Just to be clear in regards to the mixer you speak of; is it common then to take a piece of dialogue from one take and lip sync it over another?
To be absolutely clear, it would be the dialogue editor who chooses the "pieces" of dialogue, not the re-recording mixer. When you watch/listen to a high budget film, it would be quite unusual to use all the dialogue for one scene just from a single take. Realise it or not, this raises several issues which may or may not be immediately apparent:
1. The quality requirements of dialogue editing, sound editing and mixing are not fixed. The first question of audio post is "what is the distribution format?", because you need to be editing at a quality level (and therefore with a monitoring environment) equal to the quality of equipment the consumer is using and for mixing you need to have monitoring equipment and levels of quality substantially better than the average consumer. This is so that you can identify (and fix) flaws the consumers' equipment could reproduce which would destroy the suspension of disbelief and the emotional/dramatic impact of what you are trying to create. So if your distribution is the web, most consumers are going to be listening on smartphones, laptops and computer speakers, so even average priced nearfield monitors would be sufficient for editing and mixing. A DVD or TV product is going to generally be listened to on a TV, a budget home cinema system or possibly a high quality home cinema system. A BluRay product is more likely to be listened to on a high quality home cinema system. A film for international theatrical distribution is going to be played in cinemas where many hundreds of thousands of dollars have been spent on the acoustics and audio playback equipment. Each step up the ladder therefore requires higher quality audio post facilities and experience/skill. The huge, powerful, accurate sound systems found in most cinemas can reveal the tiniest of flaws, completely inaudible on even expensive studio monitors. In fact cinemas represent by far the highest quality audio reproduction most people will ever experience. This is why the top audio post facilities spend millions on acoustics and monitoring equipment, why the dialogue (and sound) editing and mixing needs to be so exacting and why audio post for theatrical release is so hugely expensive. The big problem for the micro/no budget filmmaker is when making the jump from the lowest quality audio arena (the web) to the highest quality arena (cinemas) in the case of film festival screenings for example.
2. Obviously, to use dialogue from alt takes and other sources means there have to be alt takes and other sources! this can be a problem if, for example, the actors are ad lib'ing or not following the script exactly. The actors need to be able to reproduce the ad lib'ing accurately and you as the filmmaker need to be recording two or more takes of this ad lib'ing. Ultimately, this should all be covered by good planning, during read-throughs in pre-production with the production sound mixer (PSM) present, who can then advise on workflow. Obviously if this and other sound issues can addressed during pre-production you are going to save a great deal of time, effort, frustration, ADR requirement and sub-standard audio during filming AND later in audio post. Also bare in mind that the PSM is working with a set of headphones in an imperfect monitoring environment. So even a highly experienced PSM with top quality equipment is not going to be aware of all the flaws which could become apparent in audio post.
3. The dialogue editor needs to be able to easily find all the sources of dialogue for each scene! This means good file management such as an accurate and detailed sound log, a logical and accurate file naming convention, scene and take numbers entered in the metadata and matching timecode stamps for each take. All this sounds boring but is an essential part of the PSM's role; it's common to have 3 or more different mics per take, plus there could be many takes per scene and then how many scenes are there in a film? The dialogue editor could be dealing with thousands of audio files and tens of thousands of channels of audio. If alt dialogue (from other mics, alt takes and wildtracks) cannot be easily located, imported and aligned, time constraints are going to require a great deal more ADR and/or sub-standard dialogue.
A lot of this info might appear quite discouraging to the average micro/no budget filmmaker but look on it as an opportunity. The vast majority of micro/no budget filmmakers either don't know this info, can't afford or figure out how to implement it or just don't care. Knowing this info and trying to implement it could give you a substantial advantage!
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