Premiere Pro processes each audio channel, including audio channels in video clips, as 32-bit floating-point data at the sequence sample rate. This processing assures maximum editing performance and audio quality. Premiere Pro conforms certain types of audio to match the 32-bit format and the sequence sample rate. If conforming is required, it is done when a file is imported into a project for the first time. Conforming takes some time and disk space. A progress bar appears at the lower right of the Premiere Pro window when conforming begins. Premiere Pro saves conformed audio in CFA audio preview files. You can determine where to save these audio preview files by specifying a Scratch Disk location for Audio Previews in the Project Settings dialog box.
You can work with audio files, even applying effects to them, before they are fully conformed. However, you can preview only the parts of the files that have been conformed. You cannot hear unconformed sections on playback.
These rules determine which types of audio get conformed:
Premiere Pro does not conform audio in uncompressed clips that were recorded in one of the natively supported sample rates, when you use these clips in sequences with matching sample rates.
Premiere Pro does conform audio in uncompressed clips when you use them in sequences with non-matching sample rates. However no conforming is done until you export the sequences or create audio preview files.
Premiere Pro does conform audio in uncompressed formats that were not recorded in a natively supported sample rate. In most of these cases, it will upsample the audio either to the nearest supported sample rate, or to a supported sample rate that is an even multiple of the source audio sample rate. For example, it will upsample an 11024Hz source to 11025Hz, since that is the nearest supported rate, and there is no supported rate that is an even multiple of 11024.
Premiere Pro does conform all compressed audio, such as audio found in mp3, WMA, MPEG, or compressed MOV files. It conforms this audio at the sample rate of its source file. For example it will conform a 44100Hz mp3 file at 44100Hz. However, if the conformed audio is used in a sequence with a non-matching sampling rate, as when a 44100Hz clip is used in a 44000Hz sequence, the audio will play back at the sample rate of the sequence without further conforming.
Premiere Pro does not conform a file that was conformed in one sequence when you import it into another sequence with the same audio sample rate, so long as you haven’t moved or renamed the file since it was conformed. Premiere Pro keeps the location of the conform files for all files it has conformed in the Media Cache Database.
To avoid conforming, use audio editing software, or transcoding software, to convert your files to natively-supported uncompressed formats at the supported sampling rates.
In addition to conforming some files, Premiere Pro also creates a PEK file for any file containing audio when it is first imported into a project. It uses these PEK files for drawing the audio waveforms in Timeline panels. Premiere Pro stores PEK files in the location specified for Media Cache Files through the Media pane of the Preferences dialog box.
|Recommended Audio File Formats
For best performance, use one of the following uncompressed audio file formats when editing in Final Cut Pro:
AIFF or AIFC containing uncompressed audio
WAVE or Broadcast Wave Format (BWF) containing uncompressed audio
Sound Designer II
Single-track or multitrack QuickTime movies containing uncompressed audio
Natively, Final Cut Pro captures to QuickTime movie files with one or more audio tracks.
Audio Formats to Avoid
The following formats and audio data formats should be avoided when editing in Final Cut Pro because they require real-time processing for playback:
Any file containing compressed formats such as MP3, AAC, and Apple Lossless Codec
QuickTime movies containing compressed audio, such as MPEG-4 and H.264 files
Multiplexed video and audio streams such as MPEG-2 program streams and DV Stream files (This is one of the formats iMovie can create during capture.)
Choosing Audio File Sample Rate and Bit Depth
Final Cut Pro can import audio with any bit depth and sample rate supported by QuickTime and Mac OS X Core Audio. Final Cut Pro performs real-time bit depth conversion and sample rate conversion when your audio file settings don’t match your sequence settings. However, less processor power is required when your audio file settings and sequence settings match.
Common sample rates and bit depths used in the video industry are:
32 kHz/12-bit: Consumer mini-DV camcorders can record four channels of audio using these settings. This is not recommended for most productions.
44.1 kHz/16-bit: Audio CDs and consumer DAT recorders use these audio settings.
48 kHz/16-bit: DV, HDV, and DVD all use these audio settings.
48 kHz/20-bit: Some professional video devices record natively in this format.
96 kHz/24-bit: These settings are becoming increasingly popular for professional sound and music production, although most video formats still record with 48 kHz.
For more information, see Audio Fundamentals.
Choosing Sequence Bit Depth
The bit depth setting in the Sequence Settings window determines the bit depth whenever you output or export your sequence. However, sequence audio is always mixed using 32-bit floating-point values.
Note: The Aud Format column in the Browser and Item Properties window always shows the internal mixing bit depth of a sequence, whereas the Sequence Settings window shows the bit depth used if the sequence is exported.
Mixing Sample Rates and Using Real-Time Sample Rate Conversion
Ideally, the sample rate and bit depth of your audio files should match that of your sequence settings. When you play a sequence in Final Cut Pro, any audio files with sample rates that don’t match your sequence sample rate are converted in real time. This is known as sample rate conversion, and it requires additional processing power. Clip items that require real-time sample rate conversion appear with a green render bar within the clip item. For more information, see Rendering and Video Processing Settings.
Even though Final Cut Pro can perform real-time sample rate conversion, conversions can reduce your audio mixing and effects performance. The quality of this conversion is controlled by the Audio Playback Quality setting in the General tab of the User Preferences window. Higher quality conversions reduce the number of audio tracks that Final Cut Pro can mix together in real time.
If the sample rates of all the audio in your sequence match, sample rate conversion is not necessary and the number of audio tracks that can play in real time increases. If you are working with someone who is creating music or audio files specifically for your project, you can request audio files at the settings you need to match your sequence.
However, If your audio clips don’t match your sequence settings, you can improve audio playback performance by converting your audio files to the sample rate and bit depth of your sequence.
Converting Audio Clips to Match Sequence Settings
If you are working with preexisting audio material, such as music from audio CDs, you need to convert the audio files so they match your sequence settings. For example, if you plan to use a lot of sound effects or music from audio CDs (which have a sample rate of 44.1 kHz) in a DV sequence with a sample rate of 48 kHz, it’s a good idea to convert your audio files to a sample rate of 48 kHz.
Most professional video formats, including DV, have a sample rate of 48 kHz and a bit depth of 16 (this is often abbreviated as 48 kHz/16-bit). Since these settings are so common for video post-production, they are used for most sequences in Final Cut Pro.
Important: DV sequences sometimes use 32 kHz/12-bit settings, but these settings are not recommended. As long as you don’t record your DV footage using 32 kHz/12-bit, you should not use these settings for your sequence.
Audio files can be converted using the Export Using QuickTime Conversion command.