I recently finished a documentary project featuring the use of Dolby-Digital 2/2 surround sound. Of the 4 audio channels, two were gathered by a Sony stereo microphone and recorded onto the HDV tape, while the rear two were captured by an identical Sony stereo mic and recorded onto Minidisc. This methodology has served me well for several years.
My editing process involves several steps. The first step is the usual scene placement and timing of cuts and dissolves, using just the front audio. Once that is done, I go back and locate and sync the individual rear audio segments to the front audio segments on the timeline. What once was a colossal editing/syncing task has evolved into a process that actually moves along surprisingly quickly.
The next task is to adjust front/rear sound levels, not only overall but also in relation to each other. Finally I overlay a commentary track, and the project can be encoded into ac-3 2/2 sound.
Up until the latest project, this worked well. But on this recent project, there was a day with blustery winds, and even with foam and furry Windjammers on the mics, there were numerous times where the wind noise was dominant. Okay, fine, that's what adjusting the audio levels with the rubber-band line is all about. On the Vegas timeline, playback was fine.
But not so in the final audio encode. Much to my chagrin and after a number of final disks have been produced, I discover a "feature" of the ac-3 encoding process. According to Dolby, the user has the option of determining how much audio compression takes place by adjusting the "line mode profile" parameter. If set to "none," then no dynamic compression or clamping takes place--UNLESS THE ENCODER THINKS IT'S NECESSARY TO AVOID CLIPPING. In other words, what I got on the final DVD is a very different sound than what Vegas played for me on the timeline. The problem was that wind gusts on the rear audio channels would just clobber the desired audio (a singing performance) on the front audio channels. The ac-3 encoder in its wisdom would turn down the front audio by 10+db whenever a wind gust would hit.
Okay, so I learned something. I need to carefully manage the peaks on the rear audio channels. But what shocked me was how the surround audio played just fine on the Vegas timeline. The sound from the timeline had been my gold standard. Now I've learned it's not. Rather than just encoding the audio, the Dolby Digital process will make its own changes under certain conditions.
My editing process involves several steps. The first step is the usual scene placement and timing of cuts and dissolves, using just the front audio. Once that is done, I go back and locate and sync the individual rear audio segments to the front audio segments on the timeline. What once was a colossal editing/syncing task has evolved into a process that actually moves along surprisingly quickly.
The next task is to adjust front/rear sound levels, not only overall but also in relation to each other. Finally I overlay a commentary track, and the project can be encoded into ac-3 2/2 sound.
Up until the latest project, this worked well. But on this recent project, there was a day with blustery winds, and even with foam and furry Windjammers on the mics, there were numerous times where the wind noise was dominant. Okay, fine, that's what adjusting the audio levels with the rubber-band line is all about. On the Vegas timeline, playback was fine.
But not so in the final audio encode. Much to my chagrin and after a number of final disks have been produced, I discover a "feature" of the ac-3 encoding process. According to Dolby, the user has the option of determining how much audio compression takes place by adjusting the "line mode profile" parameter. If set to "none," then no dynamic compression or clamping takes place--UNLESS THE ENCODER THINKS IT'S NECESSARY TO AVOID CLIPPING. In other words, what I got on the final DVD is a very different sound than what Vegas played for me on the timeline. The problem was that wind gusts on the rear audio channels would just clobber the desired audio (a singing performance) on the front audio channels. The ac-3 encoder in its wisdom would turn down the front audio by 10+db whenever a wind gust would hit.
Okay, so I learned something. I need to carefully manage the peaks on the rear audio channels. But what shocked me was how the surround audio played just fine on the Vegas timeline. The sound from the timeline had been my gold standard. Now I've learned it's not. Rather than just encoding the audio, the Dolby Digital process will make its own changes under certain conditions.