STELLADAT
Using DAT in the Field
by Manfred Klemme, Broadcast Engineering
February 1995
Using DAT in the Field
by Manfred Klemme, Broadcast Engineering
February 1995
Production sound is undergoing a major changeover from analog to digital technology. Although this process is revolutionary in terms of the hardware used, it is really just another part of the continuing evolution of production sound.
The history of what we call production sound - the recording of live sound during the filming of picture - is a saga of synchronization between picture and audio. Double system sound (i.e., the use of separate devices for recording sound and picture) has gone through several major changes, beginning with the introduction of Neopilotone by Nagra in the 1960s. Later, crystal sync eliminated the pilot cable, and then time code allowed every frame to be discretely identified. The current revolution is from 1/4-inch analog tape to DAT. Specifically, R-DAT (Rotary-head Digital Audio Tape).
The audio for many documentary films and videotapes is recorded on simple consumer DAT machines with the addition of small professional mic pre-amps. The pre-amps make up for these decks' inadequate analog circuitry and their lack of phantom and T-powering for professional microphones. As with any R- DAT machine, these recorders provide the necessary stability for film sound that will be transferred to magnetic film for post-production. But production sound mixers for major motion picture and TV production are increasingly using fully professional DAT machines instead. These decks feature such exotic items as SMPTE time code for synchronization, pull-up and pull-down of the basic 44.1kHz and 48kHz sampling rates, and powering for microphones. The professional DAT machines are built for the job - rugged and expensive. Converted consumer machines typically lack the robustness required in location work.
Pre-compensating for post-production
The main reason that production sound people are clambering to get into digital is to accommodate digital post-production. It is the users of the digital audio workstations and digital non-linear editing systems who demand digital audio. These users have their own peculiarities. One involved their preference to work in NTSC television. This means that the film from the production has to be slowed down 0.1% and the sound has to follow. If we slow down the audio 0.1% the digital audio output is no longer sampled at the standard rates of 44.1kHz or 48kHz, but as 44.056kHz or 47.95kHz instead. Devices that expect standard digital input signals don't react well to those sampling rates. Therefore, some professional DAT machines can record at 44.144kHz and 48.048kHz, so that direct digital transfers are possible when pulled down in the telecine.
Most music is now recorded digitally at 44.1kHz. Audio for playback on the production of a music video (which is to be shot on film) must be sped up by 0.1% so synchronization with the picture is correct after telecine. That requires the DAT machine to be able to play back at 44.144kHz.
TV production, whether for programming or commercials, is time-critical. TV editors typically use drop-frame time code so that the time code on their color videotapes will match actual running time. Once again, the pull-down, to which the sound and picture are subjected in telecine, changes the 30 frames-per- second (fps) time code that we put on the audiotape during production into 29.97fps time code. If post-production requires 29.97fps drop-frame time code, then the production mixer must record 30fps drop-frame time code on the tape during the initial recording (30fps drop-frame is not a SMPTE-standard rate).
Another example is a telecine transfer from film to D-2 digital video. The film goes through its normal pull-down in speed and its conversion to video fields, but when the sound is pulled down, it's no longer at the 48kHz sampling rate that the D-2 machine expects. If the original DAT cassette is recorded at 48.048kHz with 30fps drop-frame time code, however, the D-2 machine will get 48kHz audio and 29.97fps drop-frame time code.
A major advantage of digital over analog recording is the available dynamic range. The production mixer also must know to what medium the original material will be transferred in order to match its dynamic range to that which can be supported downstream. For example, in a film production where the mixer knows that the tracks will be transferred to magnetic film, dynamic range should be kept smaller than if a digital audio workstation were to be used for post, and the end result released in a digital film sound format. Clearly, professional sound mixers must know what will happen to their tracks in post-production.
On the level
SMPTE has determined that -20dB should be the reference level for digital recording (i.e., 20dB below maximum record level). That means that the familiar 0VU that equates to -8dB on a Nagra modulometer should be set to -20dB on a digital recorder's peak meter. This seems a bit low, but on a machine not equipped with an adequate limiter, lower is better than hotter. (With a good limiter, you can run the level hot without encountering distortion - just a little compression, perhaps. But the next digital device in the chain may not be able to use so hot a signal.)
In fact, recording "hotter" or "down in the mud" in digital is of little consequence to the actual magnetic recording level. In digital recording, that level is constant - and always at maximum or saturation level. Changes in audio levels only affect the composition of the data being recorded, not its signal strength. The level meter on the DAT machine is, therefore, not a recording level meter, strictly speaking, but simply an indicator of the dynamic range of the signal.
Because post-production staff in the film and TV industries desire to stay in a digital format as long as possible - eventually right to the release-print or air-tape - production sound mixers must have the tools (knowledge and equipment) to do their jobs properly. They have mastered the transition from optical to magnetic recording, then from magnetic film to 1/4-inch tape, then from Neopilotone to time code for synchronization. The transition from analog to digital recording is the next step, albeit a big one.