Naturally, we want our broadcasts and web streams to sound as polished as possible. We also want to avoid wasting contributor time on routine production tasks that could otherwise be spent creating great content. Two new features in Airtime 2.3.0 bring us closer to that goal - automatic silence detection and adjustable ReplayGain correction.
A frequently requested feature, silence detection in Airtime uses the new program Silan, written by Robin Gareus, to analyse an audio file when it is imported into the Airtime library. After upgrading your server to Airtime 2.3.0, any new file imported by Airtime's media monitor is checked for leading and trailing silence, as well as average level. Non-destructive cue in and cue out points and an amplitude correction factor are saved in Airtime's database automatically. When you create a smart block, a playlist, or drop the file into a running show, those cue in and cue out points and correction factor will be used in the show playout. The original length of the file is displayed for your reference, so you can see how much silence has been trimmed. Airtime uses many digits of precision to avoid cumulative timing problems, but the display of time is rounded to tenths of a second for practical purposes. The amplitude correction for each file can be viewed by enabling the ReplayGain column in the Airtime library.
Should you wish to adjust cue points manually, you can still do that in Airtime, but the tedious task of trimming and levelling each and every file in the Airtime library is no longer necessary, removing an entire step from the workflow. Perhaps you are broadcasting a news report that has been uploaded in a hurry, and the journalist has left a few seconds of silence at the end of the edit - which is easily done. Or perhaps you are streaming a music track ripped from CD where the mastering engineer has left silence at the beginning of the piece for dramatic effect. Files from different producers no longer have to be manually adjusted to match average levels.
The ReplayGain standard (originally proposed by David Robinson) was already implemented in Airtime, but with a fixed output level of -14dBFS average. For broadcast purposes, we might need to match average level with louder or quieter live streams from other systems. With this requirement in mind, a ReplayGain Modifier control has been added to the Stream Settings page in Airtime 2.3.0. This slider enables output to be adjusted up to 10dB either way from ReplayGain's reference level. Of course, going too loud is likely to result in clipping, given that an average level of -4dBFS is louder than a typical death metal album - but automated output level is now under the broadcaster's control.
Arguably, given the relative cost and limited reach of mobile data services, streaming media is less likely to be listened to in noisy environments like a car, or on the street. Therefore it may not be necessary to compress and limit Internet streams as much as FM broadcasts have been processed in the past. The demand for upmarket headphones that Dr. Dre has introduced to the wider world might engender a new appreciation of dynamic range.
Thanks to the combination of Silan and ReplayGain, Airtime playout sounds tighter, levels are balanced with plenty of headroom, and the risk of dead air is greatly reduced. We'd like to say a big thank-you to both Robin and David for their contribution to web-enabled and collaborative open source broadcasting!