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Blindspot – How to Make Digital Audio Recordings for Evidence

Thursday, February 19th, 2015

audio recordingsAs an Audio Forensic Expert, my day to day activity includes forensic services like audio enhancement and authentication, as well as voice identification. Audio enhancement is probably the most common service I provide, because more often than not, the audio evidence was not recorded in the best way possible. Audio evidence can often be one of the most important pieces of evidence for a case, so it should always be given a great deal of attention.

One of the most common ways people create digital audio evidence is by using digital audio recorders. Law enforcement will often use them for interrogations and confessions, and sometimes even out in the field as a backup for their dash cam or body cam. People outside of law enforcement use them for creating audio evidence as well.

I would like to mention that concealed audio recordings are not always legal. Federal law states that creating an audio recording only requires one person’s consent, but some states follow a ‘two-party consent’ law. This law means that all parties who are on the recording must give permission to the person recording in order for it to be used as evidence in court. I highly suggest looking in to your own state’s laws regarding concealed audio recordings before making one.

When creating a digital audio recording that is going to be used in court, there are many things one should be aware of before making the actual recording. The biggest issue I usually come across is low recording levels. While it is possible to increase the signal level afterwards through forensic audio enhancement, this is unnecessary time and money spent. This will also increase the noise floor of the recording, which can make it more difficult to hear what is happening in the recording. Creating a clean and audible original recording can make the enhancement process much easier and can often make the evidence much more useable in court.

When preparing to make an audio recording, regardless of whether it is a concealed recording or an interrogation recording, the user should always look at the settings of the digital audio recorder.

Two major settings determine the quality of a digital audio recording: sample rate and bit depth. Together, these settings also determine the bit rate of a recording. Changing these settings will affect both the quality of the audio recording and the amount of space used on the digital recorder. When creating digital audio evidence, it is necessary to balance these two in order to get a high quality recording while optimizing the amount of space on the digital recorder. Thankfully, many digital audio recorders will record in lossy compressed formats like MP3 files, which take up much less space and don’t sacrifice a lot of quality.

When recording digital audio in an MP3 file format, the two key settings to pay attention to are the sample rate and the bit rate. The sample rate will ultimately determine the range of frequencies the recorder picks up. At least two samples are needed to record any frequency, which means the sample rate must be twice as high as the highest frequency you need to record.

The range of human hearing is roughly between 20Hz and 20kHz. Typical audio recordings are done at 44.1kHz to capture the full range of human hearing. While this is standard for music and other professional recordings, it is not always necessary for audio evidence.

Most fundamental frequencies of the voice are between 100 and 500Hz with some of the most important harmonic content between 1kHz and 4kHz. This means that a sample rate as low as 8kHz can sometimes be adequate for recording a conversation, which will also save a large amount of space on the digital recorder.

Bit rate determines the amount of bits that are processed per second, which determines the fidelity of the audio. Typical MP3 files are recorded between 192kbps (kilobits per second) and 320kbps, but they can be as low as 32kbps. Just like with the sample rate, a higher bit rate means a higher quality of audio but also a larger file size. The issue that arises with low bit rates is that the compression process applied to the file can start creating digital noise in the recording. This digital noise can often cover up parts of the recording and once it is there, it is very difficult to remove.

When determining what settings to use on a digital recorder, it is always a good idea to make multiple test recordings before making an audio recording that will be used as evidence. These test recordings will let you try out the various settings and then listen back to see what sounds best and what fits your needs the most.

Another setting that is sometimes included on digital recorders is the ‘voice activation’ setting. This setting will start and stop the recording based on the amount of signal the microphone is picking up. While it can be a good way to save space on the recorder, it is not recommended that this setting be used when creating any kind of digital audio evidence. If this setting is on, the digital recorder could stop recording at a key moment in the conversation and miss a crucial piece of evidence. If extra space is needed on the digital recorder, adjusting the quality settings is a much better way to go. Recording all of the content at a slightly lower quality is a lot safer than relying on the ‘voice activation’ setting and missing important content.

