A test reviews the ITU BS.1770-2 and CBS algorithms.
Editor’s note: With this article, we begin a series of high-level presentations from industry experts on issues of importance to the science and practice of broadcast and production technology. We are pleased to launch the Broadcast Forum with an in-depth examination of loudness and measurement with a feature article by Robert Orban.
In 2009, the ATSC released “Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital Television (A/85:2009).” This was later updated as A/85:2011. A/85 specifies use of a long-term loudness meter based on the ITU BS.1770 algorithm for measuring the loudness of DTV broadcasts.
In December 2011, the FCC adopted rules implementing the CALM Act1, which, by law, forbids commercials from being louder than non-commercial program material. The new FCC rules incorporated ATSC A/85 (and, by implication, the BS.1770 meter) as an objective means of verifying that the rule was being obeyed.
Because loudness measurement per BS.1770 uniformly integrates all program material, quiet passages tend to lower the measured value. To prevent this, the ITU added gating to the BS.1770 standard, which was revised as BS.1770-2 in March 2011. The gating causes the meter to ignore silence and to integrate only program material whose loudness falls within a floating window extending from the loudest sounds within the specified integration period to sounds that are 10dB quieter than the loudest sounds. This is because humans tend to assess loudness based on the louder sounds in a given program. As of this writing, ATSC A/85 has not been updated to incorporate the BS.1770-2 standard.
The ATSC A/85 2011, ITU-R BS.1770-2 and EBU R 128 documents are available as free downloads and can easily be located with a search engine.
CBS loudness meter
For many years, Orban has used the Jones & Torick loudness controller and loudness measuring technology2 in its products for loudness control of sound for picture. Developed after 15 years of psychoacoustic research at CBS Laboratories, the CBS loudness controller accurately estimates the amount of perceived loudness in a given piece of program material. If the loudness exceeds a preset threshold, the controller automatically reduces it to that threshold. The CBS algorithm has proven its effectiveness by processing millions of hours of on-air programming and greatly reducing viewer complaints caused by loud commercials. Orban first licensed the CBS algorithm and began using it in products in the early 1980s.
Because the ATSC recommends the BS.1770 algorithm, many broadcast and cable engineers facing the problem of controlling broadcast loudness have wondered how the CBS and BS.1770 technologies compare. An earlier version of an Orban white paper compared the CBS and BS.1770-1 (non-gated) meter. This presentation was revised in March 2012 to incorporate results from tests using the BS.1770-2 algorithm and EBU - TECH 3342 “Loudness Range” algorithm. The new measurements were performed using Version 2 of the Orban Loudness Meter3. This article compares the CBS and BS.1770-2 meters because it is expected that the ATSC will eventually update A/85 to specify BS.1770-2, which will more closely harmonize A/85 with its European counterpart, EBU R 128.
A/85 and R 128 differ significantly in philosophy and recommendations. Probably the most important difference, A/85 asserts that the loudness of a so-called “anchor element” (which is typically dialog except in programs emphasizing music, like live concert recordings) is most important, while R 128 asserts that the integrated loudness of the entire program is most important4 and, therefore, program loudness should be normalized based on an integrated BS.1770-2 measurement. The philosophy behind A/85 is similar to that of Dolby Laboratories, which, for many years, has asserted that dialog anchors most film and television programs, and that listeners set their volume controls to make dialog comfortably intelligible5. (Orban agrees more with A/85 than with R 128.)
The purpose of this paper is to present, using both meters, comparative measurements of the output of Orban’s current audio processors6 with the latest refinement of the CBS loudness controller technology.7
A stereo recording of approximately 30 minutes of unprocessed audio from the output of the master control of a San Francisco network station was applied to the 2.0 processing chain of an Optimod-Surround 8685 processor, set for normal operation using its TV 5B general purpose preset. The digital output of the processor was applied to the digital input of an Orban 1101 soundcard, which was adjusted to pass the audio without further processing and to apply it to an Orban software-based loudness meter that simultaneously computes the BS.1770-2 Integrated loudness and CBS loudness. The first 750-second segment of the program material was a daytime drama with commercial and promotional breaks, while the remainder was local news, also with commercial and promotional breaks.
