SENTINEL
Surround Audio Program Monitor

Stereo audio requires minimal out-of-phase content between channels to insure proper compatibility with monaural reproduction.

Monitoring products for stereo audio often do this by using a Lissajous (X/Y scope) display of left vs. right with a correlation indicator that shows the ratio of in-phase to anti-phase content. Surround sound audio increases the complexity with the multiplied possibility for phase cancellations – all to be watched at once! The important question: “is any part of my program going to significantly change in level when reproduced in stereo or mono?” goes unanswered. Complex visual displays and additional correlation indicators – to handle up to 10 possible phase conflicts – make monitoring difficult even for personnel experienced with surround audio.

The Qualis Audio SENTINEL takes a different approach. It directly assesses stereo and mono compatibility of a surround mix. The measurements are converted to a simple green/yellow/red “stoplight” display. Error - conditions (red light), may trigger an alarm. The user is freed from continuously watching and interpreting a screen, allowing more attention to other aspects of producing, evaluating or transmitting the program.

(Click on image above for a larger view)

The DISPLAY (main) page of the SENTINEL is shown above. Standard digital and analog meter scales are user selected in the SETUP page. The bars at the left show channel levels using the selected metering standard with the maximum values represented by the horizontal line. The channels are downmixed to stereo and mono and their levels displayed on bars to the right. Above each bar is a numeric display of true peak amplitude, allowing precise metering of signals at or near clipping.

COMPATIBILITY - All input channels and the downmixes are analyzed in the frequency domain. The power of each component in the original signals is compared to its power in the downmixed signals. The results are grouped by octave band and summarized in the three COMPATIBILITY columns. If the power is lower in one or more downmixed signals there has been cancellation. Moderate reductions result in yellow indicators while severe reductions are shown in red. The frequency at which errors occur helps identify the material affected. Errors between 250Hz and 4kHz suggest dialog problems, below 250Hz generally impact special effects and those above 4kHz affect ambience.

BALANCE - Graphical multichannel displays based on the Lissajous display concept are also used to infer level balance around the sound field. The SENTINEL BALANCE display supplies this same information in a more intuitive way. The fraction of program power contributed by each surround channel is computed and sets the brightness of the corresponding loudspeaker icon. For example a program with “phantom mono” shows equally bright left and right front speaker icons.

LOUDNESS - The SENTINEL continuously measures program loudness according to the recent ITU recommendation BS.1770-1 (see above right). The resulting dB LeqLRB (LKFS) reading may be used to monitor the measured loudness value for a program, compare it to the dialnorm value or be used to set the dialnorm value. The measured loudness is available via the SENTINEL display or may be accessed electronically from another computer. No separate specialized equipment is required for the measurement; no programming is required to access and use it.