Audio Engineering Society UK

Thomas Lund, TC Electronic A/S

Thomas Lund’s background includes work as a recording engineer and musician and the study of medicine – an unusual combination which may contribute to his understanding of loudness perception. Thomas has also been involved the design of many TC Electronic’s products, he has contributed to various standardisation groups on the subject of loudness, and has authored many papers presented to the AES and other bodies.

Traditional Loudness Measurement

Recent years have seen the ‘level’ of pop/rock music, as delivered by CD, steadily increase. Thomas cited the simple way that audio level has been measured as a partial cause. Historically, audio level has often been measured by peak programme meters, and commonly used definitions of overload have been very simplistic methods such as peak-level-counting (eg three consecutive full-scale samples equals overload). Such simple techniques of measuring (and by association, limiting) the level may have worked well when systems consisted of a microphone, a preamp and an ADC but with digital processing techniques numerous methods have been devised to increase the apparent loudness of material delivered on CD while ‘working around’ the peak-level limitations, apparently (we must assume) to some perceived commercial benefit to the record industry.

Many hold the opinion that such ‘hot mastering’ techniques are severely detrimental to the overall quality of modern music releases. Thomas calls this drive for increased level whatever the cost, coupled with a high willingness of broadcasters and consumers to use large amounts of data compression (for archiving, broadcast and replay), a ‘war on music’.

The Problems of Incorrect Levels

With such hot-mastering techniques, it is trivial to generate digital signals that exceed 0dBFS in the analogue output, after the assumed reconstruction or up-sampling filters. The greater-than-0dB peak levels can cause serious problems in the reproduction chain where some processes have been implemented with the assumption that 0dBFS is the largest signal they should expect.

Thomas offered demonstrations based on a commercially available ‘professional-grade’ sample rate converter, subtracting output from input. In this experiment the output should have been silent but differences could be heard clearly, manifested as ticks and signal-related noise. Other potential problem areas, according to Thomas, include limiting in mix-busses and codecs such as MPEG 1 layer 3 . These processes can all exhibit similar problems when faced with very high level inputs, a phenomenon Thomas further demonstrated. The codec problems can depend on the implementation of the codec as well as the codec itself.

Because of these issues, Thomas recommends normalising to -3dBFS – not to 0dBFS, in digital mixing and recording situations. He pointed out that the final 3dB increase can be done in the mastering room without any real quality loss, given that most recordings use 24 bits.

Better Methods of Loudness/Level Measurement

Thomas gave a functional summary of various improved methods of measuring loudness level and showed relative results based on ITU-R BS.1770. A simple improvement is the over-sampling peak programme meter which offers a more accurate representation of the true peak level.

Thomas also presented a loudness meter available from TC Electronic as a plugin for Pro Tools as ‘LM5 Loudness Radar Meter’.

TC LM5 Loudess Radar Meter

This meter includes representations described as ‘Loudness Units’ (LU) or LkFS, ‘Consistency’ and ‘Center of Gravity’, where Center of Gravity indicates the overall loudness of the programme material or music track, and Consistency indicates the ‘intrinsic loudness changes’ present in the track, with 0 representing a steady-state signal (one which has no loudness changes at all, eg a sine-wave) and progressively more negative numbers indicate reducing Consistency. Low Consistency scores such as -4 or lower indicate that the material may have a large dynamic range.

Conclusions

In conclusion, Thomas offered the following recommendations:

• Stop Counting Samples: There are better methods of measuring peak levels than counting the number of consecutive full-scale samples
• True Peak Level: Set maximum peak level at -1dBFS using a true peak meter equipped with oversampling capability.
• Dialog Level: Suggested level of dialog is -26 to -22 LkFS.
• Music: Suggested level of music is -20 to -20 LkFS.
• Avoid Peak level normalisation

If audio level is anchored only to peak level or only to dialogue, both commonly used techniques, loudness chaos is likely to ensue with extreme level jumps between programme, commercials and other home sources.

The tools and understanding exist to provide well-balanced loudness levels between different programmes and material, providing the end-listener a more pleasant viewing/listening experience and the potential for reduced distortion and overall quality improvement. Thomas outlined the problems and offered tools and methods for solving them.

Report by Nathan Bentall (edited by Keith Howard)