Stereo requires the reproduction of signals from 20 Hz to 20 kHz. This is done with multi-way speaker systems, which use a combination of woofers and tweeters to achieve full-range response. These speakers are connected via a crossover network to route the appropriate frequencies to the various speakers in the system. This may be a two-way, three-way, four-way, or even-five way system, but in each case, the goal is to reproduce 20 Hz to 20 kHz evenly.
Today’s Dolby® Digital consumer decoders include a bass management system to do just that. Just as with the old stereo and Dolby Surround systems, the goal is to be able to reproduce all frequencies within the system. The five main channels and extra LFE channel provide more possible combinations of speakers, including five full-range main speakers and a subwoofer for the LFE; five small speakers for the main channels and a subwoofer for both the LFE and bass redirected from all five main channels; and various combinations of the above examples.
Studios must be able to reproduce all reasonable frequencies from each full bandwidth channel. Crossovers, subwoofers, and main speakers should work together to give flat response for each of the five main channels.
Many manufacturers of near-field monitors make complementary subwoofers to complete the system. Larger rooms may dictate the need for more than one subwoofer to achieve adequate bass response.
When using the LFE channel in a mixing situation, it is important to band-limit the information for this channel. During the Dolby Digital encoding process, the encoder will brickwall filter the LFE signal at 120 Hz. This is true for both professional Dolby Digital encoders, as well as the interactive encoders found in game consoles such as the Playstation 4 and Xbox One.
Consumer decoders take the LFE signal and add any channels in need of bass management, as determined either by product design or user selection. The five main channels are then highpass filtered at either a fixed frequency of 80 Hz or a selectable frequency of 80, 100, or 120 Hz. The summation of the LFE and any other channels is lowpass filtered at the same frequency.
While the Dolby Digital encoder and decoder together will handle bass management in decoding, it is often not feasible to use them in this way when mixing, due to the delay through the encoding and decoding process. Therefore, it is necessary to have a separate crossover system in place to handle the bass management. Many manufacturers now offer such devices for this purpose.
To replicate what the consumer will hear, a third-order (minimum) 80 Hz filter in the LFE audio signal path to the recorder is recommended. It is advisable to include this filter in the console output before the monitor such that both the recorded information and the heard information are band-limited. Failure to include this filter will result in hearing substantial bass information above 80 Hz in the mix that will not be present in the Dolby Digital encoded version.
Once the ability to reproduce all frequencies in each channel has been met (as described in the Bass Management section), the room must be calibrated. For each of the five main channels, pink noise is adjusted for 79 dB C-weighted slow. Many developers might find this level too loud, and may need to calibrate to a lower level.
The LFE channel is calibrated such that each 1/3 octave band between 20 and 80 Hz is 10 dB higher than the equivalent 1/3 octave bands for any of the full-range speakers, assuming that the full-range speaker is ideally flat. This level is read from a real-time analyzer (RTA), rather than a sound pressure level (SPL) meter.
If an RTA is not available, an SPL meter may be used to approximate the level. If an SPL meter is used with band-limited pink noise, then calibrate the subwoofer between 4 and 6 dB (C-weighted slow) higher than any of the full range speakers.
A properly calibrated room will result in mixes that will sound correct when played back in a consumer environment. An improperly tuned room will result in mixes that will sound fine in the mixing facility, but will be incorrect in other situations. Using the guidelines above will result in a properly tuned room.
The following guidelines offer commonly accepted practices for setting up multichannel audio monitoring systems for game creation.
Multichannel sound systems add a Center speaker to the Left/Right (L/R) pair used in stereo systems. To promote good imaging, all three speakers should be identical, just as conventional Left and Right stereo speakers must be matched. If all three cannot be the same model, the Center speaker may be a smaller model from the same product line.
The front speakers should be equidistant from the listener, with their acoustic centers in the horizontal plane—that is, on-axis to the ear.
The Center speaker needs to be positioned below a video monitor, forcing the acoustic centers of the three front speakers out of alignment. If this occurs, attempt to situate the speakers so the tweeters are in as close to a horizontal straight line as possible. This may require either an inverted or lateral orientation of the Center speaker, as well as rotating the tweeter (when possible) to maintain the proper dispersion characteristic.
If the Center speaker is not equidistant with the L/R pair, signal delay may be used to obtain coincident arrivals.
All front speakers must exhibit the same acoustic polarity. It is highly recommended that electronic signal polarity be maintained throughout the entire monitoring system.
Whenever possible, use the same speakers all around to achieve uniformity. If this is not feasible, the surround speakers may be smaller than the front speakers but should maintain the same character—for example, they might be smaller speakers from the same manufacturer.
The front and surround speakers should be equidistant from the listener, with their acoustic centers in the horizontal plane that is on-axis to the ear.