Monitoring the battery life on the digital recorder is another very important thing to keep in mind. In some applications, like recording an interrogation, the digital audio recorder can simply be plugged into the wall so it will not run out of power. In other cases where you do not have this option, make sure the battery is fully charged or you have put in new, good quality batteries. Keeping extra batteries with you is also good practice, just in case the recorder does run out of battery and needs a replacement.

When creating the actual recording, try to be as close as possible to the person being recorded. As I mentioned before, one of the biggest issues with audio evidence is a low volume or record signal level. The farther away from the source the microphone is, the lower the signal level and the lower the signal to noise ratio. This means that less of the desired signal and more of the unwanted background noise will be recorded. Background noise can include any extraneous sounds such as furnaces, refrigerators, air conditioners, televisions or even the internal sound created by the digital recorder itself. These sounds can detract from the quality of the recording and often make the desired signals unintelligible.

Placing the digital recorder in a good location is key for making a good digital audio recording. Keep a few things in mind when making your recording. First, the microphone should always be aimed at the subject that you are recording. When placing the recorder in a pocket or a purse, aim the microphone towards the subject. Also make sure that the digital recorder is relatively stable in its location, because any movement of the recorder will be picked up by the microphone and can cover up other parts of the recording. Pay attention to any materials that may be in between the microphone and the sound source; the thicker the material, the more damping there will be on the signal, which will decrease the record level.

Many digital audio recorders have a microphone input which allows you to use an external microphone. The external microphone is always the best option to use if the recorder is going to be placed inside something. When using this option, it is always a good idea to use a high quality external microphone.

There are many different types of microphones that will work better for different situations. Lavaliere microphones are extremely helpful because they are small and usually omnidirectional. This means that they will pick up sounds from all directions and they can be placed anywhere on your person while the digital recorder stays in your purse or pocket. Other microphones, such as directional microphones, may work better during police interrogations because the subject will not be moving during the recording.

As I mentioned before, always create a test recording before making the recording that will be used as evidence. Testing different microphones, microphone placements and locations will help you learn how your digital recorder works and responds to different environments. If possible, try conducting the test recording in the same place that you will create the real audio evidence so you can prepare for any extraneous background noises and other obstacles. After making the test recordings, listen back so that you can make sure the desired sounds can be heard and the sound quality is high enough.

 

Knowing Your Digital Audio Recorder

Thursday, December 18th, 2014

digital audio recorderWith digital audio recorders, there are a lot of options when it comes to the quality of the audio recording. Despite the easy access to these options, they are often overlooked. People are either unaware of these settings, or simply forget to check them when they begin a recording. While most settings on a digital recorder will yield a good enough quality recording, I have come across digital recorders with very low quality settings that could result in very distorted or unintelligible recordings. If you are using a digital audio recorder, it is important to have a basic understanding of what contributes to the quality of your audio recording.

Two major settings to be aware of are the sample rate and the bit depth of your recording. The sample rate determines how often a sample is taken from an incoming waveform. The bit depth determines the number of bits for each one of these samples. Together, these settings and the number of channels will determine what the bitrate is. The bitrate is how many bits are processed per a period of time. Bitrate plays a bigger part in lossy audio files.

Sample Rate

There are a few standard sample rates used in most recorders, often including 44.1kHz, 48kHz, and 96kHz. Audio is usually recorded at 44.kHz to capture the full range of human hearing. An audio waveform has a positive and negative pressure area; therefore a minimum of two samples must be taken from a frequency to reproduce it. The range of human hearing is generally given as 20Hz to 20kHz, though it can vary depending on the person. With a sample rate of 44.1kHz, frequencies as high as 22kHz can be recorded, which more than covers the average person’s hearing range. Higher frequency ranges such as 96kHz are used to capture twice as many samples and therefore create a higher quality recording, though most would argue that it is almost impossible to hear any quality difference unless using professional audio equipment.

