The ITU-R standard BS1770-1, released in 2006, describes a fundamental loudness measurement algorithm. The basic technique was validated through listening tests on varied program material, performed in labs around the world.
In late 2010 the ITU committee which maintains BS1770 accepted (after much negotiation) changes submitted by the EBU. The result is a significant improvement in the calculation of loudness, making it much more sensitive to the loud portions of an audio segment. The effect is to prevent advertisers from significantly increasing the loudness of a portion of a commercial by manipulating the loudness elsewhere, for example where an announcer screams at widely spaced intervals throughout a commercial in an effort to get the viewers’ attention: Though the average loudness might be reasonable, the peak loudness could be quite annoying. The new method puts more emphasis on the louder portions and assesses this spot as louder than the original method would have.
This new version of BS1770 has just been published, and meters in use will need to conform to the revised specification. Most meter manufacturers have been following the developments in the EBU and ITU and have upgraded their software accordingly. Unfortunately they may not have done it correctly, or else the update might not have been installed in the meter you are using.
As a user, how do you assess whether a meter you are considering meets the revised specification? If it’s one you already have, how do you know if it has been modified to conform to the new requirements? We will describe a suite of tests developed specifically to check every aspect of a meter’s design and to give diagnostic information about any implementation issues that exist.
Our test suite comprises signals whose parameters change dynamically so as to stress individual portions of the measurement in isolation. Each test can then maximize its sensitivity to the specific implementation errors it was designed to detect. The signals were developed using mathematical models of the algorithm, including various intentional implementation errors, and were optimized to give the largest difference between readings obtained by correct and incorrect implementations.
The new BS1770 algorithm operates on a free-running 100ms measurement interval, so readings differ slightly with variations between the start of this interval and the start of the signal. Consequently the results for some tests are given as a range of acceptable values.
Understanding the ITU standard
The original BS1770-1 loudness measurement algorithm is diagrammed in Figure 1. The audio channels (except the LFE) are independently filtered with a low frequency roll-off to simulate the sensitivity of the human ear, and a high frequency shelf to simulate head diffraction effects. The combined response of these filters is referred to as “K weighting” and is illustrated in Figure 2. Surround channels are given a 1.5 dB boost to account for the relative gain provided by their position on each side of the listener. The individual channel powers are summed to obtain the surround program’s total power. This is averaged over the entire program yielding a single reading for the program loudness. If a “dynamic” indication of loudness is desired, a 3 second moving average is typically used. Readings are reported in LKFS (Loudness, K-weighted, relative to Full Scale) which may be thought of as “loudness dBFS”.
The BS1770-2 revision maintains the same filtering and power measurement, but changes the way measurements are averaged and presented. The integrator stage of Figure 1 is replaced with the processing shown in Figure 3. The power is summed over 400 ms intervals. These intervals overlap by 75% so a new value is obtained every 100ms. Results are gated with a Start/Stop control to allow selection of the audio segment to be measured. An absolute gate of -70 LKFS is applied, which eliminates lead-in and playout portions of isolated audio segments.
The algorithm focuses on the foreground portion of the audio by a two step averaging procedure (the yellow elements in Figure 3): The 400 ms measurement values are averaged over the content being measured. The resulting LKFS value is decreased by 10 and used to gate the 400 ms values a second time. The values which pass the second gate are averaged to form the final reading called “Integrated” loudness. This “relative gate” focuses the assessment on foreground sounds, which generally dominate viewers’ judgments of program loudness.
Loudness Meter Evaluation
The test suite described here is available for testing BS1770-2 compliance of any loudness meter. It may be downloaded as a dScope III script and supporting files from www.prismsound.com, or as a series of wave files and documentation from www.qualisaudio.com. More complete documentation of the tests and their expected results are included in the download package. Any new tests will be added as they are developed.
The menu for the script-based implementation is shown in Figure 4. The suite currently consists of 16 tests. All tests except Test 2 are stereo signals and should be applied to the LF and RF channels of a surround loudness meter. All tests except the first three comprise a 1 kHz sinewave at varying amplitudes. When the expected result is a range rather than a specific target, this is due to the 100ms alignment uncertainty.
The ITU BS1770 standard also describes a true-peak meter, which is checked by Test 1 as described in the technical note accompanying the test suite.
Test 2 is Dolby Digital encoded and stimulates all channels simultaneously, including the LFE. Power summation is checked by using sinewaves of slightly different frequencies. A compliant meter reads -23 LKFS. If the meter incorrectly sums in the time-domain, the reading will cycle. If the meter is affected by the LFE, the reading will be too high.
Test 3 checks the filter response at six frequencies: 25, 100, 500, 1 k, 2 k and 10 kHz using sinewaves of varying amplitudes to give a constant reading of -23 LKFS.
Test 4 alternates between -69.5 and -90 dBFS to exercise the absolute gating function. A compliant meter reads -69.5 LKFS. A meter which does not implement absolute gating reads -71.5 to -72.7.
Test 5 steps the amplitude between -23 and -6 dBFS at intervals between 0.5 and 1.4 seconds. A meter which correctly implements relative gating reads -7.7 to -8.2 LKFS; a non-compliant meter reads between -13.2 and -13.5.
Test 6 checks aspects of relative gating missed in test 5 by alternating between -36 and -20 dBFS. A compliant meter reads between -22.0 and -22.5 LKFS, whereas non-compliant meters read between -24.3 and -24.7.
Tests 10 through 15 evaluate an EBU-developed measurement called Loudness Range (LRA) which has not yet been accepted by the ITU. For more information refer to the technical note available with the test suite.
Test 16 measures the meter’s relative gate threshold, displaying -8 or -10 as appropriate. This is useful to determine if your meter is designed around the old standard and needs updating.
If a loudness meter gives the expected results for all of the tests, there is a very high likelihood that the implementation is compliant with BS1770-2.
Richard Cabot is the CTO of Qualis Audio and was previously CTO of Audio Precision. He was Chairman of the AES digital audio measurement committee for the development of the AES-17 standard. Ian Dennis is Technical Director of Prism Sound and as Vice-Chair of the AES digital audio measurements committee wrote the document which became the true-peak meter specification in BS1770.