A picture may say a thousand words, but without audio to accompany the picture, the impact of the material is subdued. Viewers strive for higher audio quality within their home theater systems. This technology evolution creates the need for multichannel audio monitoring solutions. In particular, audio and video professionals need monitoring displays that help them visualize the auditory image that viewers will experience.
Traditionally, audio professionals have used level meters, Lissajous (phase) displays or correlation meters to monitor the audio channels. These methods work well for monitoring stereo signals but are difficult for operators to quickly gain an understanding of surround-sound systems. Monitoring multiple channels requires a new display. Ideally, it would give the operator or engineer, at a glance, an interpretation of how the mix will sound to the end customer.
Surround-sound setup
A 5.1 multichannel audio system locates the speakers as shown in Figure 1. The left (L) and right (R) channels drive the speaker pair in front of the listener and carry most of the music in the program. The center (C) channel primarily carries dialog, as producers usually want listeners to perceive this in the center of the video field.
The left surround (Ls) and right surround (Rs) channels drive the left and right speaker pair placed to the side or behind the listener. They typically handle the sound effects and ambient noises that create the aural illusion of a particular environment.
The Low Frequency Effect (LFE) channel delivers low-frequency non-localized special effects and creates the dramatic effects within the material (e.g., explosions). The LFE channel drives a high-power speaker (a subwoofer) that has a restricted frequency below 150Hz. The subwoofer is typically positioned in front of the listener. Although the speaker device is called a subwoofer, in a surround-sound system, it is often referred to as the LFE channel because it will have different responses depending on the size of the speaker system being used by the viewer.
Displaying surround sound
Germany-based RTW developed the surround-sound display available in the Tektronix WFM700, WFM6100 and WFM7000 series waveform monitors as well as in the WVR6100 and WVR7100 series waveform rasterizers. The display is configured to be similar to the speaker setup in viewers' home theater systems. Figure 2 shows how the waveform monitors display information. The L, R, Ls and Rs form the corners of the display, and audio signal amplitude is displayed diagonally from the center level of -65dBFS to 0dBFS at the outer corners of the display.
The response of the audio level can either be based on the linear root mean square amplitude of the signal or on the use of an A-weighting filter, which adjusts audio amplitude relative to the frequency response of the human auditory system. The test level is also noted by a mark at -18dBFS or -20dBFS level to aid in the setup of audio levels through the system. Cyan lines connect between each of the audio levels (L, R, Ls and Rs) and provide a total volume indicator. This indicates the balance level between the channels.
A bending of the cyan line that connects the amplitudes of the channels indicates the correlation between channels. Figure 2 shows the display within the WFM700 waveform monitor. The WFM700 audio module offers the ability to monitor the digital audio signal in either embedded or external AES/EBU digital inputs. A straight line connecting the audio level indicators of two adjacent channels indicates these channels have uncorrelated signals (i.e., a correlation value of 0.0).
Applying a lineup tone to all channels produces an octagon shape within the display. (See Figure 3 on page 96.) As the correlation between the two signals increases toward +1.0, the line connecting the audio level indicators bends outwards away from the center and toward the phantom sound source. As the signals move toward an out-of-phase condition (i.e., correlation approaches -1.0), the line bends inwards, toward the center, indicating the destructive interference and reduction in total sound volume associated with the out-of-phase signals.
The center channel has a special role within the surround-sound system and is denoted by a yellow vertical line positioned between the left and right audio channels. The display forms a center volume indicator by connecting lines drawn from the center channel to each of the left and right channels.
Phantom source indicators positioned around the perimeter of the display offer additional help in visualizing sound localization and correlation between channels. Four PSIs placed on each side of the display indicate the nature of potential phantom sound sources formed by L/R, L/Ls, Ls/Rs and R/Rs adjacent channel pairs. Additionally, a fifth Phantom Source Indicator (PSI) located above the L/R PSI indicates potential phantom sound sources formed by L/C and C/R.
Monitoring audio signals
Within digital audio there are a number of phenomena an audio engineer needs to monitor to ensure the audio signal quality and prevent distortions. Tektronix waveform monitors incorporate audio monitoring options that allow video and audio to be monitored simultaneously. The WVR7100 and WVR6100 offer a variety options for monitoring digital, analog and decoding of Dolby data streams. This gives the user the ability to monitor the audio signals at all the different layers within the facility.
In the digital domain, a clip can occur when a number of consecutive audio samples are at full scale. This could lead to distortion of the audio signal when it is reproduced in the analog domain.
A mute can sometimes occur if an audio packet is determined to be either non-audio data or if it contains an error. The number of consecutive samples before the alarm is triggered is user configurable.
The audio engineer can also set up his own selectable conditions for when the audio exceeds a certain level for a period of time an over condition occurs. The operator can also set up a level where below which he considers the audio level to be silent. When these conditions occur, the audio monitor then warns the operator of the specific condition within the audio display and provides a summary and log of the events. Figure 3 shows the error log display from the WVR7100 instrument with the various audio error events related to time code. An audio session display provides a summary of these errors within the material.
Unlike analog audio, in digital audio, it is important for the equipment to be synchronized to the same clock, either a 48kHz Word Clock or an AES/EBU digital audio reference signal (DARS). This digital audio reference must be supplied to all digital audio equipment in order for synchronous operation of the audio within the facility. Then, when signals are switched, they all have a fixed relationship to the DARS, and the equipment does not require a finite period to lock to the new signal.
There should also be a fixed relationship between video and digital audio signals to ensure complete synchronization of the facility. Within audio monitoring equipment, it is useful to apply the DARS to allow measurements related to the reference. The equipment should also measure the relationship between the audio signal and video signal and flag an alarm if there is an error.
Conclusion
Traditional audio monitoring tools can be used to monitor multichannel audio signals and provide simple audio session summaries of errors present within the material related to time code. However, a traditional Lissajous display is not sufficient to quickly interpret the interaction of multiple audio channels.
The surround-sound display provides a quick interpretation of the interaction of multiple audio channels. It offers an indication of the total loudness of the program, correlation between channels and indication of the dominant sound within the material.
This display can help audio engineers and operators more easily visualize the interaction of audio channels within a surround-sound environment. Understanding audio measurement techniques can help all engineers and operators more carefully monitor the audio signal.
Mike Waidson is a video applications engineer for Tektronix.