Bit Depth

The bit depth, as mentioned, determines the resolution of each sample that is taken. A 16 or 24 bit setting is most commonly used; depending on what medium is being used. Audio CD’s, for example, only use 16-bit audio. The bit depth will determine the signal to noise ratio of a recording depending on a logarithmic formula. The signal to noise ratio is the comparison of the desired signal to background and internal noise. A 16-bit recording will have a 96dB signal to noise ratio, while a 24-bit recording will have a 144dB ratio. While 24-bit does have a higher SNR, the 96dB range of a 16-bit recording is often more than enough to create a good quality recording.

Bitrate

When using a format such as an MP3, bit depth no longer applies because of the lossy compression format. This is when bitrate becomes a more important factor of a recording. The bitrate is the number of bits processed in an amount of time, typically written in kilobits per second. The bitrate of an uncompressed audio file, such as a .WAV file, can be determined from the bit depth, sample rate, and number of channels. A CD with 44.1kHz, 16-bit stereo audio has a bitrate of 1411kbps. MP3 and other lossy audio files typically have much lower bitrates, which is why they are so much smaller than uncompressed formats. They achieve this through perceptual coding, which essentially removes parts of the data that are found to be unnecessary and unperceivable by the human ear. Typical MP3 music files have bitrates between 192kbps and 320kbps in order to maintain good quality. Digital recorders that record lossy formats will often have optional bitrates as low as 32kbps.

When choosing what settings to use for a recording, it’s important to consider the purpose of the recording. Music production is usually done with at least a 44.1kHz sample rate and a 16-bit depth. WAV and AIFF files are typically the file formats used for the master recording. When later compressed to MP3, as mentioned before, a bitrate between 192kbps and 320kbps is used to maintain the highest quality possible after compression. When a digital recorder is being used for another purpose, such as recording a conversation, other settings may optimize the performance and memory of the unit while still maintaining a high enough quality.

Whenever a smaller sample rate, bit depth or bitrate is used, the recording will always take up less space on the memory of the recorder. This can be very important to someone who may need to leave the recorder on for long periods of time. When capturing audio evidence, a recorder may need to be left on for hours or even days. If this is the case, and a lower quality file needs to be used, it is important to know how to go about maintaining quality while optimizing the memory.

Options and Limitations

While the range of human hearing covers up to 20kHz, fundamental frequencies of voice do not fall in the higher end of the frequency range. The human voice is strongest in the 1kHz to 4kHz frequency range. Because of this, it is possible to capture a completely audible and intelligible recording of people talking with a sample rate of only 22kHz. This would mean the highest frequency recorded would be 11kHz, which is still much higher than the most important frequencies in the voice. Some recorders can even be set to an 8kHz sample rate. While this does save a lot of space on the recorder, this means the cut off frequency would be 4kHz. This may be acceptable for some applications but may also cut down on the clarity of the voices. When a large amount of background noise is present, the higher frequencies between 4kHz and 10kHz can add some needed clarity to the voices. It is always a good idea to test the different sample rates before using them to make sure that the quality will be adequate for its purpose.

When trying to optimize the memory on a digital recorder, it is almost always a good idea to use a lossy compression format, such as an MP3. This means that determining the bitrate rather than the bit depth will be a factor in the size of the recording. As mentioned before, a bitrate between 192kbps and 320kbps is often very good quality for an MP3. When recording only a voice in which the content of the recording rather than perfect quality is the concern, lowering the bitrate can be very helpful for conserving space. One should be cautious when lowering the bitrate because the data compression may begin to affect the intelligibility of the recording. When too much compression is introduced, digital noise become easier to hear, which can sometimes cover up the desired signal. I have heard 32kbps recordings that had so much added digital noise that the much of the conversation in the recording had become unintelligible.

In summary, digital audio quality is determined by its sample rate and bit depth or bitrate. There are many options for these settings and not all of them may result in a good quality recording. It is always important to check these settings and be aware of the limitations each setting comes with before beginning a recording. Take into account the content of what you are recording and the quality of audio that is needed. The better you know your digital recorder, the more effective it becomes.

