Audio Data Rates for DTV: Getting More Bang for the Bit
When the ATSC rules were first published in 1995, the maximum data rate for a so-called Complete Main (CM) program that was Dolby Digital (AC-3) encoded was set at 384 kbps by standard A/53. The DVD-Video format was finalized at nearly the same point, and data rates up to 448 kbps were supported from the beginning.
Many of the very first DVD discs were released at 384 kbps and this may be attributable to some of the early Dolby Digital (AC-3) encoders defaulting to this rate. Interestingly, this data rate quickly increased to 448 kbps to take advantage of certain audio improvements and possibly even more likely to take advantage of the increased marketing opportunities afforded by advertising this higher audio quality. In a slightly later revision of the A/53 specification, this maximum data rate was increased to support the 448 kbps to match the capabilities of DVDs.
The increases in quality by moving from 384 kbps to 448 kbps for 5.1-channel audio may be worth the slight increase in data rate. One example of the improvement would be the frequency response of the main channel audio. At 384 kbps the bandwidth of these channels is limited to about 18 kHz, while at 448 kbps it is increased to slightly more than 20 kHz. The coding margin, or how close artifacts are to being audible, is increased. This is important for networks and affiliate stations that use Dolby Digital (AC-3) system for distributing audio. In these cases, two or more passes through Dolby Digital (AC-3) coding are common, and quality can be better maintained with enough data rate at each stage.