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Bit rate conversion

Discussion in 'Converters / Interfaces' started by trainwreck, Sep 23, 2005.

  1. trainwreck

    trainwreck Guest

    I recently recorded a session on Pro Tools 6.4 TDM with the session set up at 48k. I didn't realize that the Big Ben time clock was set to 44.1k. Does anyone know how to convert this session back? When the mix is dumped to a cd it plays back fast. I could dump it to a DAT and bring it back in at 48k, but isn't there an easier way?
  2. ghellquist

    ghellquist Member

    Somewhere on the Digidesign forum there is a pointer to freeware program that you can use to modify the file header of the Wav file to put it down to 44.1 No conversion needed than. Once that is done, setup a new 44.1 project and import the file.

    Sorry, cannot be more precise, lost the link myself.

  3. Reggie

    Reggie Well-Known Member

    Does Protools not have an option to change the sample rate of the project without converting the files? I have done it several times with Cubase......
  4. trainwreck

    trainwreck Guest

    Pro Tools can change the sample rate, but the problem is the only option is to go from 48 to 44.1. When I recorded the song, the session is set to 48, and the time clock forced the song to be recorded at 44.1. SO this means if I convert the song sample rate setting to 44.1, it'll play even faster. I need to somehow go the other direction.
  5. trainwreck

    trainwreck Guest

    Which digidesign forum should I search for this freeware app? Sounds like what I'm needing.
  6. hociman

    hociman Active Member


    If you are using Mac OS X, go to versiontracker.com and search for a program called Soundhack. You can use that program to change the header info for your sound files to the correct sample rate.
  7. trainwreck

    trainwreck Guest

    Re: soundhack

    Thanks. Soundhack's header change work perfectly.
  8. Sidhu

    Sidhu Active Member

    i need a little education here... :lol:

    If the master (big ben) was set at 44.1 and the PT session at 48...

    1) I would expect PT to give an out of sync error ? No?

    2) Would not PT record at the sample rate of 48 ? How would it record at 44.1, when the session is set to 48.

    3) If PT did record at 48, how would changing the header in the audio file to fool PT affect the sound ?

  9. hociman

    hociman Active Member

    Pro Fool

    No. The sample rate of the session does not get checked against anything. This is why the most foolproof way to clock your studio is with a SYNC I/O. When you set your session sample rate in Pro Tools, it sets the sample rate of the SYNC I/O to what you have chosen. If the SYNC I/O is your master clock, and everything is clocking from the SYNC I/O, then this problem never occurs.
    The converters would operate at 44.1, and the data transmission from the interface to the host computer would be at 44.1. Again, Pro Tools does not check the session sample rate against the converters in this scenario, so as far as Pro Tools is concerned, you are recording at 44.1 and so the sound files it creates are tagged as 48 in their headers because the session sample rate is 48.
    The sample rate is a measure of how many samples of the signal are taken in one (1) second. If PT records files at 48,000 samples per second, but you were only ACTUALLY recording 44,100 samples per second, your audio will not sound correct when you play it back. It will be pitched up a bit because you have increased the speed of playback by playing an additional 3,900 samples per second.

    So, by changing the sample rate, which is how many samples are played per second, you have affected the speed of the playback, and thus its pitch. By changing the header of your soundfiles to the sample rate that the converter was running at, that file is now played at the sample rate of the original A/D converter that was used while recording, and its pitch is what you would expect as a result.

    Remember that each sample represents the input signal. All that you are doing by changing the header info is changing how many of those samples get played back in one second. In this particular situation, too may samples were being played back in one second. The header needed to be changed to the sample rate of the A/D converter (44.1) and then the files had to be imported into a 44.1 session (or converted to 48 AFTER changing the header) to play back correctly.

    The moral of this thread is to use a SYNC I/O as your master clock.

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