THE X CURVE & Double-Head Screenings

The name sounds like some magic preset, and in a way it is, but I’m not sure if magic is quite the right description. Althought I had heard mention of it before I only became aware of the true nature of its existence last year when mixing the film Bridge To Terabithia. On every film project once we get through the lengthy process of premixing all the elements (2-4 weeks) and then final mixing (1-2 weeks) we do what is termed a “double head screening” to enable us to check the mix in context, prior to delivery of the finished soundtrack. The term double head refers to the fact that the film and its soundtrack are still seperate entities, since while we have been mixing the picture side of things has been going through its own final stages: conforming, VFX integration, grading & test film outputs. So for the screening hopefully we are watching a first graded film output, while the mix will be running off a synced hard disk recorder.

The double head screening is a CRUCIAL part of the process & is something I would always insist on with any project I am involved with. This isnt a problem here in NZ as it is always built into the schedule & budget, but talking to people in other countries I know some do not do double head screenings & considering the investment in getting the film to this point it seems like a false economy. The main reasons for a double head screening are for us to hear the soundtrack in context ie in a “normal” theatre (not a mixing theatre) and in a continuous run. By doing this we are able to check the changeovers between reels (films are made in 20 minute reels) but more importantly to watch the film as one continuous entity and discover whether the choices made in mixing the film, scene by scene, were the correct/best choices. Because we are also hopefully watching an actual film output, rather than compressed video, it is also our final sync check. If a line of ADR seemed a little bit loose when mixing to the pixelated video, when you see it against the beautiful clarity & detail of film it soon becomes apparent as to whether it is a problem or not.
After the double head screening we have an informal meeting & talk through the film from start to finish & everyone voices their concerns, starting of course with the director. Schedule-wise we then tend to spend two more days on the mixing stage making the final adjustments that are required. If I was a director I would insist on this process, otherwise any of these issues will not be apparent until the film is at the premiere & the sudden benefit of hindsight kicks in – too late… but the process also serves everyone on the film, as both a technical reality check & a conceptual/thematic overview… And I have known more than a few directors who, when budget allows, have then demanded another double head screening to check the changes and then allow time for more fixes… Which is fine – in post production you soon realise that you could keep working on the same film for the next five years & possibly/probably keep making it “better.” As with producing music the eternal question is always “when is it finished?” and its often only schedule or budget that provide that answer.

Anyway back to the X-Curve: during the doublehead screening of Terabithia both myself and the dialogue supervising sound editor noticed that some lines of ADR had a high pitched whine behind them. When we discussed it later we came to the conclusion that it was actually just one characters ADR & thus deduced it was probably the line frequency whine from a TV set that was in the room with the actor when he was re-recording his lines. Thankfully most ADR studios now have LCD or plasma screens in their recording rooms for playback, but if ever you see a TV in the booth make a note that you are also recording a nice high pitched whine. If the ADR was recorded in the USA, which this was, then the line frequency will be 15.75kHZ (NTSC) or if recorded in a PAL country it would be 15.625kHz – heres why….
Anyway the problem was added to the list of fixes to be made and so later back on the mixing stage a we worked our way through the other fixes in the film, every time we come across a line of ADR by this character we planned to notch out the offending frequency… And on the first occurence was when I learned about the X-Curve – we played the line & there was no whine!!!! What the hell???? After replaying it & getting everyones attention & listening attentively we all agreed there was definitely no whine….. hmmmm… ok so to just make sure that we werent imaging it, we zipped down to a scene where we had DEFINITELY heard the whine in the screening & sure enough, no whine. For a moment we were stumped until I had the brainwave to suggest playing the suspect line at half speed (shift spacebar in ProTools or CNTRL 2 on the numeric keypad) and BINGO – there was the whine we heard, at half its normal frequency!!! I knew we weren’t hallucinating that whine but I couldnt understand why we couldnt hear it at normal speed. So while the mixer notched the frequency out, a converstaion followed about the existence of the X-Curve…

Turns out that for partly historical but mostly consistency reasons film mixing stages & picture theatres use a monitoring EQ called the X-Curve. The EQ curve above is borrowed from an interview with audio pioneer Loan Allen, on the editors guild website. He explains better than I can why the X-Curve is used, and how it evolved from its predecessor, the Acadamy Curve… but in a nutshell the X-Curve is an EQ curve applied to monitoring in both film mixing stages & in commercial picture theatres which rolls off high frequencys at a fixed rate in an attempt to ensure consistency. The exact X-Curve used is based on room size and THAT is the real motive here, to provide the same perceived spectral response.

For what its worth there is also a specification for the playback level for films, which is treated almost religiously in film mixing stages but we can only wish the same was true of commercial movie theatres, which it seems is more based on who complains the loudest ie either “ITS TOO LOUD” (turn it down), or “I can’t understand the dialogue” (turn it up). Sadly the former complaint is often caused by amps & speakers that cannot handle the volume & what the people are actually complaining about is distortion, not level… I experienced such problems first hand a few years ago when a director complained about how his film was sounding in the theatres. He had been very happy throughout the mixing process & double head screening, so was mostly concerned that it was a print problem. I agreed to join him at one of the theatres where it was screening so we could check it out. We arrived between screenings, got the manager involved & asked them to play the first five minutes for us. It took two minutes to see what the problem was – this theatre was so crap it had its monitoring set up so the Left, Centre & Right speakers were all coming out one centre speaker, and the stereo surrounds weren’t stereo at all – they were mono! Aside from that, when they stopped the projector there was an earth hum coming from the speakers which was so loud that if it was on a piece of dialogue we would have cued it for ADR. All I could do was quietly suggest that film maker never screens his films in that theatre again & I made a mental note to never go & see a film in that theatre. After the millions of dollars spent making the film, here at the last step it was being reduced to less than what you might expect from a no budget film with practically no sound post production. And they have the nerve to charge the same price as a properly functioning theatre!?! ARG!!!! May they rot in hell!!!!!

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