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Audio Forensic Synchronization – What Happened When?

Wednesday, June 24th, 2015

Audio Forensic SynchronizationGenerally speaking, any device that captures video is also capable of simultaneously capturing audio. This audio can be crucial to the Forensic Expert, as it can show the expert more clearly “what happened when,” when it comes to a crime scene.

Picture this: a young man has just assaulted an older woman, and a police officer is in pursuit. As the young man begins to run from the police, another man on the street begins shooting video from his smart phone. The police officer is recording from both the police-car dash-cam, along with a body-worn camera, which he switched on when he began his pursuit.

As the young man continues to run, the officer announces “Taser!” and fires, activating the camera built into the officer’s stun gun. After sprinting around a corner, the young man is found dead. How did this happen? Who was responsible? When did the death occur? This is where audio can come in major handy.

As the appointed Audio Forensic Expert, you are tasked with determining what you believe happened in this situation. The evidence available to you includes the smart phone video from the witness, the police officer’s dash-cam, the on-board camera from the taser, and the body-worn camera.

This is where the video as well as audio is valuable to the investigation. An audio forensic expert can synchronize the audio in chronological order. Forensic audio enhancementcan help hear the events that occurred and remove any unwanted sounds to help synchronize the recordings. Begin with the event that took place the earliest. In this case, it would be the police officer’s dash-cam, as it is always running. Next, find the portion of audio that starts the witness’ footage. Listen for a certain sound or yelled phrase for reference, and when you find that sound source from the officer’s dash-cam video, you’ll know that this is where the witness began recording.

This can also be done by visually inspecting the waveform. Large, quick spikes in the level can make it very easy to quickly sync the audio. Most software will also allow you to zoom in closely on the waveform so you can line the waveform up as closely as possible.

Next, you’ll want to find the point where the body-cam began recording. Again, this will require critical listening and visual analysis to align the sounds from the body-cam evidence and the witness video to get an idea of when the officer began recording.

From there comes the last piece of evidence: the taser camera. Remember when the police officer announced “Taser”? Well, the second the trigger on that stun gun is pulled, the in-board camera kicks on. The body-camera audio will give you an idea as to when the officer announced his taser use, along with when the video clip begins.

This will give you the most accurate occurrence of events. The actions of the taser camera are the most recent recording of the event, and in it you notice a loud sound that couldn’t be heard in the other recordings. That sound was the sound of a pistol, which another officer around the corner pulled out to shoot in an attempt to detain the criminal. This is what the body-camera, witness video and dash-cam, did not see. However, due to the alignment of the audio, the expert is able to see, in chronological order, the events as they occurred.

Synchronization does not always go so smoothly. Sometimes different frame rates are used in different videos, which can alter the speed of how the different videos play back. Most modern digital recording technology is self resolving and does not have this issue, but there are still devices that do not. These can cause the video and audio to be in sync at one point, but slowly drift apart throughout the video. It’s important to be aware of this so scientific adjustments can be made to make sure that all the events are synchronized as accurately as possible.

The audio makes it much easier to synchronize all of the pieces of evidence together. Some of the time the audio must be forensically enhanced in order to better hear the events as they occurred. The audio can provide both auditory and visual cues, through viewing the waveform, to use as reference points so an accurate sync can be completed. With only video, the different perspectives and qualities would make it extremely difficult to find exact reference points to line up.

Not all cases will give the expert this much to work with, but when working with multiple clips of the same occurrence, having a critical ear can be invaluable to understanding the timeline of the situation. Video can be powerful, but its direct counterpart, audio, can be essential to finding the cause of confusing and misleading investigations.

How to Set Up a Microphone for CCTV Systems

Tuesday, March 10th, 2015

CCTVClosed circuit television systems have become a major contributor of evidence to court cases. While the video from these systems is often very important, the audio can often play just as much of a role in the investigation. At Primeau Forensics, we often are hired to enhance not only the video from a CCTV system, but the audio as well. As an audio forensic expert we are qualified to perform forensic audio enhancement.

Clients typically hire us for enhancements because the original CCTV system was not set up properly and was capturing less than ideal quality audio and video. Many times, the audio is more valuable than the video because of what was said during the event. While enhancing the audio is possible, setting the microphone on the system correctly can be extremely beneficial when an incident does occur. Getting a good, clean signal from a microphone relies on two key principles: microphone gain and microphone placement.

Setting a proper gain structure for a microphone will always yield the best result for any kind of recording. Gain is applied to microphones because microphones have inherently low levels. A preamp is used to amplify the signal before it is recorded into a system. When setting the gain, the goal is to get a high enough level that the signal is audible, while also making sure that the level does not clip the system or preamp. Clipping means that the signal has exceeded the capabilities of the system and begins to distort. This distortion hurts the quality of the audio and can make it very difficult to understand what people are saying in a recording.

Gain structure is often set based on the input signal, which makes setting a surveillance system microphone difficult. The input signal of a surveillance system is always changing and cannot be manually reset whenever people enter or leave the area. When setting the gain for a surveillance system microphone, it is usually a good idea to test different levels of sound in the room. Having someone talk or even yell in the room while you set the level can ensure that the recording will not clip when it is recording later on.

We recently recovered some surveillance video evidence that required an audio enhancement. When we received the audio, we found that the gain had been set too high and the entire recording had clipped. We also found that because the room was small and filled entirely with hard surfaces, there was a buildup of reverberant sound. Reverb consists of reflections of sound off of surrounding surfaces. Some reverb is always present, but too much can begin to cover up the direct sound. Direct sound is the original sound coming directly from the source, such as a person speaking. In this case, the gain should have been set much lower on the microphone. This would have produced a much cleaner and more audible recording. Reverb is a more difficult issue to combat and relies much more on the microphone placement.

Different microphones will have different pickup patterns, which means that they will pick up sound in different directional patterns. Typical microphones used for surveillance systems are either cardioid or omni-directional microphones. Cardioid microphones pick up sound from one direction and reject sound from the opposite direction. Omni-directional microphones pick up sound from all directions. Knowing what kind of microphone your system is using is the first step to setting up a microphone. If you are using a directional microphone, it should be aimed at the area where the sound sources will be. Omni-directional microphones are easier to set up because they do not need to be aimed in any direction.

When placing the microphone, it is important to be aware of other extraneous noises in the room. We often see CCTV systems placed near the ceiling and in corners so they can obtain the best view point of the area. This is not always the best location for the microphone depending on how the room is designed. If an air vent or another electrical device is near the microphone, they will add a large amount of noise to the recording and can cover up desired sound. If a directional microphone is being used, try aiming the rejection end of the microphone at the unwanted sound source. This means the least amount of the unwanted signal will be picked up.

Reverb can also be an issue in smaller spaces that have no absorptive surfaces. When the direct sound is buried by the reverb, it can make the desired signal muddy and undefined. Acoustic treatment can be added to a room to deaden the amount of reverb, but this is more often an approach for musical spaces. A typical fix for a surveillance system can be placing the microphone closer to the desired sound. Placing the microphone in the center of the ceiling instead of in a corner could cause the microphone to pick up more of the direct sound, resulting in better and clearer audio. Because sound and reverb tends to build up more in corners, placing the microphone away from the corner will also prevent it from picking up those extra reflections.

Audio evidence is a very prominent part of many investigations and court cases. Setting up a microphone properly for a surveillance system can often make a huge impact on whether that audio can be used as evidence or not. As an audio forensic expert, I come across many audio recordings that could have been much more audible if the system had been set up properly. When installing a surveillance system, setting the gain for the microphone and placing the microphone properly will always improve the quality of the recorded audio.

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.

 




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