Posts Tagged ‘audio clarification’

Audio Enhancement vs Clarification; The Audio Filtering Process

Wednesday, February 6th, 2013

FilterOftentimes I’m given digital audio recordings by litigators to “enhance”. The request is for enhancement – they want to hear the voices in the recording better than what they currently hear. By the way, in litigation it should be termed ‘audio clarification’ – ‘enhancement’ has a negative connotation, implying that the audio has been in some way altered. Clarification is a better word – courts like it better and you don’t open yourself up to having your investigation questioned, or an accusation that your ‘enhancement’ somehow ‘altered’ the recording.

Some of the time, when I receive the files the first thing I discover is that the file is not an original recording and there are filters that have been previously applied to the file.

The other day I received a file from a litigator from another country and I agreed to do a pro bono listen, as I often do when I’m learning what their expectations are versus what reality will allow. (Television shows like “CSI” have given a false perception of what is and is not possible when it comes to audio enhancement).

The file I received from the International pro bono job had a very heavy noise reduction filter applied to it. I can tell just because I can recognize the sound of noise reduction on an audio recording. It would have been a complete waste of time to try to work with that file so I asked if they had an original version of the file, or a copy of the file that had not had any processing on it. The next morning there it was in my in basket – a link to the complete file without the noise reduction.

My message here is you can try things on your own but always save the original unprocessed audio file before filtering. Then, apply one filter at a time to the maximum capability of the filter to help your clarification.

As an audio forensic expert I sometimes apply several filters through the course of an investigation for a clarification project. However, amateurs tend to select a lot of different filters and not use them to their optimum potential before applying another filter.

This is why it’s important to seek the advice and retain an audio forensic expert to handle your evidence, have a chain of custody that can be reported at the end of the investigation, and properly apply audio engineering best practices and techniques, forensically. Following protocol that is accepted in the scientific community, allowing you to maintain integrity on your audio evidence will bring you the best results for your clarification process.

If your budget doesn’t allow for an audio forensic expert keep in mind a secondary message in this blog post: apply one filter at a time to its maximum potential when restoring the low quality in an audio recording.

photo credit: JPS NRF-7 NF Filter via photopin (license)

Audio Clarification 101: The Science of What the Ear Perceives

Thursday, April 26th, 2012

audio clarification Ever since I began my career as an audio engineer back in the 70’s, noise and sound quality have always been a focus mine as an audio expert. I remember when my interest first started back in the day when records—those twelve inch round black things—would play at parties and the background  noise was never a point of focus. But today as vinyl is making a comeback, audio philes all over the world are starting to collect vinyl again. Last week, in fact, The Who released a 10 inch vinyl record to celebrate National Record Store day, but I digress…

There are several things that I do with almost every audio recording that needs to be clarified. Step one is to listen to a good portion of–if not the entire–audio recording and make observations about the noises that I feel can be removed. The second step is to determine what tools I have available that I can use to remove those unwanted sounds.  Lastly, I begin the process of noise reduction in the steps of audio clarification.
Almost every software program that’s available has a noise reduction program in it. Once you learn how to use that noise reduction program, you first select the audio file you are working with and begin by removing unwanted noise. You’re only going to be able to remove one type of that unwanted noise, so my advice is to try different settings within the noise reduction program. Then, the next step is to use equalization to first remove additional unwanted sounds and then use a second equalization process to increase the desired or wanted sounds. When that’s complete, the last step of the restoration and clarification process is compression. Compression basically helps to make all of the remaining sound even so that if what you’ve done so far has, for instance, decreased the volume of the digital file to such an extent that the volume is very low, the output of the compressor plug in will help you raise that volume to a more desirable level.

Also, if it is an audio recording that you are trying to hear a conversation of, the playback of what you have done with all of these tools is almost as equally important as the entire clarification process is. I have found that listening to a very hard to hear audio recording on different devices helps my ability to create a transcript or to hear all of the sounds more clearly. For example, if I am restoring an audio that has bad or hard to hear conversation on it even after I have finished the restoration process, I will then burn that file onto a CD and listen to it, for instance, on a boom box or on a clock radio. Because both devices will play that recording back differently, my physical sound perception will pick up those sounds differently and aid in the transcription process.

This whole process is all about the science of what the ear perceives. Because the perception of sound is different from person to person, it is important to listen back to the hard to hear conversations, music in the background, any other sound that you’re trying to identify (gunshots, screams, etc.) on more than one playback device. The main reason is because headphones and the various types of playback devices that have external speakers will reproduce that sound differently based on each device that you’re listening to the sound on.

So in conclusion, follow the steps that I’ve outlined above as you are using your software program and do the best you can with the tools that you have to work with when restoring your audio file. It is also almost as equally as important to listen to it played back on multiple devices, which will help your clarification process out.

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Clarification of Audio Recordings for Authentication

Thursday, October 14th, 2010

digital audio recordingAll recordings–both digital and analogue–have a noise floor. The term originated when manufacturers of analogue audio recorders referred to the extraneous noise that their machine created in addition to the desired recorded audio signal.

Often a background noise constitutes most of the audio recording and covers a portion of speech that needs to be audible in order to determine a series of events pertinent to the case. These noises can often be removed by the audio forensic expert to help determine facts about the series of recorded events.

