Hi Hugh,

I've had extensive discussions about the preparation of audio for entries submitted to two prominent film festivals, speaking with the technical director of one of the festivals and the programming manager of the other festival. What do you think?

Thanks.

Robert

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According to the technical director:

Most of the entries, whether on DVD for programming consideration, or on HDCAM for projection, have most of the audio between -10 dBFS and -40 dBFS, although peaks might be at full scale, and ambience or “room tone” might be at -60 dBFS. Normal dialog should follow the Dolby specs, even for PCM audio, i.e. it should be at an average of -31 dBFS, or -27 dBFS, which are two levels often adhered to.

The DVD may be watched on anything from a laptop computer to some super system with great audio reproduction. [This would suggest that a narrow dynamic range should be used for suitable playback on mediocre systems, such as a computer, although, he said that the audio used on the DVD and the HDCAM tape usually originate from the same mix, with the same relative levels among the various constituents, dialog, Foley, music and ambience.]

The HDCAM tape must be prepared so that it is Pro Logic compatible. By default, Dolby audio, prepared according to Dolby’s “e” specifications, will be compatible, and indeed, any Dolby audio on the HDCAM tape must be of the “e” type. In addition, a PCM, two-track mix is also required on the HDCAM tape, but it too must be Pro Logic compatible. (Dolby audio is not required for the tape, but PCM audio is.) Basically, the Pro Logic system will attempt to place all dialog on an additional track, to be reproduced on speakers that are centered in the theater; however, when this done, problems may arise due to phase relationships in the original two-track mix. [He did not explain how the dialog of a two-channel mix is identified by the Pro Logic system. Just because it’s centered, or because, in addition, it’s at a certain level? But regardless, wouldn’t that mean that any sound (e.g., Foley) that was centered and at the same level as the dialog would also be processed as the dialog was and thus be placed on the “center track”?]

During projection, the volume setting on the Pro Logic system will first be set to “7”, then adjusted upwards or downwards, if required, as the HDCAM tape plays. The volume scale on such a system is from 0 to 10.

I suggested that someone using Pro Tools might create a mix that has three submixes, or “stems”, as they are called in the audiovisual world, and he said that that would be appropriate: a mono stem consisting of dialog only, where that dialog was all recorded in mono; a mono stem consisting of Foley and ambience, where those elements were recorded in mono only; and a stereo stem consisting of music, thus making for four tracks in total. It is these stems that one would take to a post production facility to create audio for HDCAM, and from those stems, Dolby audio as well as Pro Logic compatible two-track PCM audio could easily be created (at the post production facility), where the latter would be without the phase problems mentioned earlier. [I suppose that relative levels between stems could also be adjusted then, compression or expansion applied to stems, etc.]

He also said that any latency in the digital projection system (which results in the image lagging the audio by as much as two or three frames) will be compensated for by the projectionist, and that any need to advance the audio with respect to the image, due to the slower speed of sound and thus its delayed arrival at the audience, will be done, once again, by the projectionist.

He added that the sound at the back of the theater may be 3 dB or so quieter than that at the front, and that theaters have an “x-curve”, which refers to its frequency response, and which results in there being a roll-off at the high end and at the low-end.

The festival has a spec. sheet concerning the audiovisual requirements for the HDCAM tape.

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According to the programming manager:

He said that the DVD for programming consideration will be viewed by the judges in a variety of environments, using a variety of playback equipment. Some judges view DVDs at home, perhaps on an “ordinary” TV, or on a home-theater system. If the submission makes it far enough “up the ladder”, then several judges might view it together in a small theater, having perhaps a 12-foot diagonal screen. The sound quality in such a theater is as good as that of a commercial cinema.

The sound quality in the theaters used for projection during the festival is also as good as that of a commercial cinema. People are well behaved and so “crowd noise” is minimal. The noise from the amplifiers is low. If the volume has to be changed during projection, it is usually adjusted downwards. Those theaters seat anywhere from 200 to 1,400 people. The 200-seat theater has a projection screen that is perhaps 30 feet across, measured diagonally. Short programs are typically shown in the smaller theaters.

He suggests PCM audio only for the DVD, since Dolby requires one to fiddle with the playback system, and he implied that the judges will not do that.

The festival has a spec. sheet concerning the audiovisual requirements for the HDCAM tape.

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Robert Eidschun

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