Introduction to IP Audio
by: Lawrence Harte

IP audio is the transfer of audio (sound) information in IP packet data format. Transmission of IP audio involves digitizing audio, coding, addressing, transferring, receiving, decoding and converting (rendering) IP audio data into its original audio form.

Figure 1.1 shows how audio can be sent via an IP transmission system. This diagram shows that an IP audio system digitizes and reformats the original audio, codes and/or compresses the data, adds IP address information to each packet, transfers the packets through a packet data network, recombines the packets and extracts the digitized audio, decodes the data and converts the digital audio back into its original video form.

Monoral (Mono)

Monoral audio is the generation and reproduction of sound in a single channel of audio. Monoral signals are produced by a microphone that is located in a single point near the audio source that captures a sample of the audio wavefront at that location. A wave front is the outermost 

combine, resulting in interference. Wave interference is the result of the interaction between two or more waves (such as sound waves) where the combining of the waves distorts or changes the shape of the wave. The interaction of sounds is an important consideration when designing listening areas such as concert halls or home theaters.

Wavefronts have crests (high points) and troughs (low points). Crests are compressed areas of medium and troughs are rarefied (expanded) areas of a medium. When these areas meet with other wave crests and troughs, they either ad or subtract. When compressed signals meet other compressed signals, it has a constructive effect (constructive interference). When a compressed signal meets with a rarefied (expanded) signal, it has a canceling effect (destructive interference). 

Sterophonic (Stereo)

Stereo is the generation and reproduction of a 2-channel sound source (left and right). The use of stereo can add the ability to hear audio where the relative position of sound sources (such as instruments in a band) can be determined. Stereo signals may be transmitted through the use of independent channels or be sent by sending a single (mono) channel along with channel difference signal(s) that can be combined with the mono source to produce the two separate channels. Use of a 

Stereo recording usually involves the use of two (or more) microphones that simultaneously sample different audio wavefront locations. The small differences in these sound recordings (time delays in combined sound signals) allow the user to determine the positions of audio sources.

The microphones may be located at different locations (separated by distance) or separated by an angle or a combination of the two. When stereo microphones are separated by a distance, it is called time of arrival stereophony. When microphones are separated by an angle, it is called intensity stereophony.

Time of arrival stereophony is the creation of stereo signals using two microphones (can be omnidirectional) that are separated by a distance. These microphones pickup the time delayed signals from multiple audio sources. Intensity stereophony is the creation of a stereo signal using two directional microphones that are separated by an angle (usually 90 degrees). These microphones pick up the combined wavefronts in specific directions from multiple audio sources. There are other microphone type and location options available for stereo recording.

Figure 1.2 shows some of the different methods for audio stereo recording. This diagram shows that intensity stereo recording involves the use of two directional microphones that are separated by an angle