for at least 10 minutes. 20 minutes are better, but not essential.
At the end of the period, (timing not critical), make a second similar 'clapperboard' reference point. Allow a few more seconds recording, and then stop both devices.
Step 4: Download the video from the camera, and the audio from the recorder to your computer.
Step 5: Open the video file in 'AoA audio Extractor', and extract the audio from the video. Convert this audio file (if necessary) to a 48KHz 16 bit LPCM (wav) file, using the 'Switch' program linked to above. This file will be your reference audio file.
Step 6: Open the WAV file saved in the previous step in 'Audacity' - a free audio editor.
It is now necessary to save only the part of the file between the two 'clapperboard' references, which hopefully stand out clearly (if you hit the pencil hard enough in steps 2 and 3!).
This must be done accurately, to within 100th of a second, or better.
Click and drag the file to highlight the beginning section, including the first 'clapperboard' waveform. Use the 'fit selection to window' tool (immediately to the right of the 'zoom' tools) to magnify. Delete as much as possible before the 'clapper' reference. Repeat the operation until you can clearly identify the first distinct waveform of the 'clapper' reference waveform. Click, hold and place the cursor directly over the first full (clipped) waveform, where it crosses the centre line. Delete every thing to the left of that point, by clicking and dragging to highlight, and then deleting with the scissors tool.
Step 7: Repeat the above procedure for the end part of the waveform, again using the start of the second 'clapper' waveform as your reference cutting point. In this case, cut everything to the right of your selected reference point (not the left, as in the first case.)
Step 8: It is necessary to record the exact length of this new cropped waveform. This information is listed at the bottom of the window, but in a rather inappropriate format. Locate the toolbar
at the top of the window, click 'Edit'-'select all' and then 'Effects'-'change tempo' . Note the length of the file in seconds in the right hand box (the left hand box is greyed out).
Record this figure. It is the reference file length
Step 9: It is necessary to convert the file recorded on the audio recorder into an identical format to the camera audio track, namely 48 KHz 16 bit PCM. If the track is not already in this format, open it in Switch, a free file transfer program, and select the output encoding as 48 KHz 16 bit, mono or stereo to fit your audio file. This program will then convert whatever format your input file is (MP3, WMA, 44.1 KHz PCM, etc) into the required 48 KHz, 16 bit format required.
Step 10: Repeat the procedures in steps 6 and 7 for this new file. Edit the 'clapper' points with as much accuracy as before, but notice that the 'clapper' waveform will appear different from the first, although hopefully with as clearly defined first ' zero crossing' point.
Step 11: Repeat the procedure in step 8, and you will notice that the overall file length is different.
Overwrite the original file length into the right hand box, and take careful note of the resulting figure in the percent change box. Make sure you record whether it is a plus or minus value - it could be either! This is the most important figure, because it represents the percentage variation between the two units.
Once you have obtained this value, simply apply it to all files recorded with the same recorder and this camera. Providing you can align the start of the file with a single 'clapperboard' mark, there is no need to repeat the rest. Simply change the length of the external audio file by the correct percentage value, to allow the two audio tracks to remain synchronised.
It may seem a long winded process, but it only needs to be done once, and could then save quite a lot of time, against the option of trying to align tracks manually in your NLE, in small sections.
You may of course wish to use an alternative audio editor, and the details of this precedure may then be slightly different, but the principle remains the same.