 

Release or Destroy Audio Evidence After Litigation

Wednesday, January 16th, 2013

audio evidenceAn audio forensic expert listens to audio recordings of litigation points during a forensic investigation. Oftentimes, these audio recordings are very sensitive in nature. They are crucial to the litigation and must be destroyed or returned to their provider once the litigation is complete. Generally, in civil litigations, settlements are reached based on a series of conditions. One of those conditions is often that all of the evidence be destroyed that was used to process that litigation. It is very important that the audio forensic expert stay in good communication with their lawyer contact to make sure that they clearly understand what to do with the audio evidence once the litigation is complete.

If that audio evidence were to become available to the wrong people after the litigation settlement it could violate and have other ramifications to the terms and conditions of the settlement. That’s why it is very important for the forensic examiner to know what is expected of them from the attorney and other litigators regarding that forensic evidence.

In the past I’ve received letters from attorneys asking me to dispose of the audio evidence I used to investigate their case. In my professional opinion, there are only two options for an audio forensic expert for dealing with the evidence they examined upon reaching a settlement in the litigation.

One, return the evidence back to the attorney or your contact person in the case. Signature required, delivery confirmation is essential so that the forensic expert has proof that it was returned to the appropriate party. Second, destroying the evidence by physically breaking or destroying the playing surface on a compact disc, which is the most common media for delivering the audio recordings to be examined by the forensic expert. Or delete files stored in computer hard drives or other external storage devices, such as a thumb drive.

So, we either return the evidence to the contact person, signature required so that we have confirmation and proof of delivery; or we dispose of the evidence by destroying the compact disc or delete from external storage device. It is best, if possible, to get their instructions in writing (particularly if the instructions are to destroy the evidence), just to protect yourself and make sure all your bases are covered.

I recently had a case that settled and I was contacted by someone who identified himself as a friend of one of the litigators, and claimed to have permission to listen to part of the audio recording that I investigated for this litigation. I denied the (alleged) friend access to the recording and contacted the party who retained me for the investigation and let them know I was being contacted. The huge problem here is that, had I given the audio evidence to the person that contacted me not only could I have jeopardized my reputation in the forensic community if this person had released that evidence, it would also have had a huge ramification on the settlement for my client because, that audio evidence being compromised, would also violate the conditions of his litigation settlement (not to mention opening myself up to a possible lawsuit for releasing the audio evidence to an unauthorized party).

Bottom line, when the forensic expert completes a forensic examination, it is the forensic examiner’s responsibility to determine what to do with the evidence that they have investigated so that it does not fall into the wrong hands and violate the terms of the litigation settlement.

photo credit: Salat via photopin (license)
 

Maintaining Audio Evidence Integrity

Tuesday, June 5th, 2012

evidenceThroughout the course of my professional career, the necessity to maintain the integrity and the chain of custody of the original evidence has never changed. As a forensic expert, I am often asked to authenticate and clarify audio and video evidence that is not original, and that does not have a chain of custody that has been maintained. When working for defendants, I make it a point to bring this out in my reports and in my testimony. I often find challenging audio and video evidence containing anomalies or inconsistencies when conducting my forensic examination.
When I work for government agencies and police departments, I guide them through the process of maintaining integrity and a proper chain of custody on audio and video evidence. In fact, I have recently had several cases that involved audio clarification for different police departments.

Frequently, clarification of in-car video or audio tracks simply requires separating the radio chatter from the officer’s microphone. There are a couple of ways that I can do that as a forensic expert. First, in many cases, the radio chatter is on a separate audio track. I load the in-car video in its entirety into my forensic computer and then remove the audio tracks and work on them in Sound Forge to remove the radio chatter. This helps the prosecutor learn more about the audio portion of what went down and why that is important to the case. Other times, I apply compression and equalization to help remove unwanted noise and bring out the speech. Lastly, I lay that audio track back onto the video and export a new digital video file in an mpeg format so that it can be burned to a DVD. This process does not change the integrity of the evidence and my forensic documentation contains of all of the steps that I took so that I can maintain the chain of custody on the evidence as well as its integrity. This processes that I use is acceptable in the scientific community as well.

