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04/10/2013

ZEN And The Art Of Loudness Metering

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Television loudness has been a viewer’s concern for decades, but recent ITU standardisation has focussed the attention of programme creators on achieving compliance. Chromatec pioneered the first widely-used product to measure loudness, based on research carried out by Thames Television in the 1990s.
RFV ran a Q&A with Chromatec to learn more about the company...

When was Chromatec Video Products formed and what were its first products?

Chromatec was formed in the late 1980s by Neil Garner and a few years later the business was purchased by audio enthusiast and business entrepreneur Mike Stevens, who could see its development potential. Based in Bromley, Kent, Chromatec was set up to design and manufacture a new range of pioneering audio and video metering products. The original models were soon replaced and the AM Series were the first products designed that allowed four channels of audio metering to be superimposed on a through-going composite PAL or NTSC image with alarms displayed in-picture and simultaneously output on GPIO lines.
Subsequent products expanded the number of inputs supported and modularised the design. Support for IP based alarms monitoring was offered and Chromatec loudness was added first with the analogue AM-4, which was then migrated to the digital video products that followed. Later developments saw the same high quality metering introduced into a range of modular multiviewers.

What is loudness measurement and why was it introduced?

Viewers initially highlighted programme loudness as a problem during commercial breaks. Despite the introduction by the Independent Broadcasting Authority (IBA) of recommended peak levels for different types of programme content, viewer complaints persisted. Further tests at the IBA confirmed that the problem did not necessarily correspond to a peak level indication but that perceived loudness is dependent on both amplitude and frequency.
In late 1993, the Independent Television Commission (ITC) engaged Thames Engineering to design a loudness meter and produce prototypes for assessment. Working with John Emmett at Thames and Charles Girdwood at ITC, Chromatec engineered the prototypes which were successfully evaluated in several locations and later shown at IBC in 1994.
Channel 4 were one of the first users of Chromatec loudness metering and used it extensively throughout their facility, as well as several of their advertising providers.
In 2006, the ITU released BS.1770, specifying the algorithms used to measure subjective programme loudness, and true-peak signal level.

How does Chromatec loudness differ from R128/ITU-R BS.1770?

There are several differences including the frequency weighting curve, as shown in the graph (pictured). The Chromatec loudness meter was limited to two channels but BS.1770 allows for surround channels to be included.
In some respect, Chromatec loudness more closely resembles R128 short-term loudness because there is no audio gating and the three-second sliding rectangular time window is similar to the Chromatec four-second attack and decay time.
Whilst the unit interval is the same for both types of meters, where one unit is 1dB, the Chromatec loudness meter is a relative scale type. For R128, both relative and absolute scales are defined. The units used for relative scales are LU and for absolute measurements it is LUFS. But perhaps the main difference is that BS.1770 allows for an integrated loudness measurement that can be used to verify compliance of a complete programme.

Do Chromatec products support ITU-R BS.1770?

Absolutely. All Chromatec loudness metering products support the latest version of BS.1770-3 but most products also support Chromatec loudness which is still used as a reference standard with some content providers.

What are Chromatec's products?

The MD-16 and IPM-16 are both fully featured in-picture audio metering products supporting the display of embedded audio (or optional external audio) as vertical bar graphs superimposed on through-going video. They provide an effective in-picture alarm system for both audio and video errors. Two EBU R128 compliant loudness meters may be displayed, with each one allowing up to five channels to contribute to the loudness levels. The MD-16 card, which is currently offered in the openGear modular format, can be used as a stand-alone device or in multiples to form a multi-channel system with external control and alarms reporting via the openGear Dashboard® application. The IPM-16 exists as a stand-alone mini-converter with HDMI output and is configured via USB connection.

What are the key features of Chromatec's audio metering products?

Chromatec in-picture meters are designed to efficiently convey accurate level and loudness measurements. Accuracy is assured by up-sampling the audio to 192kHz using the guidelines provided in BS.1770-3 to obtain an estimate of the maximum true peak level. The loudness measurements are carried out at the incoming sample rate with an overall loudness tolerance of +/- 0.1 LU. Following BS.1771 recommendations, peak level meters are displayed separately from loudness meters to avoid confusion.
Loudness meters may be reset, paused and restarted again using the control interface. This affects both the integrated loudness and loudness range measurements, thereby allowing the loudness and the accumulated duration of a programme to be measured.

Image: MD-16 card for openGear and stand-alone IPM-16-EXT mini-converter

www.chromatec.com
(IT/CD)
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