The BS.1770-2 meter was adjusted to produce a 10-second integration window in which, per the BS.1770 standard, all data are equally weighted. The CBS Loudness Gain control was set to -3.12dB. Data logged every 10 seconds and included the maximum meter indication produced by both the BS.1770 and CBS meters in each 10-second interval. (See Figure 1.) This produced 165 data points, which were imported into a scientific plotting application.8
Orban’s experimental long-term loudness measurement, based on the CBS meter and first published in 2008, was also included in the measurements and is also shown. This algorithm attempts to mimic a skilled operator’s mental integration of the peak swings of a meter with “VU-like” dynamics. The operator will concentrate most on the highest indications, but will tend to ignore a single high peak that is atypical of the others. This algorithm can be seen to share certain characteristics with the floating gate first introduced in EBU R 128 and later adopted in BS.1770-2.
The algorithm displays the average of the peak indications of the meter over a user-determined period: 10 seconds for these measurements. The average is performed before dB conversion. All peak indications within the period are weighted equally with the following exceptions:
If the maximum peak in the window is more than 3dB higher than the second-highest peak, it is discarded.
All peaks more than 6dB below the maximum (or second-to-maximum, if the maximum peak was discarded) are discarded.
Because the CBS long-term measurement discards a single peak if it is more than 3dB higher than the second-highest peak, the CBS long-term measurement tends to be biased about 3dB lower than a measurement that shows the maximum peak indication of the CBS meter in a 10-second period. The amount of bias depends on whether or not the loudness applied to the meter’s input is well controlled. This bias can be seen in the figures in this article. Because the Orban meter allows control of the level applied to the CBS algorithm via the “CBS Gain” control, setting it 3dB higher could better match the CBS long-term measurement to the BS.1770-2 Integrated measurement at the expense of moving the “maximum peak loudness” indication 3dB higher.
To provide a baseline for discussion of the loudness-controlled results, we measured the unprocessed audio that was applied to the Optimod 8685’s input. The measurement results show the loudness of the unprocessed audio both as a function of time and as a histogram. The histogram sorts the meter outputs into 0.25dB or 0.25LK-wide slices9 and shows the number of measurements that fit into each of these slices. (See Figure 2.) Thus, the histogram portrays loudness consistency. When the histogram is clustered tightly within a few bins, the loudness is more consistent than it is when the histogram is spread out into more bins.
With all meters, the histogram of the unprocessed audio shows a wide spread. This is consistent with the EBU Loudness Range measurement for the entire clip, which was 16.5LK, while the LRA for the daytime drama alone was 19.2LK (including commercials). The BS.1770-2 Integrated loudness was -20LKFS, integrated over the entire measurement period, although the inconsistencies between the loudness of program material and commercials are large enough to make this 30-minute measurement essentially meaningless.
In general, the loudest parts of the unprocessed audio are commercials and promos, both network and local. These are anywhere from 5dB to 10dB (or LK) louder than the rest of the program material. This inconsistency was not a problem because the station in question was using an Orban automatic loudness controller on-air, which smoothed out loudness differences before its input.
While the general shapes of the CBS and BS.1770 loudness vs. time curves are similar, there were some significant differences. For example, at approximately 1250 seconds, the CBS measurement shows a sharp loudness spike that was caused by a network news report that was equalized to emphasize frequencies around 2kHz to 3kHz, where the ear is most sensitive. The BS.1770-2 measurement did not indicate this as being louder than the surrounding program material although to our ears, it clearly was.
Editor’s note: This Forum paper will conclude with “Part 2, Measured Results,” in the September issue of Broadcast Engineering magazine.
The CALM Act applies only to U.S. broadcasters and cable providers.
Jones, Bronwyn L.; Torick, Emil L., “A New Loudness Indicator for Use in Broadcasting,” J. SMPTE September 1981, pp. 772-777.
This software is available for free download at http://orban.com/meter/.
EBU – TECH 3343, “Practical guidelines for Production and Implementation in accordance with EBU R 128,” version 1 (February 2011), p. 29
Riedmiller, J., Lyman, S., Robinson, C., “Intelligent program loudness measurement and control: what satisfies listeners?” AES Convention Paper 5900, 115th Convention (October 2003)
Optimod-Surround 6585 and 8685, Optimod 6300 (with version 2.0 and higher software), and Optimod-PC 1101 and 1101E (with version 2.0 and higher software).
For a further discussion of the CBS and BS.1770 technologies, see http://orban.com/meter/Technology.html. The ATSC A/85:2011 document also discusses the BS.1770 algorithm.
PSI Plot: http://www.polysoftware.com/plot.htm
Unfortunately, two terms for the same loudness units have been used in different standards documents. For convenience, we will use LK and LKFS (as used in ATSC A/85); these units are the same as LU and LUFS (used in EBU R 128 and BS.1770) respectively.
—Robert Orban is chief engineer, Orban.