In This Section
Journal of the AES
2009 September - Volume 57 Number 9
Small frequency variations in the electrical power network, which are the same through the area serviced by the network, can be used to assess the integrity of audio and video evidence. Small amounts of hum usually leak into a recording, and this provides a unique time signature. By archiving power line frequencies over many years, the time of a recording can be determined by comparing it to the information in the archive database. Similarly, discontinuities in hum frequency or multiple frequency components provide additional forensic evidence.
The formal procedures for the scientific authentication of audio media in investigative matters and legal proceedings have been updated as a result of the widespread conversion from analog to digital technology since the 1990s. Topics discussed include recommendations regarding handling examination requests, laboratory space and technical equipment, digital authenticity protocols, evidence handling, and expert testimony. Additionally, four case studies illustrate how this formal process has been applied to real-world examples.
The mechanical vibration of a loudspeaker drive unit can be represented by a set of distributed parameters as a set of linear transfer functions. This representation can be used to distinguish mechanical and acoustical problems. Starting with an analysis of mechanical modes such metrics as accumulated acceleration level (AAL) can be used to interpret the vibration patterns. The smoothness in AAL is useful for predicting peaks in the sound pressure level (SPL) curve.
Directional Audio Coding (DirAC) is a perceptual method for reproducing spatial audio using existing microphones and surround sound configurations. In the analysis phase, the direction and diffuseness are continuously estimated in frequency bands. This information, together with input signals remapped into a set of virtual microphones corresponding to the reproduction configuration, is used to recreate the spatial audio experience in the listening setup. Subjective listening tests in both an anechoic chamber and in a standard listening room showed that the mean opinion scores were almost always good to excellent, which is higher quality than traditional techniques.
Standards and Information Documents
AES Standards Committee News
Libraries and archives; audio metadata; loudspeaker modeling and measurement
Analog audio needs a separate physical circuit for each channel. Each microphone in a studio or on a stage, for example, must have its own circuit back to the mixer. Routing of the signals is inflexible. Digital audio is frequently wired in a similar way to analog. Although several channels can share a single physical circuit ( e. g., up to 64 with AES10 ), thus reducing the number of cores needed in a cable. Routing of signals is still inflexible and any change to the equipment in a location is liable to require new cabling. Networks allow much more flexibility. Any piece of equipment plugged into the network is able to communicate with any other. However, installers of audio networks need to be aware of a number of issues that affect audio signals but are not important for data networks and are not addressed by current IT networking technologies such as IP. This white paper examines these issues and provides guidance to installers and users that can help them build successful networked systems.
Audio forensics is a growing field with increasingly sophisticated tools. Gunshot recordings can be analyzed acoustically to reconstruct information about the timing of events and the geometrical layout at the crime scene. Electric network frequency data, picked up on recordings made at crime scenes, can be used to uniquely identify the time at which events occurred. Furthermore, voices of speakers recorded at crime scenes can be automatically matched with specific exemplars or entire databases of records with an increasingly high chance of accurate identification. Such systems typically employ a number of metrics extracted from voice signals.