The surround speakers should achieve coincident arrival with the front speakers either as a result of equal path lengths or through alignment with signal delays.
The Surround speakers must exhibit the same acoustic polarity as the front speakers. It is highly recommended that electronic signal polarity be maintained throughout the entire monitoring system.
For Dolby Atmos® configurations, there are two options for speaker types that can be used to playback the height channels; discrete and Dolby Enabled. Discrete speakers are just as described, a physical speaker mounted on or in the ceiling, one for each height channel. Discrete speakers are required for Home Entertainment and Games studios in a control room. Dolby Enabled speakers are speakers that use licensed technology from Dolby that are positioned on the listener’s horizontal plane but reflect sound off of the ceiling, again one for each height channel.
Again, it is generally recommended for a control room to use discrete overhead speakers whenever possible. Dolby Enabled speakers have the potential of creating a more diffuse image at the listening position depending on placement and ceiling height. While this is very complimentary for playback in the consumer’s living room, it may not be suitable if more positional precision is required for content creation. Discrete speakers offer more precision when mixing and panning, which allows for a more controlled final mix that will playback great for the consumer on any Dolby Atmos renderer.
Whenever possible, use the same speakers all around to achieve uniformity. If this is not feasible, the ceiling speakers may be smaller than the front speakers but should maintain the same character—for example, they might be smaller speakers from the same manufacturer.
The height speakers should achieve coincident arrival with the front speakers either as a result of equal path lengths or through alignment with signal delays.
The height speakers must exhibit the same acoustic polarity as the front speakers. It is highly recommended that electronic signal polarity be maintained throughout the entire monitoring system.
The LFE channel requires the use of at least one subwoofer in the monitor system.
The bass from any channel that is not reproduced in the main speaker for that channel must be redirected to the subwoofer(s). There are now various products and techniques that handle bass management (crossover filters, bass mixing, and combining with the LFE channel in the proper mixing ratio) that can help achieve a proper monitor setup in the studio.
It is essential to correctly integrate the subwoofer(s) with the main speakers to ensure a wide, smooth, and uniform frequency response from all five main channels. In addition, it is critical to have the LFE channel reproduced in the proper relation to the other channels.
Positioning the subwoofer(s) often can be an arduous task and the relative location(s) will not be the same for all rooms. A certain amount of experimentation should be expected particularly when retrofitting an existing production room.
Initially, place the subwoofer(s) near the listening position. Play program material with significant low frequency content and listen at likely subwoofer locations in the room. Locations delivering the smoothest bass response are apt to be the best choice for final subwoofer placement.
The International Telecommunications Unit, Radiocommunication Sector (ITU-R) has specifications for a listening room layout designed for the critical evaluation of multichannel programs. These recommendations are a good starting point for a mixing room setup as well. Aside from signal alignment, a specific geometry is described. With the Center speaker directly in front, position the L/R speakers 30 degrees from center (forming a 60 degree angle) and the Surround speakers 110 degrees off center.
Discrete overhead speakers should be mounted between 2 and 3 times the vertical position of the main listening-level speakers. The angle of elevation from the listening position to the Ltf/Rtf and Ltr/Rtr overhead speakers in a 7.1.4 reference layout should be between 30 and 55 degrees. This will tend to be the case when the front-to-back separation of the overheads about half the length of the room layout. If the mounting height is so high that the angle of elevation exceeds this range, the overhead speakers should be moved further apart (towards the front and back walls) until the angle of elevation is in the recommended range and delays should be set for all speakers to ensure coincident arrival. If the ceiling height is very low, it is still recommended to keep the front-to-rear separation of the overhead speakers at half the length of the room.
Dolby Enabled speakers with upward firing drivers are available as separate modules or integrated into the cabinet of the listening-level speakers. Modules should be placed on top of the front Left/Right speaker cabinets for the corresponding Ltf/Rtf channels. Modules should be placed on top of the Left/Right Back surround speaker cabinets for the corresponding Ltr/Rtr channels. Whether using separate modules or speakers with integrated upward-firing drivers, the top of the Dolby Enabled speakers must be at or above “ear level” at the listening position. This ensures that the listener is receiving more of the reflected sound from the ceiling and not the direct sound from the driver itself.
Each studio design has its own unique approaches and sets of challenges. Contact us to consult with our team about how best to meet your goals.
Our Dolby® Game Studio program can help ensure that you get the most successful results from Dolby technologies and your multichannel mix so that your audience receives the best audio experience possible.
- Measurement and analysis of your development studios, and use of this data, with your input, to set a practical and achievable audio standard for such items as acoustic room tuning, reference recording levels, and reference SPL levels.
- Consultation and referral services for acoustic design and audio hardware for your development studios.
- Dolby Laboratories’ calibration for all completed facilities on an ongoing basis as determined by Dolby and you.
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