Visual audio-signal monitoring
Jan 1, 2010 12:00 PM, By Michael Kahsnitz
The introduction of HD video in broadcast has triggered an increasing demand for quality control and monitoring of audio signals. Here's what you need to know.
Figure 6: This SSA view shows a polarity error on the left front channel.
The correlation scale ranges from -1 to +1; the zero point is at the center of the scale. “Correlation” refers to the degree of correspondence of two audio signals. Entirely identical signals (for example, a mono signal on both stereo channels) have a correlation of +1; completely unrelated signals have a correlation of 0. The same value is displayed when a channel fails. The correlation meter also allows for concluding the “width” of a stereo signal. A displayed value of 1 refers to a mono signal located in the stereo center; on the other hand, 0 indicates a signal reproduced on the channel sides only; no sound from the stereo center is heard. Stereo mixes normally have a correlation between 0.3 and 0.7.
A stereo signal with a negative correlation is normally considered as technically defective. When two channels of the stereo signal are identical but their phases are reversed by 180 degrees due to polarity reversal, the correlation meter shows a value of -1. A value between 0 and -1 results from a stereo mix that contains phase-modulated components as generated by effect units, delay units and electronic sound generators. Downmixing such signals to mono causes drastic sonic changes due to phase cancellation.
Stereo-image display
Stereo-image displays (which are also referred to as goniometers or audio vectorscopes) provide comprehensive information about phase relations, intensity, stereo width and directions; however, the user requires specific basic knowledge for correctly interpreting the screen display. This is because these devices are not as intuitive and easy to understand as the simple ±1 bar graph of a correlation meter.
Stereo-image displays originally were modified oscillographs featuring a monochrome display tube. Today, these have been replaced by modern high-quality multicolor flat screens — for example, TFT displays. While those devices cannot replace acoustic checks of a production, they are still very useful for supporting the user in assessing the balance of a stereo mix. Stereo-image displays show the phase relations of signals contained in the mix in real time and allow for discovering errors caused by polarity reversal or clipping. Current devices are even capable of displaying the phase relations and levels of the input signals at the same time, which is quite convenient.
Since the practically utilizable display range of those units is relatively small, a vectorscope must include an automatic gain control (AGC) circuit to keep the signal level within an appropriate range regardless of the actual input level. On the other hand, this means the instruments constantly readjust the processed level. Therefore, this instrument is not suitable for assessing the absolute level or even the loudness of a signal; it deals only with level and phase relations between the left and right channels.
Experienced users immediately notice whether a stereo signal has an appropriate stereo width, is shifted from the center and contains out-of-phase components. In general, a wide presentation hints at many out-of-phase portions, while circular images suggest a large stereo width and an appropriate phase. The position of that “ball” quickly shows tendencies toward a side. A mono signal results in a line; its direction on the display indicates the signal position within the stereo panorama.
Real-time analyzer (RTA)
Another important tool for visual audio QC is a real-time spectral analysis. Typical applications of RTAs are found in the area of sound reinforcement; these not only include examinations of room and speaker-system characteristics but also allow for quickly localizing sudden feedback using a peak-hold function on the analyzer display. An RTA can also serve users in production and mastering of music programs and continuity by allowing them to assess the spectral balance of the program and to adjust it as necessary using an EQ. In addition, an RTA provides for localizing interfering resonant frequencies that occur, for example, when recording sources in small speaker booths. Experience has shown that a real-time analysis based on individual third-octave band filters as used in acoustics measuring matches the characteristics of the human ear particularly well and is therefore capable of providing a meaningful representation of the spectrum. Of course, users working with a sample rate of 96kHz in production are particularly interested in the effective bandwidth enhanced to 48kHz. None of us will aurally perceive 36kHz noise produced by the defective fan of an air-conditioning system; however, such spectral components (and their interference) may subsequently cause undesired artifacts in the mix. Therefore, in addition to the spectral representation of the audio range, a summing display of spectral components above the audible range up to half the sample rate would be desirable.
Conclusion
A main objective of any measuring is the comparability of the results. This is achieved by specific standardized measurement units and methods. Unfortunately, many established measurement standards suffer from lack of uniqueness in practical use; this is true for audio technology, too. Scopes of standards remain subject to interpretation, which makes true comparability rather difficult. In this context, the critical point is that the same measuring standards must be applied everywhere within the immediate working area. All instruments must be calibrated accordingly. Never lose sight of the fact that setting up a meter or another analyzer tool will never change the actually examined audio signal — only the personal “view” of it.
Michael Kahsnitz is head of engineering at RTW.
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