Dolby Tech Page

2.1 Dynamic Range Requirements

The most obvious feature of the new high-fidelity sound-tracks, analog SR and digital SR•D, is a significant increase in dynamic range. The potential for louder sounds requires attention to loudspeakers, power amplifiers and sound isolation between screens. Reproduction of quieter sounds requires attention to sound isolation (again), and background noise.

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2.1.1 Power Amplifier Size

1.  Each channel of the sound system should have a power handling capability easily capable of playing back an SR film recorded at 100% modulation level at all frequencies throughout the audio bandwidth. This should be the very minimum capability for Dolby digital playback, and a crest factor (safety margin) may require a significant increase in power amplifier size.

The dialog level on both the analog and digital SR•D sound-tracks will be at the same acoustic level as that of the dialog of a conventional Dolby Stereo film. Occasional sound effect "stings" and music, though, can have a level far greater than found on conventional films, and this increased peak level capability is one of the great advantages of both analog SR, and the digital sound-track on a Dolby Stereo SR•D print. Depending on the signal content, the peak levels on an analog SR sound-track can be 3dB higher, rising to 9dB higher at frequency extremes, as shown in Figure 2.1. The digital sound-track can provide a peak level 12dB above conventional A-type Dolby Stereo films; it is also important to note that this peak level capability (around 103 dBC for each stage/screen channel) is constant with frequency.

As a very rough guide, analog SR sound-tracks in small and medium sized theatres, peak levels will require power amplifiers for the screen channels with total power output ratings of at least 250 watts. The mono surround channel for SR playback will probably need at least 400 watts.

Large theatres will require even more power -- however, the incremental cost increase of a 500 watt power amplifier when compared with a 250 watt unit is trivial. If in doubt, a theatre should always install amplifiers with a power safety margin.

It is unlikely that any film mix will take full use of the SR•D digital capability for more than an occasional "sting". But examination of analog SR tracks shows regular clipping at 100%. So, for a very minimum, power amplifiers should be able to reproduce the typical levels of analog SR. More power is required if a safe margin is to be provided for SR analog, and for playback of the digital Dolby Stereo sound-track.

Total insurance of enough power for a Dolby SR•D digital sound-track can be derived from Fig. 2.2. Unlike some optimistic nomograms for power requirements, this model takes no account of room volume and reverberation time. Transient sounds (of short duration) are not augmented by reverberation, and the required power for a given sound-pressure-level at a specific seat is directly controlled by the direct sound field -- an inverse-square law characteristic based on how far the listener is from the loudspeaker.1

The power requirement derived from Fig. 2.2 is the total power needed for a screen channel. If the system is bi-amped (virtually essential) the power for each HF and LF section should be approximately two-thirds of that shown.

For an analog SR sound-track, power amplifier headroom capability can be tested by installing a Cat. No. 85C Dolby pink noise generator into the Dolby processor, and assuming Fader 7 is the normal operating level, turning the fader up by 3 points, i.e. up to Fader 10. Examine the power amplifier output signal on the oscilloscope, and confirm that the signal is not clipped, which will be evidenced by squaring of the signal peaks. Do not run this test for extended periods of time, as loudspeakers could be damaged. The test should be repeated for each stage loudspeaker channel.

To verify power amplifier capability for a fully modulated SR•D digital sound-track, the following test can be used...

A tone burst generator is used to insert a signal into the theatre sound system. The output level of the generator is set such that it would generate a playback level in the theatre of 103dB. Ideally, there should be some means of bringing the level up from no sound to the maximum 103dB level at a reference seat two-thirds of the way back in the house with a fader. The easiest way is to run the signal into the cinema processor and then use the house fader to control the level. The tone generator should be able to output tones at the following frequencies: (for Left, Center, Right, and each Surround Channel) 63Hz, 200Hz, 1kHz, 4kHz, and (for subwoofer) 50Hz. The different frequencies will give a better idea of the capabilities of the system if it has speakers that have a deficient response and therefore need excessive eq (which limits headroom). The tone should be gated with a synchronous (zero crossing) switch that has an "on" time of 750 msec (³/₄ sec) and an "off" time of 2 sec. In regards to damaging speakers from this high-level signal, the short "on" time with a relatively long "off" time should allow for a realistic test of the sound system headroom without damaging the speakers (or, less likely, the amps) in the process. A storage scope is connected to the output of the power amp for the channel to be tested. The tone-bursts are switched on for that channel and the fader turned up while observing the scope trace. As the fader is raised to the 103dB level look on the oscilloscope for signs of the amplifier clipping. If the maximum level can be reached with no sign of clipping and with no audible sign of stress from the loudspeaker, the channel will probably be satisfactory for the demands of a digital soundtrack.

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2.1.2 New Jiffy test film

An updated version of the well-known Jiffy test film is now available from Dolby Laboratories. The new Jiffy test film contains both an SR soundtrack and a digital soundtrack. There are six minutes of subjective audio tests, in some cases different for the analog and digital soundtracks, which include high-level tone bursts for checking power amplifier and loudspeaker capability. The test film also contains some quick visual tests, to verify framing and checking for ghosting and shutter problems.

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1 One of the difficulties in calculating sound pressure levels comes from the increased reverberation times in large rooms. This increased reverberation will only be applied to steady-state, or quasi-steady-state, signals. Dialog is mainly made up of short staccato sounds (like t's, k's, p's). Consequently a mix balance created in a small room, with a short reverberation time, will sound different in a big room, with those music components with sustained sounds being augmented by reverberation. The dialog may sound relatively quieter with respect to the music. Sound pressure levels for the playback of Dolby Stereo are set up using pink noise, a quasi-steady state signal source. Theoretically, this means that in a big auditorium (say 1000 seats or more), the music will play at the same level as in a small room, but the staccato elements of dialog will play lower. It can be argued on a theoretical basis that in a really big theatre, for dialog to have the same level as in a small theatre, the reference SPL should be increased from 85dBC to (say) 86 or 87dBC. The result would be a consistent dialog playback level, but an increase in level of steadystate signals, sustained music or effects.

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