File Conversions (Filter) System

[Craig Maier]

File Conversions


The File Conversion filter is not really a filter at all, but a way to convert mono files to stereo and visa-versa. It can also be used to adjust the channel balance or reverse the channels of a stereo recording, or convert a mono source into a stereo file. It is useful in converting stereo recordings made out of phase (such as old vertically recorded acoustic discs) into a stereo or mono file that is compatible with modern systems. The Time Offset feature can be used to correct for group delay problems, correct azimuth errors from tape recordings, or to create special effects. The phase inversion function can be used to correct files that contain an inadvertent 180 degree phase shift of one channel with respect to the other (only one of the phase inversion checkboxes need to be checked to accomplish this correction on a relative basis). Phase inversion sometimes occurs when a balanced microphone (XLR or Cannon type connector) had been wired backwards. When paired with a properly wired microphone, the out of phase microphone can produce unusual phasing issues when both are mixed together. The phase inversion feature will correct this problem.


The File Conversion Window
The “File Conversion” Filter includes three slider controls. One set of two controls allows you to adjust the Amplitude (gain) levels for each channel. The third control, called “Time Offset”, provides a means for azimuth correction, Forensics audio enhancement, and stereo simulation. A final important use of the file conversion filter is to simply copy parts of the source file over the destination file. This is one way to revert back to the original source file (undo) following a bad filter application.

This sometimes will be the first step that you will perform in a sound restoration project. This File Conversion Filter provides a number of file conversion options that can provide a certain degree of noise reduction in and of themselves. Gain adjustments can also be performed during the file conversion process. The following File Conversion options are available:

From Stereo to -

• • • • Mono (L + R): This option adds the .wav file Left Channel Input to its Right Channel Input before feeding it into the Destination workspace. It produces a single channel output signal.
      • Mono (Left Only) & Mono (Right Only): These options choose only one of the two .wav file inputs to be used in the file conversion-processing step and yields a single channel output.

• • • • Mono (L - R): This option subtracts the Right .wav file signal from the Left signal before feeding it into the Destination workspace as a single channel signal.

• • • • Stereo: This option maintains the .wav file in stereo through the file conversion-processing step. It can be used to adjust the gains of the two signals if they are incorrect while maintaining their independence.

• • • • Stereo Reverse: This option will reverse the left and right channels during the .wav file conversion-processing step. It can also be used to adjust the gains of the two signals at the same time, if they are incorrect.

• • • • L = L+R & R = L-R: This option provides monophonic mix to the left channel, and provides the ambient signal from a stereo recording to the right channel.

• • • • Stereo (Left Only): This option takes the Left Channel input and applies it to both (stereo) output channels and yields a dual channel output signal.

• • • • Stereo (Right Only): This option takes the Right Channel input and applies it to both (stereo) output channels and yields a dual channel output signal.

From Mono to -

• Mono: This option merely provides a clone of the original .wav file.

• Stereo: This option converts a monophonic single-track file into two single-track monophonic files.

Here are more details on the various File conversion options and their specific application:

• Mono (L + R)

This is generally used to convert a lateral cut record (like a typical 78, or a monophonic LP) which is monophonic to start with, but which has been transferred to the hard drive with a stereo cartridge, and convert these two signals into one signal on which you will perform further processing. The advantage of this simple conversion is that some of the noise content of the record will cancel out during this process, in particular, low end rumble, and even some higher frequency surface noise. This process alone can provide up to 6 dB of signal-to-noise improvement (depending on the condition of your source) compared to the use of only one of the lateral groove walls (i.e. using the left only or the right only signal).

Important Note:
It is advisable to set both gain controls to - 6 dB to avoid overloading of the Destination channel during this mixing process, unless your recording is extremely under-recorded to start with. Minus 6 dB is the default value for the two gain settings in the Mono (L + R) File Conversion feature.

• Mono (Left Only) & Mono (Right Only)

Sometimes, 78-rpm laterals are worn unevenly due to years of improper tracking of the tone arm that played the particular record. Therefore, it is sometimes useful to compare the Left Only groove wall with the Right Only groove wall to hear if that is the case. If you hear a significant difference between one of the two groove walls, you should then compare the quieter of the two with the Mono (L + R) signal for comparison. Choose the quietest of the three possibilities for your Destination file.

• Mono (L - R)

This feature takes the algebraic difference between the left channel and the right channel audio signals and feeds it into the destination file. It has four significant applications:[LIST=1][*][FONT=Courier New]If you have transferred vertical cut records such as cylinders or Edison Diamond Discs utilizing a stereophonic cartridge, and haven't previously extracted the vertical signal component from that signal, this feature will enable you to do so. Just as you would have done with the laterals, it is useful to listen to the Left Only signal and compare it with the Right Only signal to make sure that no significant tracking damage has been done to the record over the years. Choose the quieter of the two for subsequent comparison to the Mono (L - R) signal. Generally you will find that the Mono (L - R) signal has the best signal- to-noise ratio for vertical (hill and dale[FONT=Times New Roman]) recordings. Path