Background noise and noise floor extraneous sound can consist of a heating or air conditioning fan running, refrigerator motor, window fan, clock, fluorescent lighting, wind, rain, car running and even radio or television. All these sounds contribute to the background noise and noise floor of a recording and aid the forensic examiner in authenticating a recording. However, this background noise can interfere with the forensic examination. Clarification is part of the forensic examiner’s job. It is appropriate for the forensic examiner to remove these background sounds in order to authenticate or clarify an exhibit of audio recorded evidence.

Some of the recordings experts are asked to authenticate are confession recordings created by law enforcement agencies. Defendants exclaim, “That is not what I said, they edited it” or “There is more I said that has been edited out of the recording.” Due process entitles both parties in litigation to examine any evidence presented in their case. However, original recordings are not always available for examination. How do you as a law enforcement official feel about the absence of original recordings?

I have worked on cases where missing “original evidence” was considered spoliation of evidence. Personally I believe that circumstances of each case should be considered by the forensic examiner before any decision has been made by either party.

If the forensic examiner observes characteristics that are noticeably questionable, then the expert must notify the officials in charge of their findings during the preliminary examination phase of the forensic investigation. Original recordings are required, and if not produced, a motion to suppress the evidence should be filed.


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Equalization for Audio Clarification

Monday, October 11th, 2010

equalizationEqualization settings can be adjusted to help the forensic expert recover lost or poorly recorded audio.  This blog post covers basic audio clarification.

The sound spectrum is measured in frequencies. All audio sounds like a car horn, gunshot and spoken words have frequencies associated with them. Music has the most broad frequency range and covers 80HZ to 12,000 Khz in most cases. In an attempt to keep this blog post easy to understand, I am going to focus on spoken word audio.

Forensic spoken word recordings range in frequencies from 1Khz to 3Khz. Most sound spectrum frequencies above 4Khz and below 800 Hz do not contribute to the spectrum of spoken word recordings. The forensic expert should remove frequencies above and below 1Khz to 3Khz when background noise is present to increase the clarity of spoken word recordings.

Beautiful spoken word fidelity can include frequencies from 400 Hz to 5,000 Khz when recorded in an audio recording studio using state-of-the-art digital audio recording instruments like mixing council, pre amplifiers, microphones and digital computer recording software. However, when it comes to forensic audio, most recordings that need authentication, clarification and identification have been made on digital pocket recorders and wireless interception devices.

These recordings are subject to extreme background noise because they are recorded outside the professional studio environment, next to noisy furnaces and air conditioners and in public environments like restaurants and other places where unwanted chatter and background noise can spoil a recording. Equalization helps the forensic expert begin the clarification and restoration process and is a very important element of restoring the audio recording.

The best way to determine what frequencies need to be adjusted is to first listen to the audio recording. The trained critical listening skills of the audio expert can identify both wanted and unwanted frequencies during the initial listening pass.

Next, the expert will remove frequencies where unwanted sounds exist. Background hums in room ambience, air conditioners and furnace blower motors exist in low range frequency spectrum’s like 250 Hz all the way down to as low as 60Hz.

Radio frequency interference is probably the most difficult distortion to remove from the audio recording. When radio frequencies have static, it is best to first try noise reduction before setting equalization’s settings to clarify the spoken word recordings. The best methods are those that are tried and tried again until the desired outcome is achieved.

It is best for the expert to save the original audio file before applying any equalization treatments just in case it becomes necessary to go back and start over. Also, every step of equalization should be noted in the expert’s work notes so they can recreate the process for the court should it become necessary.



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Noise Reduction for Sound Clarification

Friday, October 8th, 2010

noise reductionThe first step when restoring audio is to remove background noise. More often than not, a recording in need of sound clarification or restoration has background noise covering the sounds that are desired to be heard. Noise reduction is the process of reducing and often eliminating that unwanted background sound. Sound like wind, motors, lawnmowers, electronic hums and buzzes and other sounds that may be louder than the spoken word and cover that speech so it is not audible.

Once the background noise is removed the desired spoken words can be heard. There are many levels of clarification acceptance. Some of the time the recording needs to be clarified so a jury can hear it. Other times the recording may only need to be audible to a transcriptionist or court reporter. Then, once the transcription is created, the audio forensic expert can go back, listen to the clarified recording comparing to the transcript and correcting any discrepancies using the expert’s critical listening skills.

The recording can then be certified by the forensic expert and an affidavit created as to the genuineness of the transcript for the legal proceeding. That way, if the audio recorded evidence is difficult to hear in a court room, the audio expert not only certifies the audio recorded evidence but can also testify on its accuracy.

This is a good example of a strategy between lawyers and expert witnesses. It is the expert witness’s job to suggest strategies to lawyers, public defenders, police and other government agencies that retain the expert witness. The lawyer will have difficulty and not represent the client to the best of their ability if they do not understand the audio forensic process and retain an audio forensic expert. The audio forensic expert of course cannot develop a strategy without a lawyer who is licensed to present the law case in the court room.

Before beginning your case that includes audio evidence, consult with a qualified audio forensic expert.

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