Recent Coursework and PPM meters

Yesterday saw the main hand in for my module on Audio Post Production whereby I asked the students to create a complete sound track for either a 3 minute extract from a film, or for some of their own created footage. The assignment stated that every sound must be originally recorded and that any dialogue must be replaced using ADR techniques. The finished 3 minute piece then had to be submitted on DVD together with two other audio layers: a 5.1 mix and a stereo mix complete with supporting "directors commentary" added. Some of the students also investigated the use of "ducking" techniques whereby the commentary automatically dips down the level of the main soundtrack when present and then when not, the main soundtrack returns to unity.

One of the other main areas I wanted my students to ensure, was that their levels did not breach broadcast "safe" levels, which in Europe is PPM6 maximum (which equates to approx. -10dBFS), with the digital reference tone being PPM4 (-18dBFS). This area is important and it is something I plan to cover more on next year. It's simply not the case of checking levels on a PPM meter - calibration is necessary as otherwise, like with all uncalibrated meters, the readings are useless. A student asked me the other day, why, even though they had normalised their levels to -10dBFS (so that nowhere was their peak outputs higher than what is effectively PPM6) that the meter was overloading. This is an interesting area, as again, it's not as simple as generating a 1kHz sine wave at a digital level of -18dBFS and then adjusting the software's master fader until the meter reads PPM4. This method would produce a monitoring situation which did effectively show correct levels within the studio that it was carried out in, however, take the soundtrack anywhere else and it won't be. It is important to realise that the ELECTRICAL output level entering the PPM meter must be such so that for a sine wave with a level of -18dBFS it shows PPM4. If the sine wave is generated and then, say, the master fader within the software is adjusted, then the tone is obviously no longer -18dBFS. All faders within the session must be set to unity (i.e. 0dB) and then the actual ELECTRICAL output must be adjusted. Some professional audio interfaces (the Digidesign 002 for example) are set so that -18dBFS = PPM4 (or 0dBu, 0.775Vrms) so this is not a problem, however when using other audio interfaces, especially cheaper models, or ones powered solely by USB, the outputs simply can't output sufficient level. To overcome this, a small mixing console can be used to amplify the electrical level of the audio interface so that, for -18dBFS, the PPM reads accurately.