Audio level control
Oct 1, 2008 12:00 PM, By Aldo Cugnini
Correct modulation requires a fine-tuned ear.
Figure 2. Loudness meter
Click to enlarge
In the 1960s, the FCC conducted a study of audience complaints regarding loud commercials on broadcast television. As a result, CBS Laboratories developed a series of loudness meters based on many of the loudness factors described above. (See Figure 2.)
By placing a loudness measuring circuit within the servo loop of an automatic gain controller, as in Figure 3, an automatic loudness controller was also developed. This kind of AGC-based analog loudness controller is not practical, however, because it will continuously force the program loudness to a constant level, resulting in an objectionable compression of the original program dynamics.
A better solution is available today with digital transmission: dialog normalization, or dialnorm. Dialnorm works by setting the ATSC receiver dialog level to match a predetermined loudness level for each source program. By measuring the long-term average program dialog level and then transmitting this as the dialnorm parameter in the bit stream, loudness consistency can be maintained from program to program and even across different broadcasters.
In the ATSC AC-3 bit stream, dialnorm is a 5-bit word, transmitted every 32ms, that sets a reference level in 1dB increments from 0dB to -30dB. At the receiver, dialnorm is then used to adjust the output level of the audio decoder. (See Figure 4 on the next page.) By equalizing to a common -31dBFS (full scale) level, the perceived loudness can be made uniform across programs and broadcasters. ATSC specifies the loudness measurement parameter as Leq(A), which stands for A-weighted equivalent loudness. It takes into account frequency dependence and loudness integration, both of which are needed to measure speech material.
Figure 3. Loudness controller
Click to enlarge
The CBS algorithm mentioned earlier also takes into account loudness addition from separate critical bands, which helps facilitate the measurement of wideband nonspeech material. The ITU-R standard, BS.1770, titled “Algorithms to measure audio programme loudness and true-peak audio level,” specifies a more recent loudness measurement, Leq(RLB), using revised low frequency B-weighting.
Although Leq(RLB) is simpler to realize than the other methods, it performs better in subjective testing. It is now being proposed to replace Leq(A) as the recommended loudness metric in the ATSC standard.
One of the challenges of using dialnorm correctly is for different broadcasters to use equivalent practices in setting up their systems. Broadcasters can use different settings for dialnorm, as long as each one is consistent in their setups; any dialnorm setting can be used, as long as it is correctly generated. For example, a -23dBFS setting will be attenuated in the receiver by 8dB, -27dBFS by 4dB, and so forth.
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