Archive for July, 2011
In the following blog post, I will help you understand a basic three part process for audio restoration. In the embedded video, I exhibit an audio file that was lifted from a video which is in need of some audio restoration. The video walks you through the process which varied from video to video in this video training set.
The first step is to either compress the audio increasing the gain during the process or removing any unwanted distortion; whichever you feel is best to lead with. You can try one process first then the second. Worse case you can apply the processes in opposite order if your initial results are not up to your expectation.
Varying the order which filters and noise reduction are applied is how a forensic examiner will proceed when restoring an audio file. Audio restoration takes time, patience and good critical listening skills.
The three filters I will demonstrate in this video are wave hammer, equalization and noise reduction. I use Sony Sound Forge software 9.0 for this example. Other audio software programs can be used and more than likely include similar filters to those found in Sound Forge.
When I first began my career as an audio engineer, we used analogue recording tape in the form of reel-to-reel, cassette and eight tracks. The tape for reel- to- reel moved across an erase head, then a record head then playback head on professional recorders. It traveled from left to right. Tape width varied from ¼ “, ½”, 1” and 2”. Editing of these tapes required a razor blade, splicing tape and an edit block to help make the tape splice consistent before you used the splice tape to put the recording back together.
Cassette tapes traveled the same direction (left to right) except the tape width was much smaller as were the channel. Cassette tapes were hard to edit but editing was possible. In fact, the same manufacturers of reel-to-reel editing tools also made cassette editing tools.
This is where my forensic career began. People would edit the original cassette recording then make a copy of the edited cassette claiming it to be the original. Audio forensic experts would have to examine the cassette (that was claimed to be an original) to determine and establish authenticity.
Today, audio authentication is much more difficult because of digital audio recordings. However, with the help of an audio expert, the authentication can be established.
If you have a recording contained on a digital pocket recorder, more than likely the recording is complete and represents the facts as they occurred. Many police agencies use Olympus digital pocket recorders to document confessions and to record undercover investigations.
More than likely these digital recorders do not have editing capability so their audio recordings can be assumed authentic. However, experts in fraud can and will find ways to alter digital audio recordings regardless of the circumstances.
One of the activities an audio forensic expert executes to determine if an audio recording is original is to discover what the original audio recording was created on. If the recording was created on a computer (for example) then the computer that created the original audio recordings must be present for examination. This is the easiest way to determine fraud.
If the original recording equipment is no longer available, then the forensic expert must use visual display and electronic measurement to determine and establish authenticity.
Audio authentication is much more difficult today than it was back in the days of analogue recorders. Authentication is still necessary in litigation because alteration of the original conversations as they occurred can lead to litigation outcomes that are altered from their true justice.