In litigation, sometimes the small facts about a video or audio recording are important. When those facts are difficult to see or hear, police departments and other litigators look to an audio forensic expert to help them better understand portions of a recording that are not as clear as was hoped when the recording was made. It could be because of wind noise, background cars passing by on the highway, or distortion from a microphone transmitter.

There are a number of things that can distort an audio recording, but a forensic expert can use multiple techniques to help restore the audio so we can better understand the facts as they occurred. A forensic expert’s role in litigation is very important. As an expert, he or she can help courts interpret the process of exporting the original evidence from the digital recorders, the chain of custody and the purpose of the processes that were applied to the evidence to help restore both sound and vision. Finally, a forensic expert presents these processes and the evidence in court–keeping it easy for the court to understand the facts as they occurred.

Audio Evidence

Tuesday, November 30th, 2010

chain of custodyAudio evidence can include but is not limited to confidential informant recordings, confession recordings, telephone intercept, voice mail and 911 calls. The goal of the forensic examiner is to be sure a chain of custody has been maintained like any other evidence and that the original recording is used as evidence and not a copy.

This was important back when analogue audio recordings were primarily used as evidence.  It is especially important today with digital audio being the primary format for audio evidence.  Once the digital audio file has been burned to a CD, it is no longer considered an original because it has been removed from its native environment (computer or pocket digital recorder). Furthermore, the audio file can be authenticated when it is in its original environment.

There is no reason that an original digital audio recording cannot be preserved when a legal proceeding is expected or may occur.  The rationale that it is ok to erase or delete the original so the recording equipment, in the case of a digital pocket recorder, could be reused is not a logical thought process. In many cases, this could be considered as spoliation of evidence.  A CD copy is not original because once the audio recording is removed from its native environment; the audio evidence is vulnerable to alteration and editing.  Often times, this alteration can go undetected even by an experience audio forensic expert. This is why preserving the original file of the audio evidence is extremely important.

Whether you are law enforcement presenting a confidential informant or confession audio recording, a private individual presenting a voice mail or concealed audio recording, always preserve the original recording so there is no doubt of the authenticity and integrity of the audio recorded evidence.  Consult an experienced audio forensic expert to assist you in authenticating the audio evidence for a fair and accurate representation of the facts as they occurred in their original environment.

Audio Evidence and State Public Defenders

Monday, November 8th, 2010

State Public DefendersI have a great deal of respect for State Public Defenders. They are lawyers who practice law for people who cannot otherwise afford legal assistance. The State Public Defender coordinates accused citizen’s legal representation to indigent people who are accused of a crime. Some cases include juvenile and others include others on appeal in cases where they have already been found guilty. Many clients of State Public Defenders are on death row. Legal representation provided by State Public Defender Offices or through private attorneys who contract with the State Public Defender or who are appointed by the Court is held in high regard by this audio forensic expert.

As a forensic expert, I have assisted state public defenders in Michigan, Ohio, Missouri and Iowa. The cases I assisted in had audio evidence that was used against their client. In many of these cases prior attorneys did not understand the processes involved with audio evidence and were not aware of how a forensic expert can help.

When I am contacted by a State Public Defender, I often begin our relationship on a pro bono basis. I believe that public defenders are a strong element in our countries “Due Process” system. Most other legal proceedings both civil and criminal treat defense guilty until proven innocent, but that’s another story.

Public defenders are about the most unbiased government employees left in the legal system and a very valuable asset to those who either have been convicted or are about to be convicted. Many clients of State Public Defenders are given another chance they would have not had in other circumstances.

The character of State Public Defenders is very professional and second to none. They exemplify true professionalism in a non self-serving capacity.

If you are reading this blog post and feel your audio evidence was not presented properly and you were not given a fair and partial trial, I suggest you contact your State Public Defender for more information.

 

photo credit: Courtroom via photopin (license)




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