iPod and mobile TV
Mobile and computer devices have a different gain structure and make use of different codecs than domestic AV devices such as television. Tests have been performed to determine the standard operating level on Apple devices. Based on 1250 music tracks and 210 broadcast programs, the Apple normalization number comes out as -16.2LKFS (Loudness, K-weighted, relative to Full Scale) on a BS.1770-3 scale. It is, therefore, suggested that when distributing podcast or Mobile TV, to use a target level no lower than -16LKFS. The easiest and best-sounding way to accomplish this is to: 1) Normalize to target level (-24LKFS); 2) Limit peaks to -9dBTP (Units for measurement of true peak audio level, relative to full scale); and 3) Apply a gain change of +8dB. Following this procedure, the distinction between foreground and background isn’t blurred, even on low-headroom platforms.
Headroom in broadcast
The ratio between max peak level and average operating level is called headroom. Using BS.1770, headroom can be regarded as the ratio between true peak level and program loudness. The amount of headroom is genre-dependent, as is shown in Figure 2.
In commercials and pop/rock music, the headroom requirement can be 6dB or even lower, while a cinema movie may need more than 20dB. Furthermore, movies and classical music only need this headroom for a short period of time, while “beat music,” in general, requires the same amount of headroom from start to end.
When a signal path offers less headroom than required for conveying a program, limiting or clipping will result.
Unfortunately, every part of the signal path may constitute a headroom bottleneck, so broadcast doesn’t sound better than its weakest link. In analog broadcast, headroom is frequency-dependent, with less at high frequency because of transmission emphasis. Analog TV has a headroom of 10dB to 12dB, while FM radio is often operated with 8dB or less.
In digital broadcast, noise is generally lower, and emphasis is no longer part of the equation. Consequently, a lower average level in combination with a higher peak level is now a possibility. With target and peak level specified by ATSC A/85 (-24LKFS/-2dBTP), a generous 22dB of headroom is available. This is more headroom than broadcast has ever had.
Used for stereo only, the AC-3 codec isn’t more sensitive than other codecs at a similar bitrate with regard to the max true peak level it handles without clipping. If a typical pop/rock track is encoded without attenuation, AC-3 clips frequently, but if the same track is attenuated so peaks don’t exceed -1dBTP, the problem is solved.
The real challenge with AC-3 and headroom is the way 5.1 is handled. The majority of consumers are listening in stereo, so if AC-3 is transmitted without an independent stereo stream, the decoder has to downmix every 5.1 program that comes along. This is where problems start. The decoder doesn’t include a transparent downmix limiter, so one option is to use conservative mix coefficients to avoid stereo overloads: L, R: -6dB; center: -9dB; SL, SR: -12dB. Now there will be no mix overloads, but systematic level jumps will occur when switching from native 5.1 to native stereo.
The real peak level problem in AC-3 doesn’t come from the data reduction itself, but from the downmix section in the decoder. If broadcasters could keep peak level low, decoder mix coefficients wouldn’t have to be conservative. On the other hand, it would be a shame if a general restriction of headroom in broadcast was inflicted because of first-generation codecs with technical design issues.
Recent experiments have pointed to a solution more tolerable than using a general limit threshold at -6dBTP. In 5.1 action movies, the center channel generally uses more of its headroom than the other channels. The AC-3 downmix solution is, therefore, simple: Use -6dBTP limiting for all the lateral channels, but -3dBTP for center.