Correcting Sample Rate Mismatches in Pro Tools

 In TSM Studio Blog
  • At the rate that various producers and engineers cycle through studio rooms with unique projects calling for different sample rate delivery specifications, recording a passage of audio at a different sample rate than the external clock being used can hinder a project from getting off the ground.
  • The moment that a studio introduces more than one digital device, clocking issues can arise. Each and every digital device needs to be kept in uniform synchronization by working off of a single, master clock source. With an improper setup, audible clicks can appear, moments of silence can intermittently pop in and out, distortion artifacts can sneak their way into the output, timing and pitch may begin to delay and modulate, and it becomes a nightmare of a work environment for the talent and engineer.
  • These issues arise because as the multiple digital sources fail to remain in constant synchronization with one another, clock jitter begins to take place between the digital signals. Clock jitter is defined as the deviation of a periodic signal in relation to a reference clock signal located in its ideal location. It’s useful to note that the clock will not always induce audible artifacts during playback or recording, even if jitter is in occurrence. These unfavorable hiccups will only be heard once the audio files are exported out of Pro Tools.
  • A more common mistake made during sessions involves accidentally setting the clock to a different sample rate than the Pro Tools session. I’ve been found guilty of this mistake, as is the case with many engineers and producers in the recording arts industry, so the remainder of this article will inform you on how to rectify this problem and hopefully prevent you from experiencing the same lapse in judgment.
  • During one of my first sessions with a new artist about a year ago, I mistakenly left the external clock’s sample rate at 44.1k and recorded all audio into a Pro Tools session with a sample rate of 48k. Although clocking issues will vary depending on at what point jitter presents its artifacts, my session went well all the way through. Upon sending a mp3 rough mix out to the client, I was informed that her voice sounded like it was “transposed” and “off-time”. Upon getting in contact with another engineer, John LaRosa, I was informed on how to correct this issue following the steps below:
  1. First off, it’s important to understand that your clock source is going to dictate the output sample rate of the recorded audio. In this circumstance, although my audio attempted to playback at 48k from the Pro Tools session, it was recorded with a sample rate of 44.1k. A key takeaway from this article should be that sample rate cannot be up-sampled. When going from, say, a sample rate of 44.1k to 48k, the file size is just increasing with no true improvement in fidelity.
  2. Now to actually amend this issue, you’ll want to begin by creating a new Pro Tools session at the correct sample rate (ie. the sample rate that the external device was set to when Pro Tools was clocking it). In this case, the new Pro Tools session should be created with a sample rate of 44.1k. You’ll also want to make sure that the new session is the same bit-depth and the file type remains the same as the original session.
  3. Within the new session, you will then select File -> Import Session Data (or use the key command: Shift + Option + I) and open the original session that contained the mismatched sample rate.
  4. Within the “Import Session Data” window, under the “Sample Rate Conversion Options” section, uncheck the box located next to “Apply SRC”.
  5. Highlight the tracks you wish to import to the new session.
  6. Click the “Ok” button and the session should now playback at the